I 1~ · PDF filebasis for preliminary scree-ing of materials. Correlation of behavior ......
147
AD-75,1 802 I NIPROV KD11 MATLRIALS AND MANUFACTURING NI I-CTIODS FOR GUN BAR RLLS. PA RT I R .l- lP. O'ShIca, Cl al I11T Rc2s2ZLCa rc 1,tl IsiutC I Pr-cpa rcd for-: 1~ ~b 1972n ;Cinind i i StRIRBU1T3" BY: Natiunbi ec~nkcaI Mfafrrnairk Service 11, S. eOYARTMrNT OF COMM¶ERCE 5285 Port tPopi Ro?'1, Sp~ingfield Va. 2'ý2151
Transcript of I 1~ · PDF filebasis for preliminary scree-ing of materials. Correlation of behavior ......
AD-75,1 802
I NIPROV KD11 MATLRIALS AND MANUFACTURINGNI I-CTIODS FOR GUN BAR RLLS. PA RT I
R .l- lP. O'ShIca, Cl al
I11T Rc2s2ZLCa rc 1,tl IsiutC
I Pr-cpa rcd for-:
1~ ~b 1972n ;Cinind
i i StRIRBU1T3" BY:
Natiunbi ec~nkcaI Mfafrrnairk Service11, S. eOYARTMrNT OF COMM¶ERCE5285 Port tPopi Ro?'1, Sp~ingfield Va. 2'ý2151
IMPROVED MATERIALS AND MANUFACTURING40-. METHODS FOR GUN BARRELS.
mjau~(PART 1)
August 1972.
I
Dr.R. . 0'Sha, ITResearch Institute
0. S Alison h tleIfe Northwest
JD.Difteaedetto,
LISA WECOM.
-BAC' DIRECTORATE
WEAONSLABORATORY USAWECOM
E Approved ANDErIN
"I: " . •.I
"DISPOSITION INSiRUCTIONS:
" Destroy this report when it is no longer needed. 'DoSnotreturn it to the originator. ,
DISCLAIMER: "." "-
The findings of this report are not to be construed-as an official Department of the Army position unless so
---designated by other authorized documents.
S. ._2__.p it. ... ... .
". . ...... .. . , ... ..
t .
I
-Unclaslsified
(S*..~p.I.*f~i~ .i~ .4DOCUMENT CONTROL DATA. - It AA *VMI 0fI~.I*Wi.91 4 O.Awle And ftd..A.4 .*.mls we#t 6& .eamod va0,&o l t1 l. Md
IONIIIINA TING ACTyIVITY(C. e SOdv a. 011POIRY 09%uqV1IY CII.A91111ICA ION
U. S. Arm~y Weapons Commend UnclassifiedResearch, Dev, and Eing. Directorate Gat V
IRock Island, Illinois 61201 ____
I 0111001T TITI.-
I XPROVED MATERIALS AND MANUFACTURING METHODS FOR GUN BARRELS (PART I)(U)
4 OR ICRIP Tow a No rag (2Tr" Ofew ~ s.,.df
R. P. O'Shea - LIT Research Institute, Chicago, IllinoisG. S. Allison - Battelle Northwest, Richland, WashingtonJ. D. DiBenedetto and K. R. Iyer - LSAVECOM, Rock Island, Illinois
* PREPOPRT CAIN mve L6A NO. OF PA696 040 wo. "COpq
Augubt 1972 134 IT 13U. C0ONTRACT7 On GRAN4T NO. Ia *I*I& 001YONS 49PORT NUUIER9IIIII
DAAF0l-70-C-0l67____________
AMIS Code 4932.06.6784_____C_ Z~. OTP090 R EPNr, PIOCSI(AIWON.A. M Lu? A0 . ... 014.d
10. ViSTRIOUIONI STATIRMICT
Approved for public release, distribution unlimited.
~.gUPp0TUNYARY.NOTES OVIOING MLIARV ACTIVITY
Deatails of illustrations inthis document may be better U. S. Army Neapons Command
studied 2n mpierofiche.. SYAYA multiphase program of the Research Directorate, Weaponsi
Laboratory, USAWECOJI, was conducted to utilize modern manufacturingmethods for improved materials when used for rapid-fire 7.62mm. Zunbarrels. An initial literature survey provided the state of the art andbasis for preliminary scree-ing of materials. Correlation of behaviorof the mate,ýials in firing tests and vented bomb experiments, results of
mechanical property tests, and economic limitations were used to selectIthree materials - chrome-plated CG-27, chrome-plated fill, and a cobaltbpss' metal powder alloy for inteitsive evaiuation. T[he hot extrusion/cold swaging production sequence was employed in barrel manufacture. Thas-swaged bore surfaces were directly chrome plated to produce a finish-ed bore. Chrome-plated CG-27 and chrome-plated fill barrels were made.Unfortunately, the cobalt base metal powder barrels could not be fab-ricated. Barrels were test-fired under a controlled schedule and wereevaluated by metallographic examination. Chrome-plated fi11 steel bar-rels showed am improvement of one-third to twice the service life overth,_ standard chrone-plated steel barrel; however, the chrome platedCG-27 barrels were inferior to the standard bai-rel. The outstandingperformance of the chrome-plated fill steel bariels was due to the hotstrength, temper resistance, good toughness of the Hill steel, and theexcellent adherence of the chromium plate. (U) (O'Shea, R. P., Allison,G. S., DiBenedetto, J. and Iyer, K.)
DD , O 47 *UPLIt@~S UI.8. too &am"IC Unclassified
Ilk--
Uncl assified
14 LI Nl K AI If
oqg 11040 OLk wI" MOL . -T t]OLK[- W
1. Gun Barrels
2. Materials
3. Rotary Swaging
4. rest-fiiing
lI
I It Ir
Unclassified
Security Clasliflcation
td
AD)
IJRESEARCHt DIRECTORATE
]
WEAPONS LABORATORY USAWECOM
RESEARCH, DEVELOPMENT AND ENGINEERING DIRECTORATE
U. S. ARMY WEAPONS COMMAND
TECHNICAl, REPORT
SWERR-TR-72-54
IMPROVED MATERIALS AND MANUFACTURING 4
METHODS FOR GUN BARRELS(PART I)
Dr. R. P. O'Shea G. S. AllisonIIT Research Institute Battelle Northwest
Chicago, Illinois Richland, Washingtonand
iJ. D. DiBenedetto
Dr. K. R. IyerResearch Directorate
Weapons Laboratory USAWECOM
August 1972
Details of illustrations Inthis document may be better
studied on mirofiche
Contract I)AAFOl-70-C-0167 AMS Code 4932.66.6784
Approved for public release, distribution unlimited.I C
1
AllSTI'IPACT
A mull ipha se program of the Research Di rec torat c,Weapons laboratory, USAWECOM, was conducted to utilizemodern manufac-turing methods for improved materials whentUsed for rapid-fire 7.62mm gun barrels. An initial liter-aturc survcy provided the state of the art and basis forpreliminary screening of materials. Corireiation of be-havior of the materials in firing tests and vented bombexperiments, results of hot hardness and thermal fatiguetests, consideration of engineering and physical propertiesof the materials, and economic limitations were used toselect three materials - chieme-plated CG-27, chrome-plated1lli, and a cobalt base metal powder alloy - for intcnsiveevaluation. The hot extrusion/cold swaging production se-quence was employed in barrel manufacture. The as-swagedbore surfaces were directly chrome plated to produce afinished bore. Chrome-plated CG-27 and chrome-plated 11l1barrels were made. Unfortunately, the cobalt base mnetalpowder barrels could not be fabricated. Barrels were test-fired under a controlled schedule and were evaluated bymetallographic examination. On the particular firingschedule the chrome-plated 1111 steel barrels showed an im-provement of one-third to twice the service life ovc,,r thestandard chrome-piated steel barrel; however, the chromeplated CG-27 ba.rrcls awere ±inf.tLoU Lu the standard barrel.The poor performance of the chrome plated CG-27 alloy barrelwas attributed to massive muzzle-end erosion-which wasprobably induced by a high-temperature nickel-sulfur re-action. The outstanding performance of the chrome-plated1ill steel barrels was due to the hot strength, temper re-sistance, good toughness of tihe ll1 steel, and the excellentadherence of the chromium plate.
ii,
FORE'WORD
This report was prepared by Dr. R. P. O'Shea of IITResearch Institute, Chicago, Illinois, in compliance withContract DAAFO1-.70-C-0167; by Mr. G. S. Allison of BattelleNorthwest, Richland, Washington, in compliance with MIPRA1-9-30372-MI-W3; and by Mr. J. DiBenedetto and Dr. K. R.Iyer of the Research Directorate, Weapons Laboratory USAWECOM,U. S. Army Weapons Command.
The work was authorized as part cf 1he ManufacturingMethods and Technology Program of the U. S. Army MaterielCommand which is administered by the U. S. Army ProductionEquipment Agency.
5 I
t"
iii
CO NTE NTS
TITLE LPACG: i
ABSTRACT ii
FO u19WORI)
TABI,Ei 01: CONTI NTS iv
LIST 01: FI(GURES vi
LIST OF TABLES xiii
1 NTROI)UCT I ON 1
lIXPITiRINMINTAL WORK 2
PHIASE I Characterization of Erosion-Corrosionin the 7.62mm Barrels 2
Literature Survey 2
Gun Tcsts 2
Vented Bomb Tests S
Mode of' Failure of Chromium-PlatedBa rre Is 12
P1iIASI: 11. Materials Evaluation 17
Vented IDomb-Gun Test Correlation 17
Correlation of Properties with Ma-terial Perfoi mance in Gun Barrels 31
Hot Hardness Determinations 31
Thermal Fatigue Testing 34
SUInmI11a ry 39
Vented Bomb Tests 48
PIIAS~i Ill. Materials Sele(tion 77
iv
CONTi-NTS (Cont inued)
PIIASE; IV. Fabrication of Selected Materialsinto Gun Barrels and Test Firing 79
Fabrication 79
CG-27 Processing 79
li1 Processing 81
Cobalt-Iron Alloy Processing 82
Summna ry 82
Test Firing 83
1P11ASE V. Analysis 83Post Analysis 83
Bore Measurements 83Metallographic E~xamination 88
Chrome-Plated Gun Steel Barrels 88
Chromc-Plated 1t11 Steel Barrels 103
Chrome-Plated CG-27 Alloy Barrels il1
SUMMARY AND CONCLUSIONS 120
RECOMMNIN)AT IONS 12 b
RIFIERIENC ES 128
!)I STR I BUT I ON 130
1)1) FORM 1473 Document Control Data R&D 133
vI
SRI
'.,
lV
FIGURES
I lPhotomicrographs Showing Chromium Plate/SteelIntLerface of a Minig n Barrel After 300 Shots 13
S2 Photiomicrograph of a Longitudinal Section* Showing Typical Chromium Plate/Steel Interface
After Firing to Failure 14
'3 Scanning Electron Micrograph Showing TypicalCrack and Associated White Layer on FailedGun Barrel 15
4 Scanning Electron Micrographs of Crack Tipand Associated White Layer 16
5 Pbotomicrograph Showing the Transverse StructureAlong the Bore Surface of Unplated Gun BarrelAfter 300 Shots 18
6 Schematic View of the Vented Bomb Fixture 19
7 Typical Presstire-Time Pulse With a 23 1/2 gCharge of WC846 Ball Propellant 20
8 Photom.icrograph Revealing the TransverseStructure Along the Bore Surface of an UnplatedRifled Insert After 5 Vented Bomb Shots With1WC846 Propellant 23
9 Photomicrograph Revealing the TransverseStructure Along the Bore Surface of an UnplatedRifled Insert After 5 Vented Bomb Shots WithIMR Propellant 23
10 Phouomicrograph Revealing the TransverseStructure Along the Bore Surface of a Chromium-01 ,...... C Arf. -.C .. d -insei "-ef 5 VenLed Bomb Shots
* C L % L.L1~.. L t1rLs
With WC846 Propellant 24
11 Photomicrograph Revealing the TransverseStructure Along the Bore Surface of a Chromium-
Plated Rif'ed Insert After 5 Vented Bomb ShotsWith IMR Propellant 25
12 Photomicrograph Revealing the TransverseStructure Along the Bore Surface of a HaynesStellite 21 Insert After 5 Vented Bomb FiringsWith WC846 Propellant 26
13 Photomicrograph Revealing the TransverseStructure Along the Bore Surface of a HaynesStellite 21 Insert After 5 Vented Bomb FiringsWith IMR Propellant 27
vi
Page
FIGURES
14 Fixture and Test Specimens for the MutualIndentation Hot Hardness Test 32
15 Gcencral View of Equipment for MutualIndentation Hot Hardness Testing 33
16 Hot Hardness Data for Selected Alloys 36
17 Schematic View of Thermal Fatigue Facility 37
18 IITRI Thermal Fatigue Specimen 38
19 Crack Propagation in Notched Samples 42
"20 Thermal Fatigue Specimen 43
21 Crack Propagation of First Crack in EachUnnotched Thermal Fatigue Sample 46
22 Dependence of Hardness and Impact Strengthof D2 Steel on Tempering Temperature 51
23 Dependence of Hardness and Impact Strengthof H].I on Tempering Temperature 53
24 Dependence of Hardness on H26 onTempering Temperature 54
25 Dependence of Mechanical Properties ofS-590 Alloy on Test Temperature 56
26 Dependence of Mechanical Properties ofL605 Alloy on Trest Temperature 57
27 Dependcnce of Mechanical Properties ofIIS-31 Alloy on Test Temperature 58
28 Photomicrographs Showing the Structure Along
One Vented Bomb Shot With WC846 Propellant 61i ':29 Photomicrographs Showing the Structure Along
the Bore Surface of an HII Alloy Insert AfterFive Vented Bomb Shots With WC846 Propellant 62
30 Photomicrographs Showing the Structure Along;. the Bore Surface of an H26 Alloy Insert AfterSOne Vented Bomib Shot With WC846 Propellant 64
S31 Photomicrographs Showing the Structure Along,'• the Bore Surface of an H26 Alloy Insert After
S•Five Vented Bomb Shots With WC846 Propellant 65;
Vi i
FIGURES
32 Photomnicrogiaphs Showing the Structure Alongthe Bore Surface of the D2 Alloy Insert AfterOne Vented Bomb Shot With WC846 Propellant 66
33 Photomicrographs Showing the Structure Alongthe Bore Surface of the D2 Alloy Insert AfterFive Vented Bomb Shots With WC846 Propellant 67
34 Photomicrographs Showing the Structure Alongthe Bore Surface of the 446 Alloy Insert AfterOne Vented Bomb Shot With WC846 Propellant 68
35 Photomicrographs Showing the Structure Alongthe Bore Surface of the 446 Alloy Insert AfterFive Vented Bomb Shots With WC846 Propellant 69
36 Photomicrographs Showing the Structure Alongthe Bore Surface of an L605 Alloy Insert AfterOne Vented Bomb Shot With *.C846 Propellant 70
37 Photcmicrographs Showing the Structure Alongthe Bore Surface of an L605 Alloy Insert AfterFive Vented Bomb Shots With WC846 Propellant 71
38 Photomicrographs Showing the Structure Alongthe Bore Surface of an HS-31 Alloy Insert AfterOne Vented Bomb Shot With WC846 Propellant 72
39 Photomicrographs Showing the Structure Alongthe Bore Surface of an HS-31 Alloy Insert AfterFive Vented Bomb Shots With WC846 Propellant 73
40 Photomicrographs Showing the Structure Alongthe Bore Surface of an S-590 Alloy Insert AfterOne Vented Bomb Shot With WC846 Propellant 74
41 Photomicrographs Showing the Structure Alongthe Bore Surface of an S-590 Alloy lnsert AfterFive Vented Bomb Shots With WC846 Propellant 75
42 Photomicrograph of a 50Co-29Fe-20W-lC AlloyRevealing the Transverse Structure Along theBore Surface After Five Vented Bomb FiringsWith IMR Propellant 76
43 Bere Profile for CG-27 Material for VariousNumbers of Total Rounds Fired 89
44 Bore Profile for CG-27 Material for 4036Total Rounds Fired 90
viii
mI
FIGURES
57 Transverse Section at 2 Inches from theChniaber of a Standard Chrome-Plated Gun BarrelAfter Firing 12,068 Rounds and Being DeclaredUnserviceable 100
58 Transverse Section. at 10 Inches from theChainber of a Standard Chrome-Plated Gun Barrel b
After Firing 12,068 Rounds and Being Declaredlinserviceable 100
59 Transverse Section at 17 1/2 Inches from the IChamber of a Standard Chrome-Plated Gun BarrelAfter Fir;.ng 12,066 Rounds and Being DeclaredUnseorviccable 101
60 Transverse Section at 19 1/2 Inches from theChamber of a Standard Chrome-Plated Gun BarrelAfter Firing 12,068 Roonds and Being Declared
Unserviceablc i01
61 Transverse Section at 1/2 Inch from the IChamber of a Chrome-Plated Nil Steel Gun BarrelAfter Firing 16,291 Rounds and Being DeclaredUnserviceable 104
62 Transverse Section at 1 1/2 Inches from theChamber of a Chrome-Plated H1I Steel Gun BarrelAfter Firing 16,291 Rounds and Being DeclaredUnser iceable 104
63 Transverse Section at 2 Inches from theCharhbec of a Chrome-Plated 1111 Steel Gun BarrelAfter Firing 16,291 Rounds and Being DeclaredUnserviceable 105
64 Trainsverse Section at 10 Inches from theChamber of a Chrome-Plated NIl Steel Gun B-rrelAfter Firing 16,291 Rounds and Being DeclaredUnserviceable 106
65 Transverse Section at 17 1/2 Inches froio theChamber of a Chrome-Plated H11 Steel Gun BarrelAfter Firing 16,291 Rounds and Being Declar-edUnserviceable 106
66 Transverse Section at. 19 1i2 Inches from theChambc.r of a Chrome-Plated Hill Steel Gun BarrelAfter Firing 16,291 Rounds and Being DeclatedUnserviceable 106
FIGURES
67 Transverse Section at 1/2 Inch from theChamber of a Chrome-Plated HIl Steel Gun BarrelAfter Firing 24,466 Rounds and Being DeclaredUnserviceable 108
68 Transverse Section at 1 1/2 Inches from theChamber of a Chrome-Plated HII Steel Gun BarrelAfter Firing 24,466 Rounds and Being DeclaredUnserviceable 108
69 Transverse Section at 2 Inches from theChamber of a Chrome-Plated HIl Steel Gun BarrelAfter Firing 24,466 Rounds and Being DeclaredUnserviceable 109
70 Transverse Section at 10 Inches from the IChamber of a Chrome-Plated HII Steel Gun BarrelAfter Firing 24,466 Rounds and Being DeclaredUnserviceable 109 j
71 Transverse Section at 17 1/2 inches trom theChamber of a Chrome-Plated HI1 Steel Gun BarrelAfter Firing 24,466 Rounds and Being DeclaredUnserviceable 110
72 Transverse Section at 19 1/2 Inches from theChamber of a Chrome-Plated Hll Steel Gur BarrelAfter Firing 24,466 Rounds and Being DeclaredUnserviceable 110
73 Transverse Section at 1/2 Inch from theChamber of a Chrome-Plated CG-27 Alloy BarrelAfter Firing 4,036 Rounds and Being Declared
Unserviceable 11274 Transverse Section at 1 1/2 Inches from the
Chamber of a Chrome-Plated CG-27 Alloy BarrelAfter Firing 4,036 Rounds and Being DeclaredUnserviceable 112
75 Transverse Section at 2 Inches from theChamber of a Chrome-Plated CG-27 Alloy BarrelAfter Firing 4,036 Rounds and Being DeclaredUnserviceable 113
76 Transverse Section at 10 Inches from theChanter of a Chrome-Plated CG-27 Alloy BarrelAfter Firing 4,036 Rounds and Being DeclaredUnserviceable 113
xi
FIGURES
Pa ge
77 Transverse Section at 17 1/2 Inches from theChamber of a Chrome-Plated CG-27 Alloy BarrelAfter Firing 4,036 Rounds and Being DeclaredUnserviceable 114
78 Transverse Section at 18 Inches from theChamber of a Chrome-Plated CG-27 Alloy BarrelAfter Firing 4,036 Rounds and Being DeclaredUnserviceable 114
79 Transverse Section at 1/2 Inch from theChamber of a Chrome-Plated CG-27 Alloy BarrelAfter Firing 7,324 Rounds and Being DeclaredUnserviceable 116
8C Transverse Section at 1 1/2 Inches from theChamber of a Chrome-Plated CG-27 Alloy BarrelAfter Firing 7,324 Rounds and Being DeclaredUnserviceable 116
81 Transverse Section at 2 Inches from theChamber of a Chrome-Plated CG-27 Alloy BarrelAfter- Firing 7,324 Rounds and being DeclaredUnsviviceable 117
82 Transverse Section at 10 Inches from theChamber of a Chrome-Plated CG-27 Alloy BarrelAfter Firing 7,324 Rounds and Being DeclaredUnserviceable 117
83 Transverse Section at 17 1/2 Inches from theChamber of a Chrome-Plated CG-27 Alloy BarrelAfter Firing 7,324 Rounds and Being DeclaredUnserviceable 118
84 Transverse Section at 19 1/2 Inches from theChamber of a Chrome-Plated CG-27 Alloy ijarrelAfter Firing 7,324 Rounds and Being DeclaredUnserviceable 118
85 Typical Structure Showing the Morphology ofWidmanstatten Precipitate in CG-27 Alloy Barrel 119
86 X-ray Pattern Kevealing CharacteristicRadiation of Elements and Their RelativeConcentration Near the Bore Surface of aCG-27 Alloy Barrel 121
87 X-ray Pattern Revealing CharacteristicRadiation of Elements and Their RelativeConcentration in the Base Material of aCC-27 Alloy Farrel 122
xii
I!
TABLES
Page
1 Comparison of Conventional and StelliteLined Cal .50 Barrels 4
2 Materials Evaluated in Vented Bomb Tests 6
3 Erosion Data for Metals Using the Criterionof Weight Loss Per Round 7
4 Erosion Data for Metals Using the Criterionof Dismetcr Increase, Inches Per Round 8
5 A Comparison of the Erosion Resistance ofMaterials With that of Gun Steel 9
6 Metallographic Observations of Fired Nozzles 10-11
7 Weight Loss of Gun Steel, Chromium-PlatedGun Steel, and Haynes Stellite 21 After5 Shots in the Vented Bomb 21
8 Weight Loss Data of Selected MaterialsA fL t 7 3 Pi r -,g s iLn t he 'Ven Lt edU B.o mtb- WtWC846 Propellant 29
9 Summary of Performance of Selected Materialsin Gun Tests 30
10 Hot Hardness Data of Selected Alloys 35
11 Propagation of Cracks in Notched Samples 40-41
12 Propagation of First Crack in UnnotchedThermal Fatigue Samples 44-45
13 Relative Thermal Fatigue Resistance 47
14 Chemical Composition of Candidate Materials 50
15 Weight Change Data of Selected MaterialsAfter Firings in the Vented Bomb WithWC846 Propellant 59
1.6 Unit Weight Change for Selected Mate-ialsAfter Firings in the Vented Bomb WithWC846 Propellant 60
17 Summary of Microexamination of SelectedAlloys Subjected to Firings in the VentedBomb With WC846 Propellant 78
18 Number of Rounds Fired on Schedule IBefore Failure was Observed 84
xiii
TABLES
19 Bore Dimensional Measurements for Chrome-Plated CG-27 Barrels After Various Numbersof Total Rounds Fired on Schedule I 85
20 Bore Dimensional Measurements for Chrome-Plated Hll Barrels After Various NumbersFired on Schedule I 86
21 Bore Dimensional Measurements for a StandardGun Barrel After Various Numbers of TotalRounds Fired on Schedule I 87
xiv
lNTRODUICT'i ON
l1ro:siou/corrosion Of barrel materials limits tihe rate offire of high-per formauce rapid-fire wcapons , and thus I imi tsservice life. The objective of this program wa•- to determinematerials and/or surface treatments capable of providing im-proved performance in the 7.62mm harrels.
For achievement of the objective, tile program comprisedthe following phases:
I. Chlh-nct erizat ion of erosion/corrosion inthe 7.62mm l'arrels.
i. tatCriaL evIi.ual ioC .
11. Materials sclect ion.
lV. Fabrication of selected materials intogun barrels and test firing.
V. Post analyses of the filed barrels.
Phase I ':onsisted of a literature survey and an exam-ination of the bore surface of chrome-plated standard steelgun barrels fired o0 a, cotrol led Z'- L ' -I
this phase was to establish the state of the art and thebaseline data.
In Phase 1I, a materials evaluation was 11ad32 to asSes±various critical material and physical parametei., also acomparison was made of the results obtained by vented bombtests and those obtained in gun tests for seven selectedmaterials, and important mechanical and physic ..t nropertiesof the materials were determined. The objective of thisphase of the work was to identify and correlate impox.tantmaterial propercies so that a rationale for selecting materialsof improved performance could be established.
In the materials selection, Phase 111, the informationdetermined in Phase II was combined with economic consider-at ions to form :1 basis for the selection of three materialsfor evaluation in gun tests. The objective of this phasewas to select cost-effective materials which would provideina, u wd performance as barrels ill thu Miiiigun.
Phase IV concerned the fabrication and the testing ofbarrels. :is
The post analyses of the fired barrels, Phase V, involvedbore measurements and metallographic examination. In PhasesIV and V, the objectives of the Research I)irectorate, WeaponsLaboratory USAWECOM were to verify the performance and torank the selected materials.
EXPERIMENTAL WORK
Phase I. Characterization of Erosion/Corrosion in the7.62rm Barrels r c1 s
Literature Survey
Two types of erosion studies have been made on selectedbarrel materials for small arms. These are the vented bombte,;t and the actual gun teSts. The majority of the ventedbomb tests were conducted during the 1940-1955 period andwere ctntered o te a peak pressure of 20,000 to 30,000 psi. Inthe actual firing tests, small caliber machine gun were used
* such as the Cal .50 or Cal .30 for the erosion studies. In" most cases, the vented bomb test environments were designed
to induce a large amount of erosion in a very few shots and,* conscq'iently, this type of test was usually more severe than
tha machine gun tests.
Gun Tests - Various metals and metal alloys have beenevaluated in gun tests. These include chromium and chromium-base alloys, molybdenum and molybdenum alloys, tungsten alloys,tantalum, cobalt, cobalt-tungsten, stellites, and nickel
S alloys.
Cobalt and cobalt alloys have been considered for linersin gun barrels. Pure cobalt was tested as a liner in aCal .30 gun. It showed excellent resistance to chemical at-,tk and ieltiig, but deforiimationi of the rifling occurred.(i)Steel barrels plated with 0.005 inch thick pure cobalt werefired in a Cal .50 gun.( 2 ) in this instance, the cobalt adher-ence was satisfactory, but melting aizd deformation of therifling occurred. Cobalt-tungsten alloys were plated ontothe bore surface of a steel gun barrel and assessed in aCal .S0 gun. Melting occurred, and difficulties with adhesionwere encountered. The cobalt-base stellites were also testedas liners. It was found that Steilite 21 was the most satis-factory member of this family. In tests in a Cal .50 machinegun, Stellite 21 showed outstanding performance as compared
2
to gun teel (see Table 1). However, in another investi-gation(•when Stellite 21 inserts were fired with double-base propellants, the bore surfaces melted.
Firing of nickel and nickel alloys as gun liners, showedthat these materials, with the exception of the nichromes,were susceptible to severe intergranular attack by the pro-pellanc gases.( 3 ) The nickel-chromium alloys with more than10% chromium were resistant to intergranular attack but wereunable to withstand deformation by the projectile. A numberof steels and other iron-base alloys have been evaluated.A 24Cr-5Ni-3Mo-0.45C-lSi steel was tested in a Cal .50 ma-chine gun and showed much better resistance to erosion thanregular 4150 gun steel.( 4 )
A 30Co-20Cr-4Mo-4W-2Ta-0.lC steel showed promise duringfiring with a Cal .50 machine gun, whereas a similar alloyincorporating 30 Ni instead of 30 Co was subject to severethermochemical attack by the propellant gases.(5)
Both pure tantalum liners (2) and steel with diffused-tantalum bore surfaces (6)have been test fired. The tantalumliners weie evaluated in a Cal .50 gun. The liners showed _
somc transgranular cracking, but no other evidence of ero-sion. The diffused-tantalum coating on the bore surface ofa steel barrel was tested in a Cal .30 machine gun. Barrelswith coating thicknesses of 0.0005 inch and 0.001 inch werefired and performed better than gun steel.
A liner of chromium electroplate was evaluated, (7)andit was found that cracking in the bore surface occurred afteronly a few rounds had been fired. The premature failure wasattribute•to a lack of cold ductility of chromium. Parkeand Bens )evaluated chromium alloys containing 20 to 30%
-irn.3 Irr", lyb11d "U1--iL JLL •U , V tL LJlo oILDLY UUentum1. ,En fthese aLC .j'uys Were lab-ricated into liners and test fired in a Cal .50 machine gun,they showed outstanding performance and did not crack ordisintegrate despite their low ductility.
Pure molybdenum liners were investigated by Smith, andfiring tests showed that the liners, as fabricated, wereneither strong nor duqtile enough to make a satisfactoryliner°(2) Other work(9-1l)showed that these deficienciescan be overcome by addition of 0.1% to 0.2% cobalt, mechan-ical working, and proper heat treatment. In test firing,molybdenum alloys so modified showed outstanding performance.After firing 2024 rounds, the bore surface was examined andthere was no evidence of erosion. There was, however,some cracking, swaging of the lands, and liner movement.
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Tungsteti-copper powder compacts(7)were evaluated. It
was found that these compacts were resistant to erosive ef-fects of thF powder gas but were unsuitaDle because of alack of sufficient strength to resist deformation by theimpact of the projectile.
Vented Bomb Tests - The vented bomb has been utilizedfor determining the erosion resistance of materials. Inthis type of test, a nozzle of the material of interest ismounted directly in front of the chamber and a blank isfired. The system is obturated so that all the burninggases flow through the test nozzle. By controlling thegeometry of the system and the propellant, the internalballistics of the system can be manipulated to simulate thepressure-time pulse of a specific weapon.
Vented bomb tests were reported in Reference 7. Inthis study the erosion resistance of pressed and sinteredchromium-copper, molybdenum-copper, and tungsten-copperpowder compacts were compared with gun steel. The best ofthe chromium compacts showed weight losse.i considerablyless than gun steel but not as low as the molybdenum ortung ten eompacts. The compacts were never considered forgun barrel liners because of the low melting point and hard-ness of copper.
Breitbart, in 1951, ( 1 2 )conducted the most comprehensiveprogram of this type. A 37mm gun was modified so that atapered nozzle could be placed directly in front of the cham-ber. The tapered nozzle had a bore which varied in sizefrom 0.707 to 0.500 inch. Double-base M2was the propellant,and charges usually consisted of 40, 50, 70, and 88 g, butother charges were also used, depending upon the results ofthe test. The 88 g load generated a pressure of 30,000 psi.
The rate of firing was limited to one round every 2 minin order to minimize any temperature effects from the pre-ceding round. The materials which were evaluated are listedin Table 2. Erosion data from the vented bomb firings aregiven in Tables 3 and 4. A comparison of the erosion re-sistance, as determined by volume of metal lost per shot, ofthe candidate materials with that of gun steel is made inTable 5. Table 6 summarizes the metallographic observationswhich were made on the various materials after firing.
In general, the steels or special iron alloyq testedhave not shown outstanding promise as bore-surface materials.Chromium plating the bore surface improves the erosion re-sistance. Cobalt-base superalloys have shown satisfactory
5
¶a
Table 2
MATERIALS EVALUATED IN VENTED BOMB TESTS (12)
Pure Metals
Iron
Nickel
Titanium
Tungs ten
Molybdenum(Pressed and sintered,
Cast and forged)
Alloys
AISI 4140 Steel25-20 Stainless Steel
Timken Alloy 16-25-6
Fe-24 0r-SNi-2.75Mo-i. OSi-0.04CStellite 21
60Cr-25Fe- l5Mo
68Cr- 23Fe-9Mo
Mo-5W
WC + 13%Co
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performance in applications in which single-base propellantsare utilized. From the information presented in this survey,it appears that molybdenum-or tungsten-base materials withoptimized mechanical properties would offer an improvementover cobalt-base alloys in the environments created by adouble-base propellant; however, the cost and fabricationdifficulties of these refractory metals dc detract fromtheir use.
Mode of Failure of Chromium-Plated Barrels
To determine the onset characteristics of erosion inthe 7. 6 2mm barrels, plated and unplated barrels were firedin continuous bursts for 1, 1.0, 50, 100, and 300 rounds.To establish the long-term characteristics a series ofchromium-plated barrels which had been fired to rejectionwere examined. In the chromium-plated barrel, relativelylarge cracks are developed in the chromium plate after oneshot. Each additional shot opens these cracks and causesthem to propagate toward the steel. In a very short period(less than 300 rounds) the crack reaches the steel, theopening and closing of tae crack is coupled o the steel atthe chromium plate!steel interface, and the accompanyingpressurc and thermal excursions iie a rn.ck in thesteel and propagation occurs. Figure I shows the platedbarrel which was subjected to the 300-round burst. It shouldbe noted that the crack in the steel is an extension of thecrack in the chromium plate and that there is no white layerpresent. The cracks present in Figure 1 were examined atmagnifications up to 20,OOOX, and no evidence of a whitelayer was seen. Figure 2 typifies a barrel which has beenfired to failure. The cracking has significantly progressedinto the steel, large regions of chromium plate and steelhave been removed, and the cracks are encased in the whitelayer. Figures 3 and 4 are photomicrographs of a crack andassociated vhite layer. Examination of Figure 4a and b re-veals that cracking occurs around the white layer ratherthan through the layer. From this evidence, it appears thatthe white layer is produced after the crack has occurred.With continued firing the white layer flakes off and theciack widens.
Microscopic X-ray spectographic analysis was performedon the white laver and the steel matrix. The spectrographictechnique for each examination was closely controlled sothat semiquantitative results could be obtained. Copper wasfound in the white layer area. The iron and chromium con-tents in the white layer were considerably lower than in thematrix. This indicates that there is a buildup of lightelements in the white layer.
12
lit
Nital EtctL 50OX(a) Transverse
kv IT
-*'
Nital Etch 50OX(b) Longitudinal
FIGURE 1 PIIOTOMICROGRAPHS SHOWING CHROMIUM PLATE/STEELINTERFACE OF A MINIGUN BARREL AFTER 300 SHOTS.
13
Nttal Etch
Nital E tch50 OXFIGURE 2 PflOTOMICROGPppl OF, A LONGITUDINAL SECTION SHOWINGTYPICAL CHROMIUM PLATE/STEEL INTERFACEAFTER FIRING TO FAILURE.
14
LIC; ~~ I~ ~~f&
Nital Etch 600X
FIGURE 3 SCANNING ELECTRON MICROGRAPH SHOWING TYPICALCRACK AND ASSOCIATED WHITE LAYER ON FAILED GUN BARREL,
I 15
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..5-ft.. . N•• • -.•", 'Vm-�,e, j
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CRAC NTI AND ASC ATE WHT LYR
161tt w I .
(b))
CRC I N ASCATE WHTtLYR
N Lta Etc (a)2000
Examination of the fired unplated barrels revealed thatmaterial removal or erosion was the prime mode of failurefor gun steel. The lands are flattened considerably by fir-ing. Figure 5 typifies the microstructure along the boresurface and shows the intersection of a land and groove.
Phase II - Miaterials Evaluation
Vented Bomb-Gun Test Correlation
An improved gun barrel material must possess a combina-tion of mechanical and physical properties whtch would enableit to resist the gun barrel environment, The material musthave enough high-temperature strength to allow for sustainedbursts without excessive wear; however, it also must havesufficient toughness so that it will not fail by cracking,It must have good mechanical and thermal fatigue properties.The material must not react excessively with the hot gasstream. This requires a reasonably high melting point anda low chemical reactivity potential with the propellant gases.
Much information is available on the mechanical andphy ica -r **t- -'of at-In-.' slu*nd *' t.i. avLa- lst , .
or known environments. However, little is available on howmaterials will respond to a gun barrel environment. Thevented bomb (Figure 6) was utilized in this program to eb-tablish the response of materials to a gun-type environment,The tes . specimen in the vented bomb is 2 inches long andhas a bore diameter of 7. 6 2mm.
The first effort was to establish the level of correla-tion of material response in vented bomb firing with thatobserved in actual gun tests. A section of a rifledchromium-plated gun barrel, an unplated barrel, and a HaynesStellite 21 gun barrel insert were subjected to five shotseach in the vented bomb. Two propellants, IMR and WC846,were employed and the peak pressure was controlled at50,000 + 2,000 psi with a rise time of less than I milli-second. A typical pressure-time pulse is shown in Figure 7.The weight loss data are listed in Table 7. The ranking ofmaterials in the vented bomb test, according to these data,is as follows:
1. Chromium-plated gun steel
2. Gun Steel
3. Haynes Stellite 21
17
Nital Etch 500XFIGURE 5 PHQTOMTCOGRAPJ-jS1jIAUT- TIJT TLVAATCVVVC t C-rTOT1(T-mr
ALONG THE BORE SURFACE OF' UNPLATED GUN BARREL AFTER 300 SHOTS.(Bore surface was nickel. pl~ated for metallographic examination.)
18
U 4is4
40
CI en
It
19
16,6 6 7 __
1mscc
r It.U")t E "i Y t-I'1LAI!1KSS i-]ju pULSL;E W1TtU A
23 1/2 gCHARGE OF WC8146 BALL PROPELLANT.
20
'a bc 7
WEIGHT LOSS OF GUN STfEL, CHROMIU!4-PLATED GUN STEEL,AND HAYNES STELLITE 21 AFTER 5 SHOTS IN THE VENTED BOM!
Weight loss,Material rrcp llant _ rams
Cr-Mo-V gun steel ID[R 0.2345
WC846 0.0059
Chromium-plated gun steel IMR 0.0002
WC846 0.0000
IHaynes Stellite 21 IMR 0.5884WC846 0.0650
II
I 2
i :21
.5
This ranking is not that observed in gun tests, in whichthe Htaynes Stellite 21 outperforms the chromium-plated gunbarrel, which in tu.n outperforms the gun steel.
The efiects of propellant on erosion can also be deducedfrom Table 7. Both of these propellants havp approximatelythe same flame temperature, 5235 F for the WC846 and 5160*Ffor the iM 5010; however, because of differences in theburning rates 29 grains of lMR 5010 and 23-1/2 grams of WC846were required to, produce 50,000 psi peak pressure in 0.6 and0.5 milliseconds, respectively. Consequently, the IMR 5010propellant produces more gas and a longer thermal pulse thanthe WC846 propellant, and represents a more severe c•osioncondition.
The relative ranking of each material r'emains unchangedwith each propellant; however, the large differences encoun-tered for each alloy with the two propellants indicate thatperformance In a gun system would be greatly dependent uponthe propellant and firing schedule employed,
Each of the three vented bomb fired materials was metal-lographically examined. Figures 8 and 9 show the bore sur-face of the unplated rifled insert•- hich ... fired w.IthIM, and WC846 propellants. The bore surface was plated withn~ckel to preserve surface effects. Both sections shown inthese figures are at a land and groove intersection. Evi-dently in the vented bomb gun steel does not crack, but
the bore surface and a rather sharp heat-affected zone pres-ent. In both cas'. , the rifling is flattened but much moreextremely with the IMR propellant. This mode of failure issimilar to thaL encountered in gun tests.
Typical land and groove intersections of the rifled.LcLrouzium-planed inserts subjected to 5 firings in the ventedbomb with WC846 and IR propellants are shown in Figures 10and 11. It is apporent that cracking from the chromiumplate has progressed into the steel, but no spalling offhas occurred. This mode of failure is similar to that en-countered in gua te.sts.
Figures 1.2 and 13 exlibit transverse microstructuresalong the bore of the Haynes Stellite 21 test insert after5 firings in the vented bomb with I-MR and WC846 propellants.The nickel plating did not adhere very well to the bore sur-faces. The bore surfaces appear to be quite irregular, andchips of metal have been removed.
22
Nita~ EtcFI£R HTMCORP EEAIGTETASES TUTR
ALN H OESRAC FA NLTDRILDISR FE
5 VETED OMBSHOT WIT WC46 POPELANT
MEMO"
.tl;1*Nital Etch 500x
FIGURE 8 PHOTOMICROGRAPIT RUI~TALTNGC THE TRAN'SVERSE STRUCTUREALONG THE BORE~ SURFACE OF AN UTNPLATED RIFLE17D INSERT AFTER
5 VETEI BO0MB SHOTS WITH I*C846I\ PROPELLANTl.
23I
Nital Etch 5 OUX
FIGURE 10 PHOTOMICROGRAPH REVEALING THE TRANSVERSE STRUCTUREALONG THE BORE SURFACE OF A CHROMIUM-PLATED RIFLED INSERT
AFTER 5 VENTED BOMB SHOTS WITH WC846 PROPELLANT.
24
.j
I
ItIS_•I . ... ..__"I_ I -__ -__ . . . .. . I II___ _ '2_ '
Nital Etch 500X
FIGURE 11 PHOTOMICROGRAPH REVEALING THE TRANSVERSE STRUCTUREALONG THE BORE SURFACE OF A CHROMIUM-PLATED RIFLED INSERT
AFTER 5 VENTED BOMB SHOTS WITH IMR PROPELLANT.
25
I
I
'4c.
iti
.,.,• • ,.-o . .v ,%.- .. .. ; . -:
. ',., - ,,,., >2, ',,t 4 -•
Nital Etch 5OX
FIGURE 12 PHOTOMICROGRAPH REVEALING THE TRANSVERSE STRUCTURE
ALONG THE BORE SURFACE OF A H-AYNES STELLITE 21 INSERT AFTER5 VENTED BOMB FIRINGS WITH WC846 PROPELLANT.
26
I
C IYAA
Nital Etch 500X
FLUUKR 13 PHOTOMICROGRAPH REVEALING THE TRANSVERSE STRUCTUREALONG THE BORE SURFACE OF A HAYNES STELLITE 21 INSERT AFTER
5 VENTED BOMB FIRINGS WITH IMR PROPELLANT.
27
To further examine the correlation between vented bombtestsand gun tests, test inserts of CG-27 alloy, A286 alloy,22-4-9 alloy, and X-15 alloy were given 3 firings in thevented bomb with the WC846 and the IMR propellants. Thesematerials were selected because Rock Island Arsenal hadfired gun barrels of each and the relative performance hadbeen established under closely controlled firing schedules.The vented bomb materials were furnished by Rock IslandArsenal, and the metallurgical analysis was performed there.The weight loss data are reported in Table 8.
In the gun tests the rate of fire was 650 rounds perminute on the following schedule:
After every 125 rounds allow for a10 second cool.
After 6 bursts cool to ambient tem-perature.
Every 3000 rounds de-copper the barrel.
The gun test data are listed in Table 9. Comparisonof these data with Table 8 reveals that there is not- n one-to-one correlation between the vented bomb firings and thegun tests. There are several probable reasons why there isnot a one-to-one correlation, The more obvious are:
1. In the vented bomb the duration ofeach single thermal pulse or shotis much longer than that encounteredin the gun tests. Consequently,melting point can become more in-
fluential.
2. The vented bomb only presents ahigh-temperature gas stream; theeffects of the projectile actingon the bore surface in a gun testwould introduce swaging and wear.Consequently, the high-temperaturemechanical properties are more im-portant in a gun test than thevented bomb.
3. Thermal fatigue properties are ofimportance in the gun tests sincea large number of cycles is induced.In the vented bomb, only a few cycles
are encountered.
28
Table 8
WEIGHT LOSS DATA OF SELECTED MATERIALSAFTER 3 FIRINGS IN THE VENTED BONE
WITH WC846 FROPELLANT
Weight Loss,
Material grams
CG-27 0.636
A286 0.3047a
22-4-9 0.0531
X-15 0.0017
Iansert galled on removal from holder.
i2
29
I11 I I I I I
Table 9
SUMMARY OF PERFORMANCE OFSELECTED MATERIALS IN GUN TESTS
Roundsto Failure
Material Failure Criterion
CC-27 5,250 Yaw
A286 4,000 Yaw
22-4-9 4,250 Barrel fracture
X-15 4,250 Yb
Cr-Mo-V steel 4,250 Yaw & velocity
Cr-Plated Cr-Mo-V steel 11,400 Velocity
30
4. The mode of failure between the guntests and the vented bomb can besignificantly different. For example,in gun tests chromium-plated barrelsfail by cracking and not by washingor reacting with the propellant gas;therefore, if enough cycles are notinduced in the vented bomb firings,no sections of chromium plate andsteel will be cracked out and weightlosses would be minimal.
•g
In view of the above factors, the vented bomb firingcan only give qualitative information on the performancein a propellant gas stream. The tests can be utilized todetermine resistance to the gas stream, indicate thermalshock resistance, and observe tendencies for a material tofail in such an environment.
Correlation of Properties withMaterial Performance in Gun Barrels
Minigun gun barrels of CG-27, X-15, A286, and 22-4-9were test fired by the Rock island Arsenal. With the fir-ing data as a base, a study was made to establish whethera correlation exists between the material performance as agun barrel and the material properties. The vented-bombtests were considered to evaluate the relative resistanceto thermal shock, chemical reactivity, and erosion resistanceto a hot gas stream. Hot hardness (strength) data and ther-mal fatigue data on the above alloys were not available inthe literature so these properties had to be determined ex-perimentally.
Hot Hardness ]efrtmlnaty'nns - The t-et amnlnytd onmeasurehot ardness was a mutual indentation techniquewhich involves the pressing together of two cylindrical spec-imens of the test material. These specimens are I cm indiameter by I cm long. A load of 500 to 3000 kg is employed.
The area of the flat surface developed at the junction ofthe test specimens is measured, and from this area the hard-ness data are derived. The anvils, anvil holders, and testspecimen are shown in Figure 14. These pieces of equipmentare loaded into a furnace attached to a Brinell machine asshown in Figure 15. The necessary load is applied, thenreleased, and the test specimens are removed from the fur-nace. The flat which is generated at the junction of thetwo specimens can be clearly seen. Brinell hardness can becalculated from measurement of the flattened area as follows:
31
A.4 4 U A"ý*JJ
p7
FIGURE 14 FIXTURE AND TEST SPECIMENS FOR THE MUTUALINDENTATION HOT HARDNESS TEST. DISASSEMBLED (LEFT)
AND ASSEMBLED (RIGHT).
32
nbest
FIGURE 15 GENERAL VIEW OF EQUIPMENT FOR MUTUALINDENTATION HOT HARDNESS TESTING. LOADING DEVICE,
FURNACE (IN OPEN POSITION), AND TEST FIX¶LUREWITH SPECIMENS ENCLOSED ARE SHOWN.
33
CP) CPBWINMi " -'•' T-WT
where C = constant (1.52)
P - load, in kg2
A - area of impression of cylinders, in mm
L - length of impression or flat, in mm
W = average width of impression or flat, in mm
The hot hardness data for the A286, X-15, 22-4-9, andCr-Mo-V alloy are given in Table 10. The 22-4-9 alloy andthe Cr-Mo-V steel deformed to such an extent at the 1800'Ftemperature that the test data were considered to be invalid.Figure 16 shows the hot hardness data. As can be seen, thereis a cross-over point at approximately 1300'F, where theCG-27 alloy has a greater hot hardness, hence strength, thanthe other alloys.
Thermal Fatigue Testing - A schematic drawing of theS therm' fatigue testing facility is shown in Figure 17. Itconsists of a 11 5 in.bh diameter . . ...... 3 situ-aLed between two 14 inch diameter intermediate-temperaturebeds.
The specimens are cycled by means of automatically con-trolled pneumatic cylinders which are sequenced by timers Iand limit switches. The facility was cycled automaticallyfor the number of cycles selected.
Each bed is fitted with four thermocouples for control,over-temperature protection, low-temperature test cutoff,and recording purposes.
The five alloys were tested over a temperature intervalof 1500'F to 600'F. One specimen of each was prepared witha thermocouple in the edge of the fin, The configuration ofthe specimen is shown in Figure 18. The test conditions foreach steel were established by varying the cycle times in thehot bed and the cold bed. The hot bed temperature was 2000'F,the cold bed 175°F. The test conditions established aregiven below:
34
Table 10
HOT HARDNESS DATA OF SELECTED ALLOYS
TestTemperature, Load,
Alloy OF kg BHN
A286 Room 3000 2561200 2000 2351400 1000 1221600 500 681800 500 25
X-15 Room 3000 3031200 1500 1901400 500 1101600 500 731800 500 24
22-4-9 Room 3000 3411200 1500 1601400 800 1191600 700 90
Cr-Mc-V Room 3000 3241200 3000 931400 500 491600 500 20
CC-27 Room 3000 2721200 2000 2001400 1000 200S1.600 500 110
35II
• 35
480 -=
0--.- SeCr-Mo-V Steel
400 --- 22-4-9
---- CG-27
0--. . A - A28
320 _ • ,.-
S240
%" 160 - .
80 -_ ._ _ _ _ _ _. ... -
0 400 800 1200 1600 1800
Test Temperature, OF
FIGURE 16 HOT FARDNESS DATA IOR SELECTED ALLOYS.
36
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' ~I-
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.. CI-
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ii../ -w
Io - '-1. . - . . .i'4 iiII .
] , :..Ii, C :"..
i 'i .}- . i "I;;i I .i 'l' • -
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r I
16
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FIGURE 18 l:TRI THiERMAL FATIGUE SPECIMEN.
i 38
O.fm
Time in Hot Bed, Time in Cold Bed,Steel sec sec
X-15 18 35
22-4-9 14.5 24
CG-27 12 16
A286 13.5 22
Cr-Mo-V 12.5 19
Table 11 lists the propagation of cracks in the notchedspecimens of the five materials. These data are shown inF igure 19. Tbe crack susceptibility of 22-4-9 alloy isreadily apparent.
Thermal fauigue testing of unnotched specimens was alsoconducted. Figure 20 illustrates the test specimens.. Thetest conditions for each alloy in the unnotched conditionwere similar to those employed on the notched specimens--i.e., hot bed temperature of 2000'F, cold bed temperatureof 175 0 F, and a temperature interval of 1500'F to 6000 F.
Table 12 lists the propagation of cracks In the un-notched specimens of the five materials. These data areshown in Figure 21. The CG-27 alloy exhibits the best re-sistance Lo crack iuitiation, followed by the A286 alloy.The X-15 alloy and the 22-4-9 alloy have the poorest re-sistance to crack initiation in this particular thermal cycle.
The relative ranking of thermal fatigue resistance isshown in Table 13.
Summary - The vented bomb tests ranked the gas erosion
rcsitatnce uf the materials in descending order as follows:
X-15 alloyCr-Mo.-V steel22-4-9 valve steelA286 alloyCG-27 alloy
SHot hardness resistance at 1600OF (in descending order)
is ranked as:
CC-27 alloy22-4-9 valve steelX-15 alloyA286 alloyCr-Mo-V steel
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4-1 41 * D0' nr-C
A&A
0 Z
91-'
0
C-,
CD
clI
42
!1
SI + I
4 64
15 RMS
I I3
300 16
DRILL FOR -- BOLTIFica(IRE 20 ThERMAL. FATIGUE SPECIMEN.
43
0 r--4 0~ C.) O
%0%00 00 CN
acoOv-4 0-4
x c)a% ri 000 00ý4
-7 C C V)~
1-4 0)
L. U u-I 1 00,C)C
340P-W 0 .al0"0 :ce 5C 0 )CN-4 -4 Cn
41i
%-.,) . 0
r4 I -f "II-A , I0 C~j0 C-4 00 Q3 r-4 00
Z u CN 0 ('4 c 0 C1400
P: 1:0O' 0-O C)r- r.00I' EX. 11.4
z EA
00 0 0 0000000-0L00Lnt 000 00
E-4 0-4 CNC)44
J~~1 00 r4
4-J Lr000
.- . .r-
5-4
(1)U ca r-4 -4 .Li 4co .- . . 3
Cl LO( 4 i)
C) 0 r4 r-4e C-4i$4. . . 14
0 C0
Q) 00
S C)
'V u u q m --4-
O"0ca 1O 0 0 C') 00
-Li
r-4
C) %0C)
U) 0
Co Lfl I-
r 4 0 -4C
45
I
-i
*I)
0
__ __ C-
4
0wLL�.. � C-,Ho
U)
______ ______ '-.,* U�3
0
_ _ _ _ HCl)
L�.
C
0
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__ __ ___ 41.
0
0
0 0 0
0
I1.�.
46
I
Table 13
RELATIVE THERMAL FATIGUE RESISTANCE8a
Matterial Notched Unnotched
CG-27 alloy 3 I
A286 steel 2 2
X-15 steel 4 3
22-4-9 5 4Stainless Steel
Cr-Mo-V steel Il
aBest resistance is designated by 1.
4 7
Unnotched thermal fatigue tests ranked the materials,in descending order:
CG-27 alloyA286 alloyX-15 alloy22-4-9 valve steel
These data appear to correlate quite well with the guntests. The erosion tests (vented bomb) show that the CG-27alloy will fail by reaction with the propellant gas streamif the firing sequence and barrel dimensions are such thatsufficient heat is generated to allow the nickel in theCG-27 to react with the sulfur in the gas stream. The otheralloys will fail either by cracking or flattening of thelands if the conditions of the firing test are such that thebarrels attain high temperature.
Vented Bomb Tests
Based on the information obtained in the correlationstudy, it was established that the vented bomb test can onlysimulate the pressure, gas erosion, and thermal shock en-countered in a gun test. The vented bomb was thereforeutilized to evaluate the resistance of candidate materials
A variety of steels and superalloys offer potential asgun barrel materials. Included in the steels are the high-carbon, high-chromium cold work steels, chromium hot worktool steels, tungsten hot work tool steels, iron-nickel-chromium-molybdenum alloys, ferritic stainless steels, andthe austenitic valve alloys. Among the superalloys, theiron-base and cobalt-base materials should provide bettererosion resistance than the nickel-base materials becauseof the tendency of nickel to react with sulfur in the pro-pellant gases.
Typical materials from the above classes were selectedfor testing in the vented bomb:
1. A286, an iron-nickel-chromium-molybdenum alloy.
2. 22Cr-4N1-9Mn, an austenitic valve alloy3. CG-27, an iron-base superalloy
4. X-15, an iron-base alloy
5. Cr-Mo-V steel, the current gun steel
48
,1.6. Chromium-plated gun steel
7. Hlaynes Stellite 21, a cobalt alloy j8. 50Co-29Fc-20W-lC, a cobalt alloy9. 1111, a chromium hot work tool steel
10. D2, a high-carbon, high-chromiumcold work steel
11. H26, a tungsten hot work tool steel
12. 446, a ferritic stainless steel 4
13. L605, a cobalt superalloy
14. HS-31, a cobalt-chromium-nickel-Iron superalloy
15. 5-590, a cobalt superalloy
Materials I through 7 were e:'amined in the correlationstudy, and only the weight loss data will be considered inthis section. The chemical compositions of the remainingalloys are listed in Table 14.
""5OoUFc2W-IC alloy boas----- bee Lut ' a T
for a tool application. This material shows unusual wearresistance in the application. The material was solution-treated and quenched to a hardness of RC 32 and then agedat 950'F to a hardness of RC 45. In this condition, thematerial had a metallurgical structure of carbides dispersedin an alloy matrix.
The high-carbon, high-chromium cold work steels havechromium and carbon as the principal alloying elements, butthey may also contain tungsten, molybdenum, cobalt, andvanadium. This class of steel is highly wear resistantwith medium resistance to heat softening. D2 was selectedto characterize the behavior of this type of steel. Hard-ness and unnotched Izod impact strength as a function oftempering temperature for D2 is shown in Figure 22. As canbe seen, D2 undergoes a secondary hardening phenomenon atabout 950'F and a tempered martensite embrittlement at 500'F.To avoid the limited ductility, a heat treatment consistingof austenitizing at 1850'F, air cooling, and a double temperat 1150'F was conducted for one set of vented bomb inserts.The resulting hardness was R 41.5. A second set of ventedbomb inserts was heat treateý as follows:
Austenitize aL 1850'FAir Cool
49
z I o o1-4 r-4 r- r4 I I
01 0" 0 A ca (N 04o
0 0
C;C
E-4 00 01 Lon cc 0vLI cc' P % CI. j
0 )0 4 r- -4 C
1-4 .,4
l0 E -4 C1 CQ C4a) 0 0
1 4 C) O N I r
05
70
S O 50
I" o 40 ° 0
30 __70
_ _ _ _ 60
I ' I0 200 400 600 800 1000 I?00
Tempering Temperature, 1F
FIGURE 22 DEPENDENCE C M IARDNESS AND IMPACT STRENGTh OFD2 STEEL ON TEMPERING TEMPER...T',I(E.
51
Spheroidize at ~'~
Air Cool
Double temper at 1.15 0 F
The spheroidizing treatment was given to Improve theIcarbide morphology and coughness. The hardness of thfis' settof samples was R 4
Hil steel. was selected to represent the chromiium hot~work class of alloys. This 6teei_ concains ctn~orium andtungsten with additions of rnoltybdetiun- an.,d vanadium, It hasgood resistance to thernr~2 softenizng because of th~e med iumchromium content supplemented by the iforkxtian oi tungsten,molybdenum, and vanadium carbides.. The low carbon and r:el-atively low total alloy content promote toughne~ss in t:'iehardness range of RC 40 to 50. The Ell alloy has found usein highly stressed bt~ructturcil parts. It has above-av~rageresistance to corrosion and exid-et:icmn as well as a relatively-low coefficienr of the-rmal expanzi.Jon. Iiardin.ss ;and CharpyV-notch impact stte,-qth as a function of tempering tempera-ture for, E-ll, steel are shown in 'Figure 2'3". H-ot Inzrdnessdata for Hi1 hardened at- 115G0 F and double tcmpv"'rneI at 1050~'Fare also shown in Figure 23. The vented bomb inserts werehardened at 1850'F aRnd double tempered at- ll510 F tc pr'ovidea hardness of RC 41,
The tungsten hot work ,ztcels are alloyed, principall~ywith cai~bor., tungsten, vnd chrocmium wit'!1 some vanadium,The high alloy content increases. resistance to high-temperaturesoftenz~ng and washing; hw¶iever, in the heat-treated statethey possess low toughness. K-126 w~s slected to typify this 'class of Lsteel. Hardness as a function of tempering tempera-ture for H26 is depicted in F'igure 24. To assess the per-iorman(ce of the H26 in the vented bomrb, inserts were 1hardenedIat 2L'O0'_, oil quea'ched, and double tempered at 1250'F.JAtLv. _ucbtitamet ube P26 had a hardnes~s of R C 41.
Thý. ferritic stainless steel 446 typifies a class ofmnaterilas which have excellent oxidation resistance at ele-vated temperatures. However, the strength of 446 is rela-tively low. The vented bomnb inserts were austenitized at)6)Or water quen~ched, an%:' aged at 900'F for 16 hr. Theaging, treatment was conducted to increase st-rength; however,a hardness of onl~y IRC. 15 was obtatned.
The wrought cobalt alloys have reasonably good mnechan-ical proiperties and oxidation resistance at elevatc-d Lem-perature. To typify these materials S-590, L605, and BS-31
52
70
S60
S50
40
0
080
I _I_ _ -.o0- £
.,_ . ... .6 0 ,
40
V LLJ 0 ot 40 0 0 0010Tempering Temnperatuxre, ?F
FIGURE 23 DEPENDENCE OF HARDNESS AND IMPACT STRENGH OFHII ON TEMPERING TEMPErATURE.
53
65
* 60
55
S50
(14
= 45 . .... .- __-_-
'40
0)
30
0 200 4,00 600 800 1000 1200Tempering Temperature, OF
FIGURE 24 DEPENDENCE OF HARDNESS OF H26 ONTEMPERING TEMPERATURE.
54
II
were selected. S-590 is used for wheels and buckets forgas turbines and for forging operations to 1500*F. L605is normally used in sheet form. Figure 25 shows the de-pendence of mechanical properties of S-590 on test tempera-ture. The S-590 was solution treated at 2200*F, water Iquenched, aged at 1400*F for 16 hr and air cooled. The re-
suiting hardness was RC 30.
Figures 26 and 27 illustrate similar information forL605 and HS-31. The L605 alloy was solution treated at2250°F, water quenched, aged at 1350*F for 50 hr, and aircooled. The hardness developed by the heat treatment wasRC 32. The HS-31 material was hot forged between 2250*Fand 1800*F. The HS-31 inserts were fabricated from the as-for&ýd material and had a hardness of RC 38.
Bomb inserts, 2 inches long, uf each material (Nos. 8to 15) were fabricated with a 7.62mm bore. WC846 ball pro-pellant was utilized at a load of 23 1/2 g to produce a
peak pressure of 50,000 + 2,000 psi at a rise time of ap-proximately 0.5 millisecond. Each material was subjectedto a one- and a five-shot sequence. The specific weight!o•dat_ ar .... rt.ed ii-1. Table i5.
Table 16 ranks the various materials on a weight changeper shot based on the multiple firing data. This table in-dicates the resistance to the erosion and thermal shock•which are encountered in the vented bomb. The ranking doesnot discriminate between weight loss due to erosion and thatassociated with cracking.
The H1I and the 446 alloys appeared to have gained weightduring the firing. The S-590 lost considerable weight onone shot; however, the five-shot seliience idates that onfollowing shots a weight increase occurred. This indicatesthat a gas-metal reaction is probably occurring. The D2 ap-peared to gain weight slightly on the first shot and thenlost weight during the five-shot sequence. The other alloyslost weight on the first shot and showed a continual losson the five-shot schedule.
To characterize the mode of failure, extensive metallo-graphic analysis on the fired inserts was performed.
In preparing the metallographic samples, a nickel plat-ing was applied to the bore surface to preserve the edges.The transverse specimen was taken 1/2 inch from the inletend and the longitudinal section from 1/2 to 1 1/2 inchesfrom the inlet end. Figures 28 and 29 show the longitudinal
5
160 ____
140 -
Tensile Strength
120
1I00
WI
f 80
Charpy V-Notch Impact Strength
Reduction in Area
30 30
20 20,
10 in a m ~100 200 400 600 800 1000 1200
Temperature, OF
FIGURE 25 DEPENDENCE OF MCHANICAL PROPERTIES OF S-590ALLOY ON TEST TMEXPERATURE.
56
ii
160
140
120
0 10
80
Yield Strenzth
40 .30'4
SCharpy V-Notch Impact Siren•th 10
__60_-_ 60
___400
Reduction in Area
200 200 400 600 800 i000 i200
Temperature, 'F
FIGURE 26 DEPENDENCE OF MECHANICAL PROPERTIES OF L605ALLOY ON TEST TEMPERATURE.
57
140
I20111ý Tensile Strength
;W 80
60
_ __
0A
2 0 'd2 0
Reduction in Area
20
200
FIG URE 27 40ED
N E O E H N CA R P R I S O S 3
Table 15
WEIGHT CHANGE DATA OF SELECTED MATERIALS AFTER FIRINGSIN THE VENTED BOMB WITH WC846 PROPELLANT
Weight Change, grams
Materials I Shot 3 Shots 5 Shots
A286 - -0.305a
22Cr-4Ni-9Mn - -0.0531
CG-27 - -0.636
X-15 - -0.002 -
Cr-Mo-V - -0.0059
Chrome plated Cr-Mo-V - -0.0000
HS 21 - - -0.0650
50Co-29Fe-20W-lC 0.000 - 0.0000
HIi +U.UUOU - +0.001
D2 -f-0.002 - -0.008
H26 +0.002 - -0.01]
446 +0.001 - +0,008
L605 -0.001 - -0,001
HS 31 -0.003 - -0.008
S-590 -0.091 - -0.009Ia-insert galled on removal from holder.
59
11 ~594
Table 16
UNIT WEIGHT CHANGE FOR SELECTED MATERIALSAFTER FIRINGS IN THE VENTED BOMB
WITH WC846 PROPELLANT
Weight Chsnge, gramsBased on Based onMateriapls 3 Shots 5 Shots
446 stainless steel - +0.0016
H11 steel - +0.0002
50Co-29Fe-20W-IC alloy - 0.0000
Chrome plated Cr-Mo-V steel - 0.0000
L605 alloy - -0.0005
X-15 steel -0-0006 -
Cr-Mo-V - -0.0012
D2 steel - -0.0016
HS 31 alloy - -0.0016
S-590 alloy - -0.0018
H26 alloy - -0.0055
HS 21 alloy - -0.0130
22Cr-4Ni-9Mn steel -0.0177 -
A286 steel -0.1016
CG-27 -0.2120
60
ILI
t .4
F I
611
tI
22 5X(a) Longi tudi~nal
22 5X(b) Transverse
FIGURE 28 PHOTOMICROGRAPHS SHOWING THE STRUJCTU2RE ALONGTHE BORE S'rRFACE OF AN HII. ALLOY INSERT AFTER ONEVENTED BOMB Sh[OT WITHI WC846 PROPE.LLANT. (PICRIC
AND HYDROCHLORIC ACID IN ALCOHIOL ETCH)
i6
(a) Longi. t ud inalI25I I
S22 5X
(b) Transverse
FIGURE 29 PHOTONMICROGRAPIIS S1IOWING THE STRtTGITRE ALONGTHE BORE SURFACE OF AN 1111 ALLOY INSERT AFTER FIVEVENTED BONB SHOTS WITH WC846 PROPELLANT. (PICRIC
AND HYDROCHLORIIC ACID IN ALCOHOL ETCH)
62
I
and transverse section of the H1I alloy after one and fiveshots. No cracking was observed anywhere along the boresurface, and this is indicative of the good thermal resist-ance of the hot work tool steels. In these photographs aheat-affected zone is present along the bore surface.
Figures 30 and 31 illustrate the bore surfaces of H26,the tungsten hot work tool steel, after one and five shots.There is no evidence of cracking on the bore surface of theinserts after either test series. There is, however, asmall heat-affected zone present along the bore surface.This is more evident in the five-shot specimens.
The bore surfaces of the D2, the high-carbou, high-chromium cold work steel after one and five shots are shownin Figures 32 and 33. In the one-shot sequence one very
fine crack was observed in microexamination. The specimensubjected to the five-shot cycle exhibited many cracks.The transverse section of Figure 33 shows two of them at amagnification of 50OX.
Figures 34 and 35 depict the typical appearance of thebore surface of the 446 alloy inserts after one and fiveshots in the vented bomb. In these firing sequences onesmall .crack was obervd. . on the bore surfLace UW the five-shot insert.
P igures4 36 and 37 are typical photomicr'ographs alongthe bore surface of the L605 inserts which had received Iand 5 shots. The surfaces have been nickel plated. Crack-ing is readily apparent in the insert which received 5 shots.
Figures 38 and 39 typify the metallurgical structurealong the bore surface of the HS-31 alloy after I and 5shots in the vented bomb. The surfaces were nickel-platedto preserve the as-fired bore surface. The cracks whichoccurred in these specimens were very fine and few in num-ber. The areas shown do not have an appreciable evidenceof cracking.
Figures 40 and 41 depict the bore surface of the S-590alloy inserts which received 1 and 5 shots. Again, nickelplating was applied to the as-fired bore so as to protectthe surface. Cracking which occurred during firing is ap-parent.
A photomicrograph showing the bore surface of a trans-verse section of the 50Co-29Fe-20W-lC alloy is presented inFigure 42. The structure of this material is a massive
63
f- 7
I?_3 4d.
'N x.•'$. .,~ So ."
"r 1
225,X
(a) Longitudinal
"-. ,-;'r •-M- -'+ 5t•- ] ' ,"' C '+'+ T ) "U•'
,. s,.,',.,.. .
~o~4X* 1gV I
?.j42W Qw~22 5X(b) Transverse
FIGURE 30 PHOTONICROGRAPHS Si-lOWING TH4E STRUCTURE ALONGTHE BORE SURFACE OF AN 1H26 ALLOY INSERT AFTER ONEVENTED BOMB SHOT WITH WC846 PROPELLANT. (PICRIC
AND HYDROCHLORIC ACID IN ALCOHOL ETCH)
64
I -- - -- "raw
ttt
225X(a) Longitudinal
- _~- -~wX
225d'b)Transvers,
FIGURE 31 PHOTOMIJCROGRAFHS SHOWJING, THE STRUCTURE ALONGTHE BORE SURFACE OF AN H-26 ALLOY INSERT AFTER FIVEVENTED BONB SI-'3TS WITH WC846 PROPELLANT. (PICRIC
AM) HYDROOFIORIC ACID IN ALCOHO TCH
~)ETCHI
6 5a
......... - -.
14
225Xa)Longitudinal •
.......... i . ..
(b) Transverse225XFIGURE 32 PHOTOMICROGRAPHS SHOWING THE STRUCTURE ALONG
THE BORE SURFACE OF THE D2 ALLOY INSERT AFTER ONEVENTED BOMB SHOT WITH WC846 PROPELLANT. (PICRIC
AND HYDROCHLORIC ACID IN ALCOHOL ETCH)
66
T A,
225X(b) Trnsvers
FIGUE 3 PHOOMIROý3RAPS SOWIN TH STITCTRE LONT117BOR SUFAC OF HE 2 ALOYINSET ATERFIV
VF.ýED OMB HOTS 'WTH WIC4o POPELANT (PCRI
AND YDRC~fLRICACIDIN L""OOL TCH
67I
• ai-. u, s-n" eraa m. ie-G4a - e n era - - - - "- - 'fltI•IB,.O-O.- -,a r n ll.' -,aa•
e , -, -" -: "- " " r S--I 7
225X(a) Longitudinal
FIGURE 34. -_PSSTRUCTURE A,
*', ;i "," ÷ o.-• .,•, -• 4.~ •.,
V E-N - , .O M , -.P-E . -0 -
* 7 :..tc ...K ,, j ~ -
,. _.:•-.. - , , , . ., , .
• .. • -. . .--. " ,
22 5X(a) Longitudina
FIUR 3 POTMIRORAH S
SUrLFURIC ACID ELECTROLYTIC ETCH)
68
%" o
"" - 'S w
.
. •. .
225X
(a) Longitudinal
""a'. aim ; 4 •
". s " * +- " -..---. at ,, ++".++ . +r .-- -, .- - 'J + .; • - • . , " 1
VEN D . .8.
.
•o ,++ .. i . " -. - S '. - . • -1 • •,;.- r;.. ., '. -:,- -':, ' , . *" . -,o . ._- . - .- -
a.-...-
-• .•, : .. 4.*' -. • - : -. ; . : S. - .. • ; .
I (b) Transverse
iFIGURE 35 PHOTOHICROGRAPHS SHOWING THE STRUCTURE ALONG+ THE BORE SURFACE OF THE 446 AlLOY TNSERT AFTER FIVE'+ VENTED BOM4B SHOTS WITH WC846 PROPELLANT. (10%
SULFURIC ACID ELECTROLYTIC ETCH)
69SW
69
412
.T. BM SHT W. (
"•-t --- . , .* .- • .~ .4 -- • .o . , .•. .
"" _ ,.. ."" • °, - "', • .4 " ," . .
22 5X(a) Longitudinal!
70
o *# /
-- 4.. , '. . ,
* .. \
o .• 0 •
4 ' .
22 5X(b) TransverseI
FIGURE 36 PJ-OTOMICROGRAkPHS SHOWING THE STRUCTURE ALONGTHE BORE SURFACE OF AN 1.605 ALLOY INSERT AFTER ONE
VENTED BOMB SHOT WITH WC846 PROPELLANT. (57%CHROMIC OXIDE ELECTROLYTIC ETCH)
70
............. ....... . ... .. .. . .-. ...
IrII l
411
.. >2
.-' • . o , . . .• ° * . . .. VC *.. : /" / , , . -. -
- -~ -.- 22 5X
(a) Longitudinal
/ 4
(b) Transverse
FIGURE 37 PJ-OTOMICROGRAPIIS SHOWING TIlE STRUCTURE ALONGTHE BORE SURFACE OF AN L605 ALLOY INSERT AFTER FIVE
VENTED BOMB SHOTS WITH WC846 PROPELLANT. (5%CHROMIC OXIDE ELECTROLYTIC ETCH)
71
I
-c I
-IN
(b) raaserse225X
. ¶÷, , '.j '%s" V" ., • .- -1 '..-
FIUR 38 PH"T. N1• .R0'RAH SHWN THE STUCUR ALONG,•' .... • •..
.. .,..S-,W T . . .4.- , • (5%
7 ~ 2
- .. •'• pM.'. - - - 4
,r . *. . .' .
4'1 -p .
I225X
4272
-- I. -•
I .••. - - • ;,: , -. . . - '. , •
4 JL,. .": >,rt-.,. -4 Q ----< : -,,-° " ". • - ,% . % ¶. V $ : N -
-41
22 5X(a) Longitudinal
225X(b) Transverse
FIGURE 39 PHOTOMICROGRAPHS SH0OWING TulE STRUCTURE ALONGTHE BORE• SURFACE OF' AN 11S-31 ALLOY INSERT AFTER FIVEVENTED BOMB SHOTS WITH WIC846 PROPELLANT. (5%CHIROMIC OXIDE ELECTROLYTIC ETICH)
73
22 5X(a) Longitudinal
0duccd rom
- a bze co" IrI
-44k :44
JV~tiYP~ 2-v%22'AXFIGUR 40L~ P1OOIRG.AP SHWN HESRCUR 7 LN
THE ~ ~ ' BOR SUFC OFA- -9 LO ISR FE N
VENTD BOB SHT WIH W846 ROPELANT (5CHROMIC OXD LETOYICEC1
- ~ 7p
. .w- t, . ý . ýL .
ddb%,fiL-- -- UM6MM A
If
.4 . .i ./$~ . .
.AitA it'
()Longitudinal 225X
Reproduced fromnbest avala.6ie copy.
I1
(b) Transverse 25
FIGURE 41 PHOTOMICROGRAPHS SHOW~ING THE STRUCTURE ALONGTHE BORE SURFACE OF AN S-590 ALLOY INSERT AFTER FIVEVENTED BOMB SHOTS WITH WC846 PROPELLANT. (5%~CHROMIC OXIDE ELECTROLYTIC ETCH)
75
Nital Etch 500XiFIGURE 42 PHOTOM4ICROGRAPH OF A 5OCo-29Fe-20W-].C ALLOY
REVEALING THE TRANSVERSE STRUCTURE ALONG THE BORE SURFACEA-RF At'PU X.TE1M' BRMR FTRTNQ" W TH MP PROPELLANITT
76
dispersion of carbides in a cobalt matrix. Again, nickelplating was employed to protect the bore surface duringmetallographic preparation. There are no apparent cracksalong the bore surface.
Table 17 summarizes the microexamination of the selectedalloys after being fired in the vented bomb. From the datait can be concluded that:
1 . The cobalt-base superalloys tend tofail by cracking.
2. The 22Cr-4Ni-9Mn alloy exhibits brit-tle behavior in the vented bomb en-vironment.
3. With the exception of D2 steel and446 stainless steel, steel alloys donot tend to fail by cracking.
4. Chrome plate on steel produces crackstarters which propagate into theSLeel OULer elienit!niL.
5. The 50Co-29Fe-20W-lC alloy and theCG-27 alloy do not appear to failby cracking in the vented bomb.
Phase III - Materials Selection
The following material were selected for evaluation asgun barrels:
1. 50Co-29Fe-20W--IC alloy
2. Chrome-plated CG-27 alloy
3. Chrome-plated HII
The rationale for selecting these materials is as fol-lows :
i. The 50Co-29Fe-20W-lC alloy showedgood erosion resistance in thevented bomb test. There was noevidence of cracking and the metal-lurgical structure ol this materialcan be controlled to produce a dis-persion of hard carbide precipitatein a tough matrix. In zhis conditiona hardness of RC 44 is obtained andthe carbide precipitate will provideexcellent wear resistance.
77
<U 0 co cc (Ij
41 0 vi 0 1 11
0i C) v4 o
(U) -r) Co((J4 U CUvU (UCU CUOU CU)I 'r-4 Cýl
:1 0 -l le -1 CO O CL) C) 0 0 0 0 0 0 C)cU
-,4 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ (lI C)C)) C a f 4 4 OCJ (ZC l4
r: Q) C) u ( 1 Pl u )Q ( iQi ) (1 (1 a >
0000
4J () C U) r 1)1 )1)'-~~~C u' U -
w1 0 r) "0) (U) u 0 C U* ~ 3 -4) F-124I (.12 En _Ile()(2(21) i U)
02 E-'- c
0- 00 aa -Y Ca) co (U) 00 ( )C'4j40 0 U~
to I- -Y Q Z4 CdZrI~ J-4 <.d<I -r-l
U) C'2 C ý1 Lfl L( f) r4f) ) C. If)c,-l ) ,-4 ý t44 , i-J 4 4-4LC o
0~~- 4-4 u tw 0 .
U 4 C) ( U r4 -4 C jQ - = 0 C U )) )ý -4 > :2-i O d 4 " T - ) q Ct M r-I 00-)i
4 w- (12 04 ý 4ý 4 P (3 4ý 4'
-- (12 H o N Q 1;CJU U) Ll ~ -1) > 1 J U) E:>, J
0't v CU) '-4 U) r0 0)r43 Z 44 4r0 0 o 0 0 tv w- co (
Za :F U) ;E Z ZU ~-4 44 CU) CU
U) 0 0-~ I
rI4( l U)~\ U2) I- 2-iS (/2 -4 Lr) r- U 4 -4 LO 0 -42 Ir 4L) - r
'> 0 f - i U
P78
2. The CG-27 alloy has good mechanicalproperties but has relatively poorerosion resistance as measured inthe vented bomb, Chrome plating theCG-27 alloy theoretically will pro-vide the erosion resistance, and themechanical properties of the CG-27alloy should provide sufficient sup-port and crack resistance to obtainexcellent performance as a gun barrel.
3. The H1I steel is a much tougher andwear resistant material than the Cr-Mo-V steel utilized in current gunbarrels. The toughness and strengthcombination in conjunction with theerosion-resistant chrome plate wouldresult in a good gun barrel system.
Sufficient quantities of the 50Co-29Fe-20W-IC alloy,CG-27 alloy, and H11 steel to fabricate 12 gun barrels ofeach material were obtained. The 50Co-29Fe-20W-IC alloywas made by the extrusion of powder into 2 inch rod byFederal-Mogui. The extruded rod was heat treated and Nhip-ped to Rock Island Arsenal. At the direction of Rock IslandArsenal 4 inch rounds of the H11 steel and the CG-27 alloywere shipped to Battelle-Northwest for fabrication into un-plated 7. 6 2mm barrels. Rock Island Arsenal chrome-platedthe H11 and CG-27 barrels.
Phase IV - Fabrication of Selected
Materials into Gun Barrels and Test Firing
Fabrication
The materials furnished by IITRI were to be fabricatedinto gun barrels by Battelle-Northwest. A formal report(1 3 )was issued by Battelle-Northwest describing the fabricationmethods. The CG-27 and H11 alloys were processed into bar-rels; however, the cobalt alloy could not be rifled by swag-ing. The following summarizes the Battelle-Northwest work:
CG-27 Processing - The CG-27 was supplied as solutiontreated, 4-1/74 inchdiameter wrought bar. This materialwas machined into the 4 inch diameter extrusion billets.The billets were preheated to 2050*F inside stainless steelcans in a SiC resistance element powered air atmospherefurnace for 2-3 hr.
-79
The cXtruision step was performed on a 1250-con press toproduce a 1.35 inch diameter extrusion containing a mildsteel sacrificial mandrel. The billet container and, diewere lubricated with "Oil Dag"* and "Phosphatherm RN."**The extrusion was conducted at a ram speed of approximately300-330 inches per minute. The extrusion was rough straight-ened while cooling in air.
The extrusions were finish straightened on a Suttonfive-roll rod and tube straightener and cut into 22-inchblanks. The extrusions were generally about 90 inches longand furnished three 22-inch blanks, one 13-inch riflingtrial blank, plus several metallographic specimens.
Four CG-27 extrusions were performed, with preheat tem-peratures varied from 2040'F to 2075°F in an effort toselect the optimum. It appears that 2050'F is optimum forthis press. Lower temperatures increase the extrusion pres-sure above the practical maximum value. Higher temperaturesresult in a severe hot-short condition which precludes ex-trusion without rupture of the extrusion. Additionally,the higher temperatures are more prone to causing mandrelfailure by necing
Two extrusions were performed without incident. One ofthe extrusions whose mandrel failed was made into usableblanks by a combination of etching and drilling to removethe pieces of mandrel and prepare a bore for rifling. Oneextrusion was unsatisfactory.
Nine full length blanks and three shouter pieces (12-14 inche,) wer.: obtained from the three extrusions used.They were all solution annealed by heating in air at 1875'Ffor 1 hr prior to quenching in oil. The hardness was re-duced trom an average level in the air-cooled extrusion of36 RC to an average of 87 RB after solution anneal. Thewarp in the blanks which was sustained in the anneal wasremoved by hand straightening in a press.
The pieces then had the sacrificial mild sreel mandrelsleeve removed from the bore by machining. On subsequentextrusions of Lhis nature this material was removed by aciddissolving. The bores of all pieces were polished by honingto remove sharp striations and scratches which would not beremoved in the swaging operation,
Proprietary graphite and oil suspension.
Proprietary glass lubricant by Alpha Molycot. Corporation.
80
The swaging operati.,;n -1a;3a e.-ýfttrmeJ on 9 Fernn., fourdie rotary swage The fotiýuoa,4 ,,-'cacts-ide a a:..1 ct.gu.with hammer blocks riding ou tLhe Th:oes. 'h os..tiAltiiraction of the hammer blocks ridi.ng c.-,.o. the volt;, drixve.ithe dies in the radial dirm:ct-icn to fcron 'Iikt work,
The mandrel, was made of Gradc 55B tungsten car.bide ,ýndwas brazed at the taper to a mild steel nositioneir rod whichextends from the swage dies through the work, t1e w'ork hs---dle,and the feed chuck to a thrust hearing anchor at therear of the power feed base.
The prepared rifling blank was welded to a hollow hoadle.The bore of the blank was lubricated with "Oil Dag and theblank and handle assembly was placed into the power feedchuck fed over the mandrel and into the rotating and radiallyoscillating dies to produce the rifling. This step requiresabout 40 seconds. The power feed chuck brake applies a re-tarding torque to the work to prevent its seizing and flash-ing between the mating surfaces of the dies. The hardnessof the blanks increased from 87 RB to 103 RB during theswage rifling.
The bores of the rifled blanks were cleaned and inspectedby air gaging and borcscoping p-ror to heat treatm•nt- Theywere aged in vacuum at 1450°e for 16 hr, then 1200'F for anadditional 16 'ir. Hardness after aging averaged 43 RC_ Re-iinspection of the bores revealed that the bore shrunk about0.001 inzh in diameter during the aging heat treatment. Thisresulted in the bores hc-ing on the minimum specified.
The acceptable rifled and heat treated blanks were cham-bered and externally machined in accordance with ArsenalDrawing 11701204, sheets I and 2, Rev. F conventional machin-I ing techniques.
Hll Processign - The processing of the HII barrels wasessentially the same as for the CG-27 except for heat treat-ment. Six extrusions were attempted with all but the firstbeing successful. The first billet which was preheated to-- 2000°F stalled after 4 to 5 inches of extrusion and wiped
all the ceramic coating from the die. The remaining fiveextrusions were preheated to 2100*F arid performed satisfac-torily. All other extrusion parameters vern the same as' CG-27.
The swaging performance and parameters for the HII wereidentical with the CG-27.
81
The Hil rifled blank, were plugged by welding end capsto them, and evacuated through tubes which were sealed.They were then hardened in an endothermic gas atmosphereby preheating Lo 1500°F for 45 min,then heating to an aus-tenitizing temperature of 1825 0 F,followed by an oil quench.They were tempered ac 1200*F for 75 min, quenched, straight-ened, tempered at 1220 0 F for 75 min and quenched. The re-suiting hardness was 33-36 RC. Very little change in boresize occurred during heat treatment.
As with the CG-27, the acceptable HII rifled blanks wereconventionally machined to final configuration. It shouldbe pointed out that they have oversize bores to accommodatechrome plating.
Cobalt-Iron Alloy Processing - The cobalt iron alloyblank's were fabricated by IITRI by extruding rods fromcanned powder billets. The rod blanks were then gun drilledat Rock Island Arsenal to form the bore.
The first two blanks received were cut in halves to fur-nish four pi.eces for establishing swaging parameters. Thehardness as measured on the OD of the blanks was 32-33R•.The swage dies, shims, and mandLels were the same as usedfor the CG-27 and HII. The four pieces were welded to han-dles. Thsrce -4re- prl-Lete for about 45 villin ai to 9Tand the fourth to IIO 0 °F. Swaging resulted in fracturingof all blanks. Hardness measurements taken on various as-swaged specimens ranged from 40 to 50 R(. The materialseemed to have high strength but was very brittle as judgedfrom the swaging performance.
The material was reheat-treatel as follows: solutiontreat at 2350'F, water quench, and age 3 hr at 1100°F. Thisdropped the as-received hardness from 44-49 RC to 33-39 RC.After heat treatment the Co alloy was swaged at 1100'F.Swaging of this piece resulted in breaking of the mandreland the back end of the blank. Work on this alloy was ter-minated pending additional development on fabrication tech-niques.
Summary
e Extrusion over a small mandrel is an excellentmethod of producing gun tube stock of CG-27 forrifling by swaging.
e Extrusion of HII steel, as conducted in thisprogram, can give tube steels suitable for
82
furthei processing by swaging to give it rifledtube. However, other methods of producing suchtube stock may be more cost effective and givea product of equivalent quality.
9 Swaging is a satisfactory method for riflingCG-27 and HI1 7.62mm gun barrels.
e A more rugged and powerful machine is requiredto rifle the Co-Fe alloy tried in this program.
* Rifling tolerances of + 0.0002 are easilyachieved in rotary swage rifling of 7.62mmbores once the parameters for a given materialare established.
e A rifling configuration with a greater numberof grooves, with the land and groove widthmore nearly equal, would be easier to swage-rifle and, therefore, more economical.
Test Firi~ng
The fabrication barrels were chrome-plated by Rock IslandArsenal and test fired there. The following firing schedulewas employed for evaluating the chrome-plated CG-27 ankdchrome-plated H11 barrels:
M134 machine gun firing at a rate of4000 rounds/min: 500 round burst,cool 10 sec, 500 round burst, cool 10sec, 500 round burst, cool 10 min instill air. Repeat until 6000 roundswere fired, and cool to room tempcra-ture.
RejecLLoti cLiteria w.!ru loý.s iu velucii. v, yaw, and barrelrupture.
Post Analysis
Bore Measurements - After Rock Island Arsenal completedthe firing tests, th'e rejected barrels were shipped to IITRIfor analysis. The rounds to failure for each barrel arelisted in Table 18. During the test sequence, silicone im-pressions of the Iore were taken at various intervals. Twobore diameter measurements were taken at 90-degree intervalsalong the length of each impression. These data are listedin Table 19 for the chrome-plat,-o" ,G-27 barrels, Table 20for the chrome-plated 1111 barrels, and Table 21 for one of
83
hI
Table 18
NUMBER OF ROUNDS FIRED ON SCHEDULE IBEFORE FAILURE WAS OBSERVED
No. of
Material Rounds Fired Reason for Failure
Chrome-plated CG-27 4,036 Cracked in muzzle area
7,324 Yaw
Chrome-plated H1l 16,291 Yaw
24,466 Yaw
Standard barrel 12,068 Yaw
11,283 Crack in breech area
84
-IS. ...: : -" • •'" " • - : • ... .• ... .... ....... ... ... ...... ... ... 1 I
Table 19
BORE DIMENSIONAL MEASUREMENTS FOR CHROME-PLATED CG-27 BARRELSAFTER VARIOUS NUMBERS OF TOTAL ROUNDS FIRED ON SCHEDULE I
Bore Diameter in.barreT-, •ariel 2
4,036 1,01 7,124Distance Total Rounds Total Rounds Total Rounds
from -os1i P o s i'- Posi- Po-sT-- Posi- 70PS-17Chamber, tion tion tion tion tion tion
in. 1 2 1 2 1 2
0 0.307 0.307 0.307 0.307 0.313 0.313½0.307 0.307 0.307 0.307 0.311 0.3111 0.308 0.309 0.307 0.307 0.314 0.3141½ 0.310 0.311 0.307 0.307 0.313 0.3142 0.308 0.308 0.307 0.307 0.311 0.3112½ 0.308 0.308 0.308 0.308 0.308 0.309"3 0.308 0.308 0.307 0.307 0.3C8 0.3084 0.308 0.308 0.307 0.307 0.308 0.3085 0.308 0.308 0.307 0.307 0.308 0,3086 0.307 0.307 0.307 0.307 0.308 0.3087 0.307 0.307 0.307 0.307 0.308 0.3088 0.307 0.307 0.307 0.307 0.308 0.3089 0.308 0.308 0.307 0.307 0.308 0.308
10 0.309 0.309 0.307 0.307 0.308 0.30811 0.309 0.314 0.307 0.307 - -
S12 0.309 0.317 0.307 0.307 0.312 0.31313 0.312 0.330 0.307 0.307 0.317 0.31814 0.314 0,340 0.307 0.307 0.317 0.32115 0.315 0.352 0.307 0.307 0.317 0.32216 0.320 0. 352 0.307 0.307 0.317 0,3261 -7 0. 3-27 0•.32 0. 0.307 0 .3 2, 2 - 0. 32
17½ 0.331 0.362 0.307 0.307 0.32] 0.32418 Muzzle 0.307 0.307 0.320 0.32218½ 0.307 0.301 0.319 0.32219 End 0.307 0.307 0.318 0.322
Cracked19L 0ff 0.307 0.307 0.310 0.32220 0.307 0.307 0.311 0.322
85
r-- 'n U ', r'n a, 1 (0, a, C e , a , o ' F w ~ w ' C'0 w ' r, 00 C' , 00 , (ON 00 .7-31 OD 00 -' -, C
-t " e C'Ii ~fvlrOC-1 -)-n v n-cn- m r-r- - ie(,m"Iv n n en a-I -n e . r r-
COO 0~ CD 000C)OOOCC 000 00000 0 0 0 0 0 CC>C)00
Lt r Y-r o -oor r o oo o55 5 5 5 5 5o oo o o D or-00 ooor - -r-
4-' n 0 ',"' rl) '. *n ) (1) fn c" , r" , All cn ) (n , C' C" M ' c ', cn (n(" (n M O "I, cn CO cn
0 -1 00 O f ý 0 0 ON00CI000 00 O) M 00070Q0NCD % oc0, , l In
Jo .- J F '~ ", ', C CDO C CO 0 0'C CD CO C) CD CO Ci CD CO C', C', (, 0
* ~ ~ ~ - 0 o . . . .~ .: .-. .~ .c . .o~ o .0 0 .~- .- .- C. .e ~ ~ ~ 0 1 -j LX 0 0' 0, 0 0 C' 00 ) 0 CO0 0 0) .' roOC - 0 -i 0 0' 0 CO 0 CC
I,)110 311- I-f Y (3% CO- W~ C0 0 Z 0Q C C CCcci 0000 0000 D000 o on qI< -) ; CD *r) (I - -)nr' -
co 121 010 1 .J .~ .'. ' . ..O * C .~OO C~~
-40I
I- r-- r-, r - r - r-, f-, r-
414 2 1 C)--D )00C D. - D D0 QC
)- Nt'.l(- N C I.C- N N .N . .N .N .s . .l .l .C' . .C C
Lx W. M .-- -) C-) n -n -- I 4'ý .C .J
08
i.4
Table 21
BORE DIMENSIONAL MEASUREMENTS FOR A STANDARI GUN BARRELAFTER VARIOUS NUMBERS OF TOTAL RObNDS
FIRED ON SCHEDULE I
Bore D)iameter, in.Distance I0,065 12,068
from Total Rounas Total RoundsChamber, Position Position on
in. 1 2 1 2
0 0.321 0,321 0.328 0,328½ 0.321 0.322 0.338 0.3401ý 0.310 0.318 0.322 0.3392 0M305 0.305 0.308 0.3092½ 0.307 0.307 0.309 0.3103 0.307 0.307 0.308 0.3094 0.307 0.307 0.308 0.3095 0.307 0.307 0.308 0.3096 0.307 0.307 0.308 0,3087 0.307 0.307 0.307 0.308R0 .307 0.307 0.307 0.308i
9 0.307 0.307 0.307 0.30810 0.307 0 307 0.307 0.308[1 0,307 0.307 0.307 0.30812 0.307 0,307 0.308 0.30913 0.307 0.307 0.312 0.31314 0.307 0,307 0.312 0.31315 0.307 0.307 0.309 0.31016 0,307 0.307 0.307 0.30717 0.307 0.30-7 0.308 0.30317k 0.307 0.307 0.307 0.3081.8 0.307 0.307 0.307 0.30818ý 0.307 0.307 0.307 0.30719 0.307 0.307 0.307 0,3079Iq 0.307 0.307 0.307 0.308
20 0.307 0.307 0,307 0.308
87
the standard chrome-plated steel barrels. Figures 43through 47 plot these data and give an excellent representa-tion of what is occurring along the bore surface during thefiring schedule.
According to these graphs, with chrome-plated 1111 bar-rels and the standard gun barrels, when failure occurs se-vere erosion is present in the barrel just forward of thebreech area. With the CG-27 chrome-plated barrels, whenfailure occurs severe erosion is present in the muzzle areaof the barrel. Another representation of these data isshown in Figure 48. Here, the maximum bore diameter--re-gardless of position--is plotted as a function of number ofrounds fired. Since two bore measurements were made at eachposition at 90' intervals, two data points for each measure-ment are shown.
Metallographic Examination
Metallographic examination was performed with the scan-ning electron microscope and the optical microscope. Theseobservations revealed that the transverse sections typifiedthe failure mode and that the optical microscope was suffi-cient for characterization of the failure mode.
Chrome-Plated Gun Steel Barrels - Examination of thebore measurements indicated that for chrome-plated gun steel,the maximum erosion occurs in the area just forward of thebreech. Additionally, there appears to be an increase inerosion in the region 11 to 16 inches from the breech. Alsorevealed in the silicone rubber molds is the general removalof the rifling at the muzzle end of the barrel. To determinechanges which were produced by firing until the two steelbarrels were unserviceable, transverse metallographic speci-mens were prepared at 1/2, 1 1/2, 2, 10, 17 1/2, and 19 1/2inches from the chamber. Phocomicrographs taken at thesepositions on the two chrome-plated gun steel barrels areshown in Figures 49 through 60. Figures 49 through 54 showthe structure of the standard chrome-plated barrel whichfailed after 11,283 rounds by a large crack extending fromthe bore surface to the outer edge of the barrel. Figure 49is a transverse section at 1/2 inch from the cha-mber. Itwill be noticed that a massive crack is present, and thisparLicular crack typifies many of the cracks in this region:all of the chrome plate has been removed; massive amountsof the steel have been removed by general erosion; the micro-structure near the crack and the bore surface appears to beferrite and carbide; it also appears aq though some decar-burizat 1 ,r along the bore has occurred; and, additionally,
88
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( I . C k, d.
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---
...... 00 ,JA•. .
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x 0 CG-27 Llnfaih.d
. CG-27 FailedS- - -- 0[ "Standard Ufeailed"
* Standard Failed
rii v ii A iH-l1 UnfailedA H-Il Failed
0 ioo 8000 12,000 16,000 20,000 24,000 2 ,000
Numher of Rounds
j.';(;•'•I :48 MA.MI1MH. CRACK OR EROSION DEPTH FOR VARIOUS GUN BARRELSAM A FUNCTION OF NUMBER OF ROUNDS FIRED.
94
•~~~......,...:-,-.. "... .4.........
9 v -' t ! ,' :•, I,, •I •
Nital Etch 16 OX
FIGURE 49 TRANSVERSE SECTION AT 1/2 INCH FROM THECHAMBER OF A STANDARD CIIROmI"-PLATED GUN BARREL AFTER
FIRING 11 283 ROUNDS AND BEING DECLARED UNSERVICEABLE.
IS•
-
. .'. -..
tct
k Nital Etch 16 OX
FIGURE 50 TRANSVERSE SECTION AT 1 1/2 INCHES FROM THECHAMBER OF A STANDARD CHROMEI-PLATED GUIN BARREL AFTERjFIRING 11,283 ROUNDS AND BEING DE1-CLARED IJNSER\,ICEABLE.
95
.,• .. ,• . .. .. . A
II
I-AE OF STNDRD r1R0F-f. DGNBRE FE
A. M.sjtV "Z
Ni~tal Etch 160X
FICGURE 51 TRANSVERSE SECTION AT 20 INCHES FROM THECHAM4BER OFl A STANDIARD CHROME-PLATED GUN BARREL AFTER
FIRING 11,283 ROUNDS AND) BEING DECLARED UNSERVICEABLE.
AN. 96
kRA
\~V .r 04 f
It t'\ t¼A~*.xtv) 4 i '.* 4 -
'- A""r~'i~..~
"-V A 'A;, *~~.js 4-
Nital Etch 16 OX
FIGURE 53 TRANSVERSE SECTION AT 17 1/2 INCHES FROM THECHAMBER OF A STANDARD CHROME-PLATED GUN BARREL AFTER
rFIRINGI 11,283 ROJUNDS'~ AMNDBIJiNG DECtLARED UNSiERVICEABLE.
AI9r'
.T* a- ... .r - .- - ;,, ,I
71. 1..
FIUR 54t TRANSVERS SETO T1 / NHSFO HCHME OF tA STNDR CRIR-'.Ai E GU BARE AFTER
CHARIER OF28 ARSTNDSAND CEINRG DELATED GUNSBARVILEAFTER
FIIN 1,23ROND AD ENGDELAEDUSEVIEALE7
I
copper alloy has been deposited in the crack. Microhardnessmeasuiements taken from near the bore surface to midway be-tween the bore surface and the outer barrel surface variedfrom Rc 23 to RC, 30 and confirm the change in metallurgicalstructure near the bore surface. This hardness differenceor softening indicates that the bore material in the steelbarrel experienced a temperature excursion in excess of thetempering temperature, about 1200*F, employed in the heattreatment of the barrel.
Figure 50 shows the microstructure near the bore sur-face of a transverse section at 1 1/2 inches from the chamber.The comments made on Figure 49 also pertain to this photo-micrograph.
Figure 51 shows the transverse microstructure at the2 inch location. The cracks are appreciably smaller; how-ever, the same observations regarding general structure canbe made on this section along the bore.
The metallographic structure of the transverse sectionat the 10 inch location is shown in Figure 52. This partic-ular site is at the intersection of a land and groove. Thethickness of the chrome plate on the land has been greatlyreduced, whereas very littie reduction ii, t........ L.c...-
curred in the groove portion. The metallurgical structureappears to be rather uniform and is quenched and temperedmartensite. No appreciable cracking in the steel was observed.
Figure 53 shows the structure of the barrel at 17 1/2inches from the chamber. The general physical appearanceof the structure is similar to Figure 52. However, it doesappear that the region directly below the land is ferriteand carbide. The bore surface at 1/2 inch from the muzzleis depicted ia Figure 54. Again, it is apparent that theland is comnletely removed, The material above the steelis copper alloy.
Figures 55 through 60 show the bore at the various sitesfor the second standard chrome-plated barrel which was de-clared unserviceable after 12,068 rounds because of yaw.
Examination of the transverse section which was cut1/2 inch from the chamber was very similar to Figure 49.The chrome plating was completely removed, and large crackswith copper alloy in them were present; however, none ofthe cracks were more than half way through the barrel wall.Figure 55 shows one of the intermediate cracks and typifiesthe area at the 1/2 inch position.
98
Nital Etch 16 OX
FIGURE 55- TRANSVERSE SECTION AT 1/2 INCH FROM THECHAMBER OF A STANDARNXD CHROME-PLATED GUN BARREL AFTER
FIRILNG iZ,0b8 ROUNI)S AND BEING DECLARED UNSERVICEABLE.
w_
~~FiAŽA~~~~~ %.t,~''N(C.i+: 4
Nital~t Ethi60
FIUR 56 TASES *E'74IO T11/ NHSFO H
CHME FASTNAD2RW--LTDCJ BARRELt AFTER
FIRING12,06 R0T.TDS AN 13FIN DFCLRE USERIEBE
I j nt
99
1v1
1 a r
Nital Etch 16 OX
FIGURE 5 7 TRANSVERSE SFCTION AT 2 INCHES FROM THECHAMBER 1206 A U~ STNDR u'OM-LAE GJXUN BARRL A(VUtF~TECHABE O AQY STNI;DCAR CHRM"= DECPLATED GUN BARVRELAFER
'W VT CC-v S% Al0 f M l flu D" -L ~ n UnV ,-
fI
ItIW. A' 14i
Nital Etch 16 OX
FIGURE 58 TRANSVERSE SECTION AT 10 INCHES FROIM THEICHAMBER OF A STANDAPD CHROME-PLATED GUN BARREL AFTER
FIRING 12,068 ROUNDS AND BEING DECLARED UNSERVICEABLE.
100
Nital Et~ch 16 OX
FIGURE 59 TRANSVERSE SECTION AT 17 1/2 INCHES FERON THlECHAMBER OF A STANDARD CLIROME-PLATEI) GUN BARR~EL AFTER
FIRING 1-2, 068 ROUN-DS AND BEING DECLARED UINSERVICE"ABLE.j
Nital Etc 16 OX
1,FIGURE 60 TRANSVE'RSE SECTION AT 19 1/2 INCHES FROM THECHAMBER OF A STANDARD CHROME--PLATED GUN BARREL AFTERFIRINC 12,058 ROUNDS AND 1BEING DECLARED UNSERVICEABLE.
101
Figure 56 shows the structure at the bore surface atthe 1 102 inch site. The land was completely removed; somechrome-plating in the grooves remained; large cracks filledwith copper alloy were present; relatively large displace-ments of the steel can be noted; and the metallurgical utruc-ture has been modified from quenched and tempered martensiteto ferrite and carbide. The white layer is present and doesnot appear to be interconnected with the ferrite.
4 At the 2 inch position, there are relatively largecracks in the chrome plate and the steel. Figure 57 showsa portion of the transverse section. The characteristicfeatures are the presence of the copper alloy in the cracks,the discontinuous whIte layer, the change in structure toferrite and carbide near the crack and bore surface, and thepresence of most of the chrome plate.
Figure 58 shows the typical appearance of the transversesection at 10 inches. The cracks in the chrome-plate canreadily be seen; however, the general integrity of the regionis good. Further examination of the sample revealed a thin-ning of the chrome plate on the barrel similar to that ofFigure 52.
Figure 59 shows the transverse section at 17 1/2 inchesfrom the chamber. The lands have been removed, some of thechrome-plate in the grooves remains, the large displacementof steel is apparent, some copper buildup is pres1ent, andthere is no evidence of a white layer or change in metallur-
gical structure.
A photomicrograph of the transverse section of the bar-rel at 1/2 inch from the muzzle end is presented in Figure 60.At this position, the lands have eroded away, the chrome platein the grooves remains, a copper alloy buildup on the boreG ... J"^=.0 . . . .jj ýOULý ....U L- W it l ye VV MU Ul gicai
S changes in structure have been induced.
From these photomicrographs, it is evident that several
events are occurring--the cracking of the chrome plating,the cracking of the steel, the apparent flaking or spallingof the chrome plate, the buildup of copper in the cracks,the presence of white layer, the thinning of the chrome platetoward the muzzle, the removal of the lands, the relativemovement of large regions of the steel, and the smearing ofcopper alloy along the bore surface.
While all of the previously mentioned observations areimportant in the general erosion of the barrels, it appearsthat two phenomena are capable of producing an early failure.
102
These are erosion, by a combination of factors, just forwardof the breech and the rapid propagation of a single crackto the outer surface of the barrel to produce catastrophicfailure.
IWith the standard chrome-plated gun steel it appears
that a rather rapid deterioration of the chrome plate isachieved in the area just forward of the chamber. Themechanism of erosion is directly related to the chrome plat-ing. As plated, the chrome deposit is extremely brittleand often has cracks present. On firing the barrel, furthercracking is produced in the chrome plate and, because ofgood coupling between the chrome plate and the gun steel,cracks are induced into the steel. As firing continues,the chrome plate flakes off and erosion and cracking con-tinue in the steel.
From this point on, a contest exists as to whether fail-ure will occur due to erosion of the steel or catastrophicfailure due to cracks extending through the wall of the bar-rel. In the erosion process, the stael is removed by reactionwith the propellant gas stream and by shallow flaking dueto propagation and interconnection of existing cracks in thearrel. The failure is observed by monitoring veluciLy,
accuracy, and yaw. In the catastrophic failure, one of thecracks exceeds the critical flaw size required for rapidpropagation under the stress field developed in the firing Icycle. In this instance the fracture toughness of the gunsteel. is sufficiently low that the pressure and thermalgradients can develop a crack of critical size which rapidlypropagates to the outer surface of the barrel and resultsin a catastrophic failure.
Chrome-Plated HIl Steel Barrels - The bore measurementson the two chrome-plate- ill steel barrels indicated thatthe erosion pattern produced was similar to the one presentin the standard chrome-plated gun barrels, the only differ-ence being that significantly more rounds were required be-fore the H11 barrels were declared unserviceable. Transversemetallographic samples were taken at the same distauce fromthe chamber as for the standard barrels. Figures 61 through72 show the bore area for two H1i barrels. Figures 61 through66 pertain to the H11 barrel which fired 16,291 rounds beforebeing declared unserviceable.
The transverse section at 1/2 inch from the chamber isshown in rigure 61. The removal of the chrome plate, the
4 massive erosion of the steel, the crack in the steel, andthe presence of white layer are apparent. The metallurgical
103
'w'3
I 16 T
Nital Etch 16 OXA:
FIGURE 61 TRANSVERSE SECTION AT 1/2 INCH FROM THECH4AMRF'BR OF A CfStflMEP-"LATED LL±±. STEELu (UTN BARRK EL AFTERFIRING 1L6,L91 ROUNDS AND BEING DECLARED UNSERVICEABLE.
Nital Etch 16 OX
FIGURE 62 TRANSVERSE SECTION AT 1 1/2 INCHES FROM THECHAMBER OF' A CHROME-PLATED 1111 STEEL CUN BARREL AFTERFIRING 16,291 ROUNDS AND BEING DECLARED UJNSERVIGEABLE.
I 104
....-....I. . :. ... .
. . . ". -. . ... . -. . : L -• ' " L :
Nital Etch 160XFIGURE 63 TRANSVERSE SECTION A 2 INCHES FROM THE
CHAMBER OF A CHROME-PLATED H1 ST EL GUN BARREL AFTER
FIRING 16,291 ROUNDS AND BEING DECLARED UNSERVICEABLE.
Nital Etch 160OX
FIGURE 64 TRANSVERSE] SECTION AT 1 0 INCITES FROM THIECHAMBER OF A CHROMfE-PLATED HI1i STEEL GUN BARREL AFTERFIRING 16,291 ROUNDS AND BEING DECLARED UNSERVICEABLE.j
N
s'i
rI1
Nital Etch 160X
FIGURE 65 TRANSVERSE SECTION AT 17 1/2 INCHES FROM TI-EF
CHIAMBER OF A CHROME-PLATED Hill STEEL GUN BARREL AFTER 44
Nital Etch 160OX
FIGURE 66 TRANSVERSE SECTION AT 19 1/2 INCHES FROM THECH•AMBER OF A CHROME-PLATED 1111 STEEL GUN BARREL AFTERFIRING 16,291 ROUNDS AND BEING DECLARED UNSERVICEABLE.
106
TV j
structure is quenched and tempered martensite with littleor no alteration. Microhardness tests were taken along thebore surface to about midway through the barrel. Uniformmeasurements of Rc 36 were obtained.
Figure 62 depicts the bore surface at 1-1/2 inches.Comparison with Figures 50 and 56, the standard gun steelat a similar location, provides an interesting contrastdespite the difference in number of rounds fired. The chromeplate is cracked but still held in place. Examinations inother regions showed the rifling to be present. The cracksare typical, filled with copper alloy and the white layerbeing present. No other changes in metallurgical structureare apparent.
Figure 63 is the bore region on a transverse sectionat 2 inches and is very similar to Figure 62. A transversesection at the 10-inch position from the chamber is shownin Figure 64. The reduction of the chrome plate on the landis obvious; however, review of Figure 52 shows that thechrome plate retained on the land of the HI1 barrel is thickerthan that retained on the standard barrel.
At the 17 112 inch position from the chamber the landsare almost completely removed, as can be seen in Figure 65;only scattered thin portions of the chrome plate remain onthe land. No copper alloy buildup is present, and the steel
microstructure is quencehd and tempered martensite.
Figure 66 shows the transverse section 1/2 inch fromthe muzzle. Again, chrome plate on the land has been sub-stantially reduced, but all portions of the land are stillchrome-plated and copper alloy buildup is present.
Fi.ures 67 th.rough 712 show the transverse sections ofthe region near the bore surface for the H11 barrel whichfired 24,466 rounds before being declared unserviceable.In general, comments made for Figures 61 through 66 applyto these figures. The cracking appears to be more extensive,but this is probably due to the firing of approximately8,000 more rounds through this barrel. Massive movementsof steel are evident at the 1 1/2 and 2 inch regions.
The major difference between the failure mode of thechrome-plated H1 barrels from tha, -f the standard chrome-plated barrels can be attributed to the apparent tenacityof the chrome plate to the HI1 steel, the support this steelgives the chrome plate, and the superior high temperatureproperties of this steel co,7pared to the standard gun steel.Even though both H11 barrels received considerably more
10107
Z:m
-'N-
:,,
Nital Etch 160X
FIGURE 67 TRANSVERSE SECTION AT 1/2 INCH FROM THE -•CHAMBER OF A CHROME-PLATED HII STEEL GUN BARREL AFTERFIRTNG 24,o ROI-S AND BEUNG DECLARED MNSERViCEABLE.
A AL
-lx-
Nital Etch 160X
FIGURE 68 TRANSVERSE SECTION AT 1 1/2 INCHES FROM THECHAMBER OF A CHROME-PLATED Hl1 STEEL GUN BARREL AFTERFIRING 24,466 ROUNDS AND BEING DECLARED UNSERVICEABLE.
108
;~~e -. i ,-!
N¶
Nital Etch 160XFIGURE 69 TRANSVERSE SECTION AT 2 INCHES FROM THE
CHAMER OF A CHROME-PLATED 111 STEEL GUN BARREL AFTERFIRING 24,466 ROUNDS AND BEING DECLARED UNSERVICEABLE.
-- v--
Nital Etch 160X
FIGURE 70 TRANSVERSE SECT ION AT 10 INCHES FROM THECHAMBER OF A CHROME-PLATED Hli STEEL GUN BARREL AFTERFIRING 24,466 ROUNDS AND BEING DECLARED UNSERVICEABLE.
109
aI
-A Y
Nital Etch 160X
FIGURE 71 TRANSVERSE SECTION AT 17 1/2 INCHES FROM THEi CHIAMBER OF A CHROME-T-.ATED H1-11 STEEL, GUN BARRELT AFTERFIRING 24,466 ROUNDS AND BEING DECLARED UNSERVICEABLE.
S
Nital Etch 160X
FIGURE 72 TRANSVERSE SECTION AT 19 1/2 INCHES FROM THE
CHAMBER OF A CHRONE-PLATED 1111 STEEL GUN BARREL AFTERFIRING 24,466 ROUNDS AND BEING DECLARED UNSERVICEABLE.
110
F
rounds fired, the complete removal of chrome plate was lim-ited to the 1/2 inch position. In the barrel which fired16,291 rounds, the size and depth of cracks were quite small.In the barrel which fired 24,466 rounds, the cracking wasmore extensive but still small, in comparison to the standardbarrel. The failure of the barrels is probably due to ero-sion just forward of the breech; however, in comparison tothe standard barrels the greater tenacity of the chrome plateto the HIl steel, the better support of the chrome plating,and the better toughw-'s of the HIl steel lead to a longertest life.
Chrome-Plated CG-27 Alloy Barrels - Transverse metal-lurgical samples were to be prepared tor the chrome-platedCG-27 alloy barrels on 1/2, 1 1/2, 2, 10, 17 1/2 and 19 1/2inch positions from the chamber. However, one of the CG-27barrels failed by cracking at the muzzle end, and on thisparticular barrel the transverse specimen was taken at 18inches from the chamber. Figures 73 through 78 show typicalareas near the bore surface.
Figures 73 through 78 pertain to the barrel which failed..y cracking ne o -te muzzle after firing 4-036 rounds. The
bore area at 1/2 inch from the chamber is shown in Figure 73.Cracking in the chrome plate is present, and associated cracksare also present in the CG-27 alloy. The integrity of thechrome plate is quite good.
Figure 74 shows the microstructure 1 1/2 inches fromthe chamber. The chrome plate is cracked, and some spallinghas occurred.
At the 2 inch site, the chrome plate CG-27 alloy asshown in Figure 75 appears quite similar to that at the 1/2inch position.
At 10 inches from the chamber, only small amounts ofthe chrome plate remain on the bore surface. As can be seenin Figure 76, erosion of the CG-27 alloy is present and ascalloping of the surface has occurred.
The structure of the bore at the 17 1/2 inch and 18inch positions from the chamber is shown in Figures 77 and78. In both positions it appears that extensive materialhas been removed from the bore surface. This can be attrib-uted to a gas-metal reaction in which the nickel has probablycombined with sulfur in the propellant gases to form a low-melting alloy which has washed away.
iii1
Acid Ferric Chloride Etch 160X
FIGURE 73 TRANSVERSE SECTION AT 1/2 INCH FROM THECHAMBER OF A CHROME-PLATED CG-27 ALLOY BARREL AFTERFIRING 4,036 RoUNDs AND BEING DECLARED UNSERVICEABLE.
Acid Ferric Chloride Etch 160X
FIGURE 74 TRANSVERSE SECTION AT 1 1/2 INCHES FROM THECHAMBER OF A CHROME-PLATED CG-27 ALLOY BARREL AFTER
FIRING 4,036 ROUNDS AND BEING DECLARED UNSERVICEABLE.
112
!A
Acid Ferric Chloride Etch 16 OX
FIGURE 75 TRANSVERSE SECTION AT 2 INCHES FROM THECHAMBER OF A CHROME-PLATED CC-27 ALLOY BARREL AFTERFIRING 4,036 ROUNDS AND BEING DECLARED UNSERVICEABLE.
I
~ I.
Acid Ferric Chloride Etch 160X
FIGURE 76 TRANSVERSE SECTION AT 10 INCHES FROM THECHAMBER OF A CHROME-PLATED CG-27 ALLOY BARREL AFTER[ FIRING 4,036 ROUNDS AND BEING DECLARED UNSERVICEABLE.
113
4pi
k Acd FericChloideEtch160
Acid Fexli Chloide Etch 10
FIGURE 77 TRANSVERSE SECTION AT 178/ INCHES FROM THEI ~CHAMIBER OF A CHROME-PLATED CG-27 ALLOY BARRE AFTERFIRING 4, 036 ROUTNDS AND REINC DECLARED UNSERVICEABLE,114
1
IFigures 79 through 84 are photomicrographs near the
bore surface of the second chrome-plated CG-27 alloy barrelafter firing 7,324 rounds.
The structure at the 1/2 inch position is shown in Fig-ure 79 and reveals that portions of the chrome plate havebeen removed, cracking in both the chrome plate and theCG-27 alloy has occurred, and a precipitate having a Widman-statten pattern is present. The precipitate is probablyformed by absorption of interstitial elements such as nitro-gen, carbon, hydrogen, or oxygen from the gas stream.
The structures at the 1 1/2 and 2 inch sites from thechamber are shown in Figures 80 and 81. Similar conrientsto those made for Figure 79 apply.
Figure 82 shows the bore surface of the barrel at the10 inch position. Portions of the chrome plate have beenremoved, and intergranular attack has occurrcd.
The structure of the bore surface at the 17 1/2 inchposition from the chamber is revealed in Pigure 83. Massiveerosion has occurred at this site.
Figure 84 shows the transverse section at the 19 1/2inch position. The structure present is simt1l'r to thatobserved at the 2 inch position, Figure 81. Figure 85 re-veals the structure of the precipitate having the Widraan-statten morphology.
The failure mechanism in the chrome-plated CG-27 alloybarrels can be directly attributed to the particular prop-erties of the chrome plate and the CG-27 alloy. The ousetof failure is brougbt on by the brittle behavior of thechrome plate which cracks and induces cracking ir the CG-27s ub stCr ate. Orce the CC- 27s ex posc -e-d t o then h ot propel lancgases, a gas-metal reaction, probably nickel pl.us sulftr,occurs; the barrel becomes 'hot short and barr'•1 materialis washed away. The result at approximvtely 11 fuches fromthe chamber is due to the thermodynamics of the gab streamand the reduction in barrel thickness at that point.
The amount of heat transferred to the barrel from thegas stream is very small compared to the energy in cxpond-ing hot gases. Therefore, along the bore surface the ten.-perature is probably reasonably constant and the barrelheats rather uniformly along the length. However, sincethe last 9 inches of the barrel has a substantial reductionin wall thickness, heating is probably more severe in thisregion and hence the greater gas-metal reaction.
115
Acid Ferric Chloride Etch 160X
FIGURE 79 TRANSVERSE SECTION AT 1/2 INCH FROM THECHAMBER OF A CHROME-PLATED CC-2:/ ALLOY BARREL AFTERFIRING 7,324 ROUNDS AND BEING DECLARED UNSERVICEABLE.
SA.,
Acid Ferric Chloride Etch 160X
FIGURE 80 TRANSVERSE SECTION AT 1 1 !' INCHES FROM THECHAMBER OF A CHROME-.PLATED CC-27 AL.i.C,1Y .'ARRETL AFTER
,IRINC 7,324 ROUNDS AND BEINC DECLAR-'IED UNSERVICEABLE.
116
I
1 4
Acid Ferric Chloride Etch 160XFIGURE 81 TRANSVERSE SECTION AT 2 INCHES FROM THEuftAbi•fER obF, A CHROME-PLATED CG-27 ALLOY BARREL AFTERFIRING 7,324 ROUNDS AND BEING DECLARED UNSERVICEABLE.
Acid Ferric Chloride Etch 160X
FIGURE 82 TRANSVERSE SECTION AT 10 INCHES FROM THECHAMBER OF A CHROME-PLATED CG-27 ALLOY BARREEL AFTERFIRING 7,324 ROUNDS AND BEING DECLARED UNSERVICEABLE.
117 1
I
Acid Ferric Chloride Etch 16 OX
FIGURE 83 TRANSVERSE £ECTION AT 1.7 1/2 INCHES FROM 7HEflt, A4fl.r, ir AI t-% CH........ L A mr) CnG -n27 ALLO\" BARREL A FTER,
FTRING 7,324 ROUNDS AND BEING DECLARED UNSERVICEABLE.
I ff
Acid Ferrisc Chloride Etch 160X
FIGURE 24 TFRANSVERSE SECTION AT 19 1/2 INCHES FROM THECHAMBER OF A CHROME-PLAIED CGC-7 ALLOY BARREL AFTERFIf.ING 7,324 ROUNDS AND BEING DECLARED UNSERVICEABLE.
118
1119
I Ij
•. ./ o
Acid Ferric Chloride Etch 400X
FIGURE 85 TYPICAL STRUCTURE SHOWING THE MORPHOLOGY
I.
119 ia
To verify that an increase in sulfur concentration wasoccurring along the exposed CG-27 bore surface, chemicalanalysis utilizing X-ray spectrometry was made on the 10 inchtransverse sample at a site near the bore surface and at thesite midway between the bore surface and the outer diameterof the barrel. Figures 86 and 87 shows the spectrometerreadout for the site near the bore surface and away from thebore surface. The sulfur Ka and the Mo La characteristicradiation occur at approximately the same energy level;consequently, the spectrometer analysis causes a reinforce-ment at approximately the same energy level; however, theS Ka and Mo Lc peak near the bore surface is higher thanthat away from the bore surface, indicating a buildup ofsulfur near the bore surface. To verify this observation,a digital readout of the S Kand Mo La peak nd the Mo Kpeak were made for the two sites of analysis. For the siienear the bore surface the peak height for the S K, and Mo Laline was 1683 counts, and the background radiation levelwas about 750 counts; this yields a peak height to backgroundof 833 counts. For the Mo Ka lines the peak height was 86counts and the background was approximately 75; this givesa count difference of peak to background of 11. The ratioOU tlhe S tL LLkr , La peak LU LhL ru peak iso.3 to
11 or about 75 to 1.At the site away from the bore surface, the combined
peak for the S Kw and the Mo La was 1.034 counts, and thebackground was about 530 counts or a difference of 504counts; whereas for the Mo K, peak the height was 163 countsand che background was approximately 80 counts, or a differ-ence of 83 counts. The ratio of the S Ka and Mo La peakheight to the M4o Ka peak height is 504 to 83 or about 6 to 1.
Since the site away from the bore surface representsthe base material, the ratio of 6 to 1 for the S Ka andMeoLa peak to the Mo K. peak typifies the proportion of Sto Mo in the base material. The determination near the boresurface shows a ratio of 75 to 1 for the L.'o peaks. Sinceit can be assumed that the ratio of the Mo La peak (alone)to the Mo Ka. peak should be constant, it can be concludedthat the increase in the ratio from 6:1 to 75:1 is due toa buildup of sulfuL near the bore sutface, and the hypothesisthat the CG-27 alloy is failing by a sulfur reaction isverified.
SUIrZIARY AND CONCLUSIONS
A multiphase program has been conducted with the ob-jective of determining materials which would be capable of
1.20
Al Ka Ti K.
Fe KN' K
(j 1000
S100
Nil K MK
CrK act
andMo L
FIGURE 86 X-RAY PATTERN REVEALING CHARACTERISTICRADIATION OF ELEMENTS AND THEIR RELATIVE CONCENTRATION
NEAR THE BORE SURFACE OF A CG-27 ALLOY BARREL.
121
Al K
Ti 1K,Fe K
10,000
om 1000o
100 ____ _
10 - _ _
NK13 Mo K cCr K '
S K
and
Mo L
FIGURE 87 X-RAY PATTERN REVEALING CHARACTERISTICRADIATION OF ELEMENMS AND THEIR RELATIVE CONCENTRATION
IN THE BASE MATERIAL OF A CG-27 ALLOY BARREL.
122
providing extended life and improved performance when em-ployed as 7.62mm Minigun barrels. The specific phases were:
I. Characterization of erosion/corrosionin the 7.62mm standard steel barrel,A literature review of the generalsubject of gun barrel erosion in high-performance small caliber weapons.
II. Materials evaluation, involving ventedbomb tests, thermal fatigue, hot hard-ness, and other mechanical propertiesof fifteen nmatrials and correlationwith gun tests of some alloys.
III. The selection of three materials fromthose evaluated in Phase II for evalu-ation in gun tests.
IV. Fabrication and gun testing of barrelsto failure.
V. Post analys-e-s- . .f barrels after
being declared unserviceable.
In Phase I, to determine the mode of failure of stand-ard Minigun barrels, a controlled firing schedule was ap-plied to both plated and unplated barrels to establish theonset of fai]ure. Metallographic examination and X-rayspectrographic analyses were performed on sections from theseveral barrels. In the chrome-plated barrels, it can bepostulated that relatively large cracks are developed inthe chrome plate after one shot. These cracks reach thesteel substrate in less than 300 rounds and, because of themechanical coupling, propagate into the steel. Further fir-ing causes crack growth in the steel substrate; as the cracksmeet, a portion of the chrome plate and steel flake off. In-variably associated with the cracks is an envelope of thewell observed "white layer." Evidence indicates that thewhite layer is produced after cracking, and spectographicanalyses shows that it is deficient in iron and chromium andprobably contains light elements. Additionally, copper wasfrequently observed within the cracks. In the unplated bar-rels, very little cracking was observed and the prime modeof fpilure appeared to be washing away of metal.
In the literature review it was ditficult to make com-parisons between various materials, inserts, coatings, etc.,because of the wide range of propellants, firing schedules,
123
calibers of weapons, velocities, and material conditionswhich were reported. However, some general trends were dis-cerned. Steels or special iron-base alloys have not shownoutstanding promise as bore surface materials. Chrome plat-ing these materials markedly improves their erosion resistance.Certain cobalt alloys have shown satisfactory performanceas liners in guns when single-base propellants were employed.It appears that molybdenum or tungsten base materials withoptimized mechanical properties would offer an improvementover cobalt base alloys in the environment created by adouble-base propellant, However, the cost and fabricationdifficulties associated with these refractory metals couldpresent a problem.
In Phase II, a comparison was made of A286, CG-27, X-15,iron-base alloys, high temperature alloys, 22-4-9 austeniticvalve alloy, and plated and unplated Cr-Mo-V gun steel ingun tests and vented bomb tests. The gun tests, employinga particular firing schedule, showed that the CG-27 barrelhad approximately a 25 percent longer life than the otherrmaterials in the unplated condition, but the chrome-platedgun steel barrel had more than twice the life of the unplatedCG-27 barrel. The vented bomb tests with WC846 propellantranked the erosion resistance of the materials (['n decr..asinigorder) as follows: chrome-plated gun steel, X-15 alloy, un-plated gun steel, 22-4-9 alloy, A286 alloy and CG-27 alloy.Hot hardness and thermal fatigue tests were also conducted.The hot hardness at 1600°F, in decreasing order, is CG-27alloy, 22-4-9 valve steel, X-15 alloy, A286 alloy, and Cr-Mo-V steel. The thermal fatigue tests ranked the materials,in descending order, as CG-27 alloy, A286 alloy, X-15 alloyand 22-4-9 valve steel. In gun tests with WC846 propellantthe ranking was determined to be chrome-plated gun steel andunplated CG-27 alloy, A286 alloy, X-15 alloy and 22-4-9 valvesteel.
Examinations of inserts and barrels after firing showedthat a correlation in the mode of failure in gun tests couldbe predicted by the vented bomb. The vented bomb erosiontests showed that CG-27 alloy will fail by reaction with thepropellant gas stream if the firing sequence and barrel di-mensions are such that sufficient heat is generated to allowthe nickel in the CG-27 to react with the sulfur in the gasstream. Sulfur attack of nickel at hi-gh temperatures (above1300°F) is exothermic and is predominantly intergranular innature. Once attack begins, the metal lattice is opened asthe sulfur penetrates the alloy and a low-melting eutectlc(aboutl70F) sulfide is formed. The reaction of nickel al-loys is somewhat similar; however, it is known that the
124
increasing chromium content does somewhat retard the sulfurattack. The other alloys will fail by either cracking orflattening of the lands if the conditions of the firing testare such that the barrels attain high temperature.
Using the correlation study as a basis, a variety ofsteel and superalloy- which could offer potential. as gunbarrels were selected:
i. A286, an iron-nickel-chromium-
molybdenum alloy.
2. 22Cr-4Ni-9Mn, an austenitic valve alloy3. CG-27, an iron-base superalloy
4. X-15, an iron-base alloy
5. Cr-Mo-V steel, the current gun steel
6. Chromium-plated gun steel
7. Haynes Stellite 21, a cobalt alloy
8. 50Co-29Fe-20W-lC, a cobalt alloy
9. HI1, a chromium hot work tool steel
10. D2, a high-carbon, high-chromiumcold work steel
11. H26, a tungsten hot work tool steel
12. 446, a ferritic stainless steel
13. L605, a cobalt superalloy
14. HS-31, a cobalt-chromium-nickel.-iron superalloy
15. S-590, a cobalt superalloy
All materials were tested in the unplated condition exceptfor the gun steel, which was evaluated plated and unplated.
"lAetallographic examination of the fired vented bombinserts revealed that:
1. The cobalt-base superalloys tend tofail by cracking.
2. The austenitic valve alloys, typifiedby 22-4-9 alloy, exhibit brittle be-havior in the vented bomb.
125
I3. With the exception of D2 (a high-carbon, high-
chromium cold work) steel and 446 stainlesssteel, steel alloys do not fail by cracking.
4. Chrome plate on steel produces crack starterswhich propagate into the steel substrate.
5. The SOCo-29Fe-20W-lC alloy and the CG-27 alloydo not appear to fail by cracking in the ventedbomb.
In Phase III, materials selection, the SOCo-29Fe-20W-lC
alloy, chrome-plated CG-27 alloy, and chrome-plated HI1 alloywere selected for evaluation in gun tests because of theirmechanical properties and economic considerations.
In Phase IV, barrels of CG-27 and HI1 were fabricatedutilizing improved hot extrusion and cold swaging methods. TheSOCo-29Fe-20W-lC alloy barrel could not be fabricated by thistechnology. The hot extrusion process reduced a 4-inch roundbillet to a 1.35 inch diameter rod c rntaining a mild steelsacrificial mandrel. After extrusion, straightening and cuttingoperations were conducted to obtain true rods. The sacrificialmild steel core was removed either by machining or acid di-s-solving to produce a barrel blank. The bore surface was honedto prepare a barrel blank suitable for rifling. Rifling wasperformed by swaging the barrel blank over a tungsten carbidemandrel to produce a premium-quality finished rifled bore sur-face. The dimensions of the bore diameter were slightly over-size to allow for chrome plating. The swaging operation toproduce a rifled barrel required approximately 40 seconds.Chrome-plated barrels of each of the two candidate materialswere fired with chrome-plated standard steel barrels in com-plement in a six-barrel Minigun weapon. The chrome-platedCG-27 alloy barrels were declared unserviceable in the firingtests atv03, 7200 0, and 7324 rounds, the standard barrel at11,283, 11,605, and 12,068 rounds, and the chrome-plated HI1steel barrels after 10,291, 24,466, and 24,583 rounds. Thuson this particular firing schedule the chrome-plated HIl steel
barrel showed an improvement of approximately 33 to 100 percentthe service life over the standard gun barrel, whereas thechrome-plated CG-27 barrels were inferior to the standardbarrel.
Phase V, post analyses, revealed that the chrome-platedCG-27 alloy barrel failed by massive erosion which wasprobably induced by a high-temperature nickel/sulfur reactionof the CG-27 alJoy. This behavior was indicated by the ventedbomb tests; the chrome plate should prevent it from occurring.
126
W-M-IIThe chrome-plated standard barrel failed in the classicmanner. The outstanding performance of the chrome-platedH81 alloy was due to the high tenacity of the chromiumplate to the Hll, the hot strength and temper resistance ofthe H11, and the toughness of the Hll steel when heat treatedto medium hardness levels for hot-work die steels. Ad-ditionally, examination of the failed barrels indicated thatan excellent metallurgical product was produced by the manu-facturing technology employed. The quality of the product 2so produced contributed significantly to the excellent per-formance of the chrome plate H11 barrels.
RECOMMENDATIONS
Chrome plating is good on a suitable substrate--meaningone which produces high tenacity and has a similar coefficientof thermal expansion, high hot strength, and good toughness.
Cost-effective ferrous alloys appear to offer potentialfor improved performance with firing schedules used in this
work. Hot hardness - high strength steels containing ap-preciable chromium and carbon, preferably with low nickelcontent, would be appropriate. Typical of the family arefill, H13, H23, and H26 steels.
The SOCo-29Fe-20W-lC type alloys could offer good po-
tential as barrel materials in the chrome-plated or unplatedcondition. However, the carbon content should be adjustedbelow the one percent employed in this study to approximatelythe 0.S percent region. This should give increased toughness
and a more fabricable alloy without the loss of other es-sential properties.
The utility of the hot extrusion/ccld swaging technologyfor the manufacture of 7.62mm barrels has been demonstrated.if cost analysis shocws thi4s production schemue to be viablethen the hot extrusion/cold swaging production method forthe manufacture of 7.62mm barrels should be adopted.
127
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REFERENCES
1. "Barrel (D-7161814) for Gun, Machine, Browning, Cal .50,M2, Heavy Barrel-Recommended for Adoption," OrdnanceCommittee, Minutes 27817, OCD, (Approved) May 1945.
2. H. N. Smith, "The Behavior of Gun Liners and CoatingsTested under Conditions of Hyperv'-locity," OSRD 6475,Rep. No. A-404, Franklin Institute, Oct. 1945.
3. "Investigation of Gun Erosion at the Geophysical Lab-oratory, Vol.Ill, Jan. 1944-July 1944, OSRD 4354, Rep.No. A-300, Geophysical Laboratory, C/W, Nov, 1944.
4. E. r. Osborn, "The Testing of Erosion-Resistant Mate-rials and the Development of Improved Machine GunBarrels," OSRD 6480, Rep. No. A-409, Geophysical Lab-oratory, CIW, Nov. 1945.
5. T. H. Gray, "Refractalloy 70 as a Liner Material forCal .50 Barrels," OSRD 6491, Rep. No. A-420, Wes.ngh.ouseElectric and Mfg. Co., Inc., Jan. 1946.
6. J. Wulff, "Erosion Tests of Materials in the Form ofShort Liners in a Cal .30 Machine Gun Barrel," OSRD6477, Rep. No. A-406, Johnson Automatics Inc., April1944.
7. "Investigation of Gun Erosion at the Geophysical Lab-oratory, Vol. 1, July 1941 to July 1943, OSRD 3448,Rep. No. A263, Geophysical Laboratory, C/W, March 1944.
8. R. M. Parke and F. P. Bons, "Development of Chromium-Base Hot-Hard Alloys as Gun Liner Materials," OSRD 6486,Rep. No. A-415, Climax Molybdenum Co., Jan. 1946.
9. P. H. Brace and J. W. Marden, "Preliminary Report onMolybdenum as a Material for an Erosion.-Resistant GunLiner," OSRD 3700, Rep. No. A-233, Westinghouse Electricand Mfg. Co., Inc., May 1944.
10. J. W. Marden, "Fabrication of Molybdenum for Use as aGun Liner Material," OSRD 6494, Final Rep. No. A-423,Westinghouse Electric and Mfg. Co., Inc., Oct. 1945.
11. P. H. Brace, "Development of Molybdenum for Gun Liners,"OSRD 6495, Rep. No. A-425, Westinghouse Electric andMfg. Co., Inc., Feb. 1946.
128
REFERENCES (Continued)
12. S. Breitbart, "On the Erosion of Metallic Nozzles byPowder Gases," BRL Memorandum Rep. No. 533, AberdeenProving Ground, Md., April 1951.
13. "Improved Fabrication Technology for 7.62mm Gun Barrelsfor High Strength Iron, Nickel, and Cobalt Base Alloys,"BNPG-1830-563, Battelle-Northwest, Richland, Wash.,Dec. 29, 1971.
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