Surfacing of 304 Stainless Steel for Liquid Sodium Service

BY R. A. DOUTY A N D H. SCHWARTZBART

The properties of type 304 stainless steel surfaced with A WS ECoCr-A type alloys for use in valves operating in mo/ten sodium at 1200F were evaluated. Specimens subjected to thermal shock cycling to evaluate the ef­fects of surfacing process, dilution and deposit thick­ness were examined by metallography, hardness tests, dye penetrant inspection and electron microprobe anal­yses

INTRODUCTION

The effort to develop, design and build a sodium cooled fast breeder power reactor necessitates the solu­t ion of a number of difficult engineer­ing problems. One such problem involves the development of valves for such a reactor. This paper pre­sents the results of a study of surfac­ing parameters relative to the antici­pated performance of valves made of

The authors are associated with the Rockwell Manufacturing Company, Pittsburgh, Pa. This work was supported in part by the Atomic Ener­gy Commission. Microprobe studies were per­formed by Battelle Memorial Institute.

304 stainless steel in molten sodium service at 1200F.

The properties of sodium which make it an excellent heat transfer medium also make it a diff icult f luid to handle at high temperatures. The low Prandtl number which results from these properties means there wi l l be litt le thermal resistance in the liquid metal f i lm and that the wal l temperature of metals exposed to or immersed in the liquid sodium wi l l be also equal to that of the bulk liquid. Also, during rapid temperature changes, the total temperature differ­ence and resulting thermal stresses wi l l have to be accommodated almost entirely w i th in the metal. In systems using liquid sodium as the

heat t ransfer m e d i u m , h ighe r thermal stresses are generated than in systems using non-metall ic fluids such as water. ' /2

In addition to these stresses, the bimetallic combination of A W S ECoCr-A type alloys and AISI type 304 stainless steel w i l l also create stresses during temperature changes because of their differences in thermal expansion at the same temper­ature. Figures 1 and 2 show the co­efficient of thermal expansion3 of the surfacing alloy and 304 stainless steel from ambient temperature to 1200F. Assuming elastic behavior, the higher coefficient of thermal expan­sion of the stainless steel w i l l cause the surfacing alloy to be in tension

7

MEAN COEF LINEAR THERMAL EXPANSION

FROM RT TO TEMP INDICATED I I I

0 400 800 1200

TEMP - F

Fig. 1—Coefficient of thermal expansion of surfacing alloy

1600 2000

-400 0

Fig. 2—Coefficient of thermal expansion of 304 stain/ess steel

400 800

TEMP - F

1200 1600

406-s I AUGUST 1 9 7 2

during heating and in compression during cooling. The stresses in the stainless steel wi l l be opposite in sign to those in the overlay and the magnitude relat ionship w i l l be inversely proportional to the thick­ness ratio. Because of the greater thickness of the stainless steel, it is unlikely that the stresses would be­come great enough to cause plastic deformation; however, jt is likely that the stresses in the overlay could be­come high enough to cause the surfacing alloy to crack. Plastic deformation would be unlikely be­cause of the low ductil ity of the surfacing alloy. 4

Considerable differences of opin­ion appear to exist as to the thick­ness and method of applying the surfacing alloy for liquid sodium serv­ice. Phillips, et al 5, state that the al­loy facings should be kept very th in (max. 0.060 in.) to avoid cracking f rom differential expansion. How­ever, 0.060 in. alloy surfacing is extremely th in and would be difficult to control w i th most we ld ing processes. Final machining could also expose the base metal in some areas and destroy the integrity of the overlay. According to Thorel 6, high dilution techniques were used w i th success on valves for nuclear subma­rines. Using this approach, the f irst pass is deposited at higher than normal heat inputs to obtain high dilution. The remaining passes are then deposited at the recommended heat inputs to minimize dilution but to effect good fusion. The resulting overlay has a chemical gradient which in turn produces a gradient in mechanical properties and an over­lay which is more resistant to thermal shock.

Although the ECoCr-A type surfac­ing alloy has been used for many years as tr im on stainless steel valves for high temperature service, data relating to its behavior in liquid sodium up to 1200F are l imited. Before the advent of the liquid metal fast breeder reactors, there was little need for this type of information. To help alleviate this lack of data, the present program was undertaken to develop information on the ability of AWS ECoCr-A alloys on 304 stain­less steel to wi thstand the thermal stresses and shocks expected in a liq­uid sodium cooled reactor.

The primary objective of this in­vestigation was to evaluate the be­havior of the surfacing alloy on 304 stainless steel applied in two dif­ferent thicknesses and w i th two dif­ferent techniques. Two processes were also used: shielded metal-arc and plasma-arc (transferred arc). Fig­ure 3 shows a block diagram of the experimental program.

PRELIMINARY TESTS

THERMAL STRESS ANALYSIS

PREPARATION OF CYLINDERS

PLASMA PROCESS

SHIELDED METAL ARC PROCESS

Jl LOW DILUTION TECHNIQUE

"I

_C

HIGH DILUTION TECHNIQUE

T

I"

LOW DILUTION TECHNIQUE

1/8" THICK

1/16" THICK

-E 1/8" THICK

1/16" THICK

_c 1

HIGH DILUTION TECHNIQUE

1 1/8" THICK

I 1/16" THICK

STRESS RELIEVE

QUENCH TESTS

METALLURGICAL TESTS

METALLOGRAPHIC

Fig. 3—Block diagram of research program

'••••77':

Fig. 4—Plasma-arc surfaced test cylinder

Exper imenta l Procedure

Process Selection The two processes, shielded

metal-arc and plasma-arc, used in this investigation were selected for two reasons: (1) from a manufactur­ing point of view the plasma-arc process provided the most eco­nomical approach and the shielded metal-arc process provided access in areas where the plasma torch could not be used, and (2) f rom previous work it was known that one of the extremes of dilution could readily be obtained wi th each process. Whe th ­er the low dilution characteristics of the plasma-arc process and the high

Fig. 5—Shielded metal-arc surfaced test cylinder

dilution characteristics of the shield­ed metal-arc process could be successfully reversed for the overlay thicknesses of interest were un­known.

Filler Metal Selection

The basic composition of the fil ler metals used in this program was se­lected because of the past successful service of ECoCr-A type alloys in liquid sodium. Prior work 7 w i th the plasma-arc process and one particular powder (Linde No 156) served as a basis for selecting the particular powder. The electrode (Cobend No 6) selected for the shielded metal arc

W E L D I N G R E S E A R C H S U P P L E M E N T ! 407 -s

studies was relatively new and our experience w i th it was l imited; there­fore, some prel iminary testing was undertaken to insure this electrode would be suitable for this study. De­posited metal from 3 / 3 2 , Vs, and 5 / 3 2 in. diam electrodes was an­alyzed in accordance w i th AWS Specification A5.13-70, ECoCr-A and were found to be w i th in specification. Addit ional tests also verified that cracking would not be a

problem even in heavily diluted first passes.

Specimen Selection and Preparation

After the preliminary tests proved the feasibility of the high dilution technique, a simplif ied thermal stress analysis was performed to determine whether the proposed specimens would generate stresses of sufficient magni tude du r i ng

quenching. The analysis, wh ich as­sumed that the ECoCr-A alloy and the 304 stainless steel have identical material properties, indicated that the proposed tests would be similar in severity to the Fast Flux Test Fa­cility (FFTF) design conditions. The stresses generated in the Vi in. thick wal l of the specimens were cal­culated to be 51,000 to 69,000 psi, and the stresses for the worst condi­tions in the FFTF valves are expected

Table 1—Welding Parameters

T E S T D E S C R I P T I O N PROCESS

O V E R L A Y L O C A T I O N

F I L L E R M E T A L S P E C I F I C A T I O N

F I L L E R M E T A L S IZE

E Q U I P M E N T MAKE & T Y P E

TORCH T Y P E

E L E C T R O D E T Y P E & S IZE

E L E C T R O D E SET B A C K

T O R C H TO WORK D I S T A N C E

TORCH OR E L E C T R O D E L E A D

T O R C H O F F S E T

T O R C H GAS & FLOW R A T E

S H I E L D I N G GAS & FLOW R A T E

POWDER GAS & FLOW R A T E

POWDER FLOW RATE

T O R C H O S C I L L A T I O N A M P L I T U D E

T O R C H O S C I L L A T I O N D W E L L

TORCH O S C I L L A T I O N F R E Q U E N C Y

N A T U R E OF E L E C ­T R I C A L C U R R E N T

METHOD OF ARC I N I T I A T I O N

WELDING SPEED OR R.P.M.

R E C O R D E D AMPERES

RECORDED V O L T A G E

NUMBER OF OVER­LAY DEPOSITS

BASE M A T E R I A L P R E H E A T

BASE M A T E R I A L INTERPASS T E M P .

I

PPHD I /8 (a)

Plasma T . A .

O.D.,

N.A. (b)

- I 40 325 Mesh

L inde PSM Plasma

Heavy Duty PT-9

2% Thor ia ted Tungsten 5 / 3 2 " D ia .

5 / 3 2 " D ia .

I / 2 "

20°

I "

Argon 8 C . F . H . (c)

Argon 5 0 C . F . H .

Hel ium I 2 . 5 C . F . H .

3 6 . 5 G R . P . M . (d)

7 / 8 "

R-2 (e) L-2

4 0 C . P . M . (f)

D.C.S.P. (g)

High Frequency P i lo t Arc

.25- .3 R.P.M.

I 65-200

27-30

4

400°F

400°F

2

PPHD I /8 Plasma T . A .

Ends

N.A.

- I 4 0 325 Mesh

L inde PSM Plasma

Heavy Duty PT-9

2% Thor ia ted Tungsten 5 / 3 2 " D ia .

5 / 3 2 " D i a .

3 / 8 "

None

None

Argon 8 C . F . H .

Argon 5 0 C . F . H .

Hel ium I2 .5 C F . H .

21 GR.P .M .

3 / 8 "

R-2 L- l

65 C.P .M.

D.C.S.P.

High Frequency P i lo t Arc

.3 R.P.M.

I 00 - I 35

26-27

5

400°F

400°F

3

PPHD I / I 6 Plasma T . A .

O.D.

N.A.

- I 4 0 325 Mesh

L inde PSM Plasma

Heavy Duty PT-9

2% Thor ia ted Tungsten 5 / 3 2 " D ia .

5 / 3 2 " D ia .

I / 2 "

2 0 °

I "

Argon 8 C . F . H .

Argon 5 0 C . F . H .

He! ium I2.5 C F . H .

36.5 GR.P .M .

7 / 8 "

R-2 L-2

40 C.P .M.

D.C.S.P.

High Frequency P i lo t Arc

.25-.3 R.P.M.

I 65-200

27-30

3

400°F

400°F

4

PPHD I / I 6 Plasma T . A .

Ends

N.A.

- I 4 0 325 Mesh

L inde PSM Plasma

Heavy Duty PT-9

2% Thor ia ted Tungsten 5 / 3 2 " D i a .

5 / 3 2 " D ia .

3 / 8 "

None

None

Argon 8 C F . H .

Argon 5 0 C . F . H .

Hel ium I2 .5 C F . H .

21 GR.P .M.

3 / 8 "

R-2 L- l

65 C.P .M.

D.C.S.P.

High Frequency P i l o t Arc

.3 R.P.M.

I 00-1 35

26-27

4

400°F

400°F

5

P P L D I / I 6 Plasma T . A .

O.D.

N.A.

- I 4 0 325 Mesh

L inde PSM Plasma

Heavy Duty PT-9

2% Thor ia ted Tungs ten 5 / 3 2 " D ia .

5 / 3 2 " D ia .

I / 2 "

20°

I "

Argon 8 C . F . H .

Argon 5 0 C . F . H .

Hel ium I2 .5 C F . H .

36.5 G R . P . M .

7 / 8 "

R-2 L-2

40 C .P .M .

D.C.S.P.

High Frequency P i lo t Arc

.25-.3 R.P.M.

I 65

27

2

400°F

400°F

(a) Plasma T.A. = Transferred arc (b) N.A. = Not applicable (c) C.F.H.= Cubic feet per hour

(d) GR.P.M. = Grams per minute (e) R = Right dwe l l , L = left dwel l (f) C.P.M. = Cycles per minute

408-s I A U G U S T 1 9 7 2

to be 60,000 psi in 1.5 in. thick plate. Eight 6- in. long cylinders were

then sectioned from 0.5 in. wal l by 8 in. diam 304 stainless steel tubing and prepared for overlaying accord­ing to the plan in Fig. 3. Preparation included thorough cleaning to re­move all cutting oi l , grease and other foreign matter which would impair the quality of the weld overlay and the locating of reference marks to control the overlay thickness during

f inal machining. The reference marks were either punch marks or machined grooves located one inch from the f inished ends.

Following preheating to 400F, the cylinders were overlaid on both ends and around the OD wi th a M in. band midway between the ends as shown in Fig. 4 and 5. Four of the above tests were made wi th the shielded metal-arc process using the com­mercial electrode referenced above

for the fi l ler metal, and the other four tests were made w i th the plas­ma-arc (transferred arc) process and the commercial powder referenced above. The 400F preheat tempera­ture was also maintained throughout the welding as an interpass tempera­ture. Upon completion of the we ld­ing, the cylinders were placed in an insulating compound, cooled slowly to room temperature, then inspected by the dye penetrant method. Pro-

Table 1—Welding Parameters 6

PPLD I / I6 Plasma T.A.

Ends

N.A.

-I40 325 Mesh

Linde PSM Plasma Heavy Duty PT-9

2% Thoriated Tungsten 5 /32" Dia.

5 /32" Dia.

3 / 8 "

None

None

Argon 8C.F.H.

Argon 50C.F.H.

Hel ium I2.5 C F . H .

21 GR.P.M.

3 / 8 "

R-2 L-l 65 C.P.M.

D.C.S.P.

High Frequency Pilot Arc

.3 R.P.M.

I00

27

3

600°F

600°F

7

PPLD I/8 Plasma T.A.

O.D.

N.A.

- I40 325 Mesh

Linde PSM Plasma Heavy Duty PT-9

2% Thoriated Tungsten 5 /32" Dia.

5 /32" Dia.

I / 2 "

20°

I "

Argon 8 C F . H .

Argon 50C.F.H.

Hel ium I2.5 C F . H .

36.5 GR.P.M.

7 / 8 "

R-2 L-2

40 C.P.M.

D.CS.P.

High Frequency Pilot Arc

.25-.3

I65

27

3

400°F

400°F

8

PPLD I/8 Plasma T.A.

Ends

N.A.

- I40 325 Mesh

Linde PSM Plasma Heavy Duty PT-9

2% Thoriated Tungsten 5 /32" Dia.

5 /32" Dia.

3 / 8 "

None

None Argon 8C.F.H.

Argon 50C.F.H.

Hel ium I2.5 C F . H .

21 GR.P.M.

3 / 8 "

R-2 L-l 65 C.P.M.

D.C.S.P.

High Frequency Pilot Arc

.3

IOO

27

3

600°F

600°F

9

SMAHD I/8 Shielded Metal Arc

O.D. Ends AWS E-COCR-A

I / 8 " Dia.

Hobart M-300

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

D.C.R.P. (h)

Scratch

2.5 I.P.M. (i

90-120

I 8-25

3

400°F

400°F

IO

SMALD I/8 Shielded Metal Arc

O.D. Ends AWS E-COCR-A

3 /32 " Dia.

Hobart M-300

N.A.

N.A.

N.A.

N.A,

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

D.C.R.P.

Scratch

1.2 I.P.M.

65-70

I 7-20

3

400°F

400°F

I I

SMALD I / I 6 Shielded Metal Arc

O.D. Ends

AWS E-COCR-A

I / 8 " Dia.

Hobart M-300

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

D.C.R.P.

Scratch

2.5 I.P.M.

90-140

I 8-25

3

400°F

400°F

12

SMALD I / I6 Shielded Metal Arc

O.D. Ends AWS E-COCR-A

3 / 3 2 " Dia.

Hobart M-300

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

N.A.

D.C.R.P.

Scratch

1.2 I.P.M.

65-70

I 7-20

3

400°F

400°F

(g) D.C.S.P. = Direct current straight polarity (h) D.C.R.P. = Direct current reverse polarity (i) I.P.M. = Inches per minute

W E L D I N G R E S E A R C H S U P P L E M E N T ! 409 -s

cedural details for the above welds are given in Columns 1 through 12 in Table 1.

After the cylinders had been dye-penetrant inspected, they were stress relieved at 600F for 72 hrs. This thermal cycle was selected to avoid precipitating any carbon beyond that which was precipitated during welding. The stress relieving

Table 2 — Specimen

Specimen code

PPLD Va PPLD 1/16 PPHD Va PPHD 1 /16 SMALD Va SMALD 1/16 SMAHD Va SMAHD 1 /16

Identification Code

Process

Plasma Plasma Plasma Plasma Shielded Metal Arc Shielded Metal Arc Shielded Metal Arc Shielded Meta l Arc

Dilutio

Low Low High High Low Low High High

Overlay Thickness

Va 1 / 1 6

Va 1 / 1 6

Va 1 / 1 6

Va 1 /16

Fig. 6—Shielded metal-arc surfacing on OD after final machining

Fig. 7—Shielded metal-arc surfacing on end after final machining

Fig. 8—Plasma-arc surfacing on OD af­ter final machining

Fig. 9—Plasma-arc surfacing on end after final machining

was fol lowed by another dye-pene­trant inspection to insure that crack­ing had not occurred during this operation.

The f inal specimen preparation step was the machining of the over­lays. Following the plan in Fig. 3, four of the specimens were ma­chined to give a f inished overlay thickness of Va in. and the other four were machined to give a f inished overlay thickness of 1/16 in. Control of the thickness was achieved by using the previously placed ref­erence marks for the end deposits

,b

trsy • /

Fig. 10—Microstructure of '/s in. low dilu­tion plasma-arc end surfacing: (a) alloy surface, (b) interface location, (c) stain­less steel metal base. 100X Marble's Re­agent.

and by measuring the diameter of the circumferential deposits. Figures 6 through 9 show examples of the machined deposits. The specimens were identified as shown in Table 2.

Quench Tests Each of the eight cylinders was

quenched three times from 1200F to 600F, 400F, and 200F for a total of nine quenches per sample. The 600F and 400F quenches were into a l iq­uid salt bath (Houghton's Draw-Temp 275 nitrate salt) and the 200F quench was into an oil bath. Heating

- -•,' \ 7 / ' . ••• 77*'

I

i

c

Fig. 11—Microstructure of '/a in. low dilu­tion shielded metal-arc end surfacing: (a) alloy surface, (b) interface location, (c) stain/ess steel base metal. 100X. Marble's Reagent.

410-s I A U G U S T 19 7 2

Fig. 12—Surfacing base metal interface of PPLD Va sample show­ing grain boundary constituents. 300X, reduced 10%: etchant. Marble's Reagent

Fig. 13—Base metal of PPLD Va sample showing stringers of un­identified exogenous material, 300X, Marble's Reagent

of the samples was done in a lab­oratory type electric furnace wi thout a protective atmosphere. After each quench, the cylinders were cooled to room temperature, cleaned, and dye-penetrant inspected for cracks.

Metallurgical Tests To evaluate the different overlays,

each cylinder was subjected to a complete metallurgical examination which included metallographic sec­tions to check the microstructure and integrity of the bond, chemical analysis of the overlay, electron microprobe scans across the overlay, and microhardness traverses which coincided w i th the electron micro­probe traverses.

Metallographic Examination The metallographic specimens

were removed at random from the test cylinders in a manner wh ich pro­duced transverse sections of the overlays for examination. To avoid burning the specimens, all section­ing was done under water w i th abra­

sive cutoff wheels. Subsequently, the samples were prepared for examina­t ion. For structural examinations, the spec imens w e r e e tched w i t h Marble's reagent. After the entire surface had been surveyed, the speci­mens were photographed in three typical areas: the base metal, the bond line, and the overlay.

Chemical Analysis To determine the overall composi­

t ion of the overlays, the alloy surface was removed from the stainless steel base metal w i th an abrasive cutoff wheel . After sectioning, the samples were etched to insure that the samples did not have any base metal on the underside of the over­lay. The analyses were performed by crushing the overlays and using X-ray fluorescence and carbon analysis techniques.

Electron Microprobe and Microhardness Examinations8

For the electron microprobe and the microhardness examinations,

additional transverse samples of the end overlays were prepared by the same procedure described above under "Metal lographic Examina­t ion . " In the as-polished condit ion, these samples were analyzed for iron, cobalt, nickel, and chromium from the machined surfaces of the overlays into the 304 stainless steel. These t raverses w e r e located approximately at midwal l to avoid the excessive melting and subse­quent dilution which occurred at the OD and ID of the cylinders. The exact locations are described in the presentation of the results. For the microhardness t raverses, w h i c h were coincident wi th the microprobe traverses, the above samples were given a light etch w i th 20ml HCI, 20ml H 2 0 , 10ml H N 0 3 , 0.5g FeCI3. The hardness measurements were made using a Knoop indenter w i th a 100 g load and were spaced 10 mils (0.01 in.) apart. The hardness measurements also started at the machined surface and extended into the 304 stainless steel to a point where the readings became uniform.

Table 3—Chemical Compositions of Quench Test Surfacings

PPLD PPLD PPHD PPHD SMALD SMALD SMAHD SMAHD Element, % Carbon Sil icon Molybdenum Tungsten Nickel Iron Manganese Chromium Cobalt

Va 1.40 1.23 0.023 3.02 0.02 4.60 0.35

29.85 59.91

1 /16 1.20 0.80 0.025 3.00 0.02 5.99 0.36

29.95 58.85

Va 1.36 0.86 0.030 2.77 0.01 5.90 0.40

29.35 59.67

1 /16 1.23 0.75 0.045 2.74 0.36

10.29 0.43

29.18 55.20

Va 0.90 0.35 0.017 4.72 0.01 6.65 0.91

29.05 48.15

1 / 1 6 0.63 0.41 0.16 3.56 1.74

25.24 0.97

27.19 39.73

Va 0.62 0.60 0.047 4.22 0.01 7.49 0.96

29.22 56.45

1 / 1 6 0.56 0.47 0.081 4.42 0.58

14.90 0.95

29.45 48.15

ECoCr-A* 0.7-1.4 0.4-2.0 1.0 3.0-6.0 3.0 5.0 2.0 25.0-32.0 Balance

•Single values indicate maximum percentages

W E L D I N G R E S E A R C H S U P P L E M E N T ! 4 1 1 - s

is , 1

Fig. 14 —As-deposited crack in overlay on sample PPLD Va which has penetrated into base metal. 300X, Marble's Reagent

Fig. 15—As-deposited crack in overlay on sample SMAHD Vs. 100X, Marble's Reagent

Results and Discussion Dye Penetrant Examination

Of the sixteen end deposits ex­amined after welding, but before quenching, two showed indications of one f ine transverse crack. The center deposits were all crack free.

Obviously, these cracks could not be examined metallographically w i t h ­out destroying the spec imens ;

however, two other alternatives were available. Either the cracks could be repaired, or they could be identified and carefully observed dur­ing the quench tests. It was decided to forego the development of a repair procedure at this t ime and to observe the behavior of the cracks. So that the cracks could be readily identified and differentiated from any which

o

19

1 '

17

l f i

tc;

1 1

1a

P

11

in

n

9

7

f.

c.

A

'i

\

\ \ \ \

(

1

\ \ \ \

\ \

I)

~s

V

\ \

\ \

(7) \ y^ (i

«>

5)

100 200 500 600 300 400

HARDNESS, KHN

Fig. 16—Hardness vs. iron content for alloy surfacing on 304 stain/ess steel

might develop at a later date, they were outlined wi th a vibratory etch­ing tool.

Quench Tests Throughout the salt and oil

quenches previously described, no additional cracks were detected and the two cracks discussed under "Dye Penetrant Examinat ion" did not appear to be propagating or opening up. Upon completion of the quench­ing tests, the two as-deposited cracks were sectioned and examined metallographically.

Metallographic Examination

The specimens shown in Fig. 10 and 11 are representative of those in the metallographic study. None of the samples were cracked, contained excessive porosity, or showed any other detr imental defects. The dark areas at some of the interfaces and grain boundaries were the result of over-etching for better reproduction. Figures 12 and 13 show the inter­face and base metal of a typical sam­ple at a higher magnif ication (300X). The grain boundary constituents shown in Fig. 12 were not analyzed; however, they appear to be either grain boundary carbides, agglomera­tion of the exogenous material shown in Fig. 13, or perhaps both. This condit ion would be less pro­nounced in material purchased to Reactor Development Technology (RDT) cleanliness specifications be­cause there would be less exogenous material, but for this investigation, the quantities of material involved and the availability of a suitable tubing did not justify attempting to purchase material to rigid cleanli­ness standards. Grain boundary car­bides could not be minimized unless the low carbon grades or the stabilized grades of stainless steel were used.

Figures 14 and 15 show the two cracks detected in the as-deposited overlays. Both of these cracks were

412-s I A U G U S T 1 9 7 2

1.90

1.80

1.70

1.60

1.50

1.40

1.30

1.20

g 1.10 GO

5 LOO

.90

.80

.70

.60

.50

.40

.30

(

0

^

•)

/ /

/ / / /

/ (

J

1

i i i

/ /

/ 1

yL

o D

Tl

P

" > 3 o

(T)

? O;

1

' ^

i T \

w

100 200 300 400

HARDNESS, KHN

500 600 100 200 300 400

HARDNESS, KHN

500 600

Fig. 17—Hardness vs. carbon content for alloy surfacing on 304 stainless steel

Fig. 18—Hardness vs. tungsten content for alloy surfacing on 304 stainless steel

in Vs-in. thick deposits, but this was the only similarity between the two cracked deposits. One was in a high dilution shielded metal-arc overlay and the other was in a low dilution plasma-arc overlay. However, the thickness did not appear to be a significant factor wi th respect to the cracking because only two of the eight Vs-in. end overlays developed cracks, and the opposite end of the same cylinders were not cracked. It is more likely that uneven preheating or cooling was responsible because this could develop uneven stresses wh ich would crack the overlays in specific areas where the stresses ex­ceeded the strength of the overlay. It is also our opinion that a higher preheating temperature would have avoided even this degree of cracking.

Chemical Analysis of Overlays Table 3 gives the results of the X-

ray fluorescence analyses of the overlays. These data show that all of the iron percentages except the one for the PPLD Vs-in. overlay are in excess of the 5% allowed by AWS A5.13 for al l-weld-metal deposits; however, considering the thickness

of the deposits this would be ex­pected. It is noteworthy that the process giving the least di lution, the technique using low dilution par­ameters, and the Vs-in. deposit did produce an iron composit ion less than 5%. All of the plasma-arc de­posits, except the PPHD 1/16- in. , did meet the RDT requirement of 6% maximum for iron content.

A l l of the shielded metal-arc de­posits were above the 6% iron l imit, but this is also to be expected since this process produces higher dilution than the plasma-arc process. How­ever, f rom the data generated in this investigation, the variation in hard­ness w i th iron content (see Fig. 16) indicate that iron contents in excess of 6% wi l l sti l l produce satisfactory overlays wi th hardnesses w i th in the RDT lower l imit of Rockwell " C " 38, provided the other elements are wi th in specification. Al though there is considerable scatter in the data, iron contents up to 12% appear to be acceptable. Figure 16 was con­structed from the average of the first four microhardness readings and the compositions given in Table 3.

Figures 17 and 18 show plots similar to Fig. 16 for the carbon and

tungsten contents versus hardness. Aiming for Rockwell " C " 43, the highest hardness achieved on our preliminary f i l ler metal tests, the per­centages of carbon and tungsten would have to be 1.20 and 4.5, re­spectively. This would mean that both of these elements would have to be at the upper end of their ranges in the fil ler metal (approximately 5% for tungsten and 1.30% for carbon) to obtain the desired composit ion in the deposit. Both of the high dilution plasma deposits are below the AWS Specification A5.13 for tungsten, indicating that the tungsten contents were not high enough in the fi l ler metal to offset the dilution.

The carbon contents of the shield­ed metal-arc deposits, except for the SMALD Va, were also below the 0.7% min specified by AWS A5.13. These low carbon percentages were obviously the result of insufficieat carbon in the fi l ler metal to account for dilution.

In general, but w i th a few excep­tions, the results of the chemical analyses showed the low dilution de­posits to be higher in carbon and tungsten than the high dilution deposits, and the Vs-in. thick de-

W E L D I N G R E S E A R C H S U P P L E M E N T l 413 -s

posits to be higher in the same ele­ments than the 1 /16- in . deposits. For elements picked up from the base metal, the reverse is true. This was the desired result, and these data confirm that the variations in dilution were achieved.

Electron Microprobe and Microhardness Examination

Figures 19a through 19h are photomacrographs showing the samples analyzed w i th the electron microprobe. The arrows in the f ig­ures indicate the location of the microprobe and m ic roha rdness traverses. Because of the varying thicknesses of the deposits, the traverses are located in the center of the overlay where, in general, the surfacing alloy is the thinnest.

As can be seen in Fig. 1 9, the plas­ma-arc process produced a smoother interface than the shielded metal-arc process; however, because of in­creased melting at the edges, the interface is curved upward in the center. The shielded metal-arc de­posits did not show as great a depth of melting on the edges, but the interfaces were not as uniform as those produced wi th the plasma process. The thicknesses of the eight samples at midwall are given in Table 4. These data show that four of the tests were wi th in 0.010 in. of the desired thickness whi le the other four were oversize from 0.019 to 0.121 in. The four overlays w i th de­posits oversized in excess of 0.010 in. were apparently the combined result of undetected errors in the punching of the reference marks and unexpected variations in penetration.

The results of the electron micro­probe and microhardness traverses are shown in Fig. 20 through 27. These traverses were made on lines which started at the ground surfaces of the hard facings and extended into the stainless steel substrate.

The microprobe results are plotted as X-ray intensities versus distance rather than the more conventional composition versus distance because of the inaccuracy of converting X-ray intensities to composition in a four-component system. The intensity versus distance plots show the gen­eral distributions of each element in the analyzed areas and this was con­sidered to be sufficient for this study. To provide some indication of the relationship between intensities and composition, the composition at the outermost edge of each sample was determined quantitatively by compar­ing the X-ray intensities w i th stand­ards. These data are given in Table 5. Comparing data in Table 5 and Table 3 shows there are con­siderable differences in the results

414-s I A U G U S T 1 9 7 2

• n Fig. 19—Photographs showing weld structure of surfacing alloy on 304 stainless steel. The arrows show the location of the microprobe and microhardness traverses

Table 4—Surfacing Thicknesses on Electron Microprobe Samples

Sample PPLD Ve PPLD 1/16 PPHD Va PPHD 1/16 SMALD Va SMALD 1/16 SMAHD Va SMAHD 1/16

Measured Thickness (Midwall),

0.135 0.072 0.144 0.090 0.246 0.071 0.134 0.141

in. Desired

Thickness, in. 0.125 0.062 0.125 0.062 0.125 0.062 0.125 0.062

42 X

41X

0 20 40 60 80 100 120 140

Distance From Outer Surface, mils (0.001 inch)

Fig. 20—Distribution of elements and microhardness across sample PPLD Vs

100 2 0 . 5 X

£ .

— Stellite overlay

x Iron o Cobalt A Nickel • Chromium • Hardness

•304 SS

Fusion line

1000

500 g

Or 40 80 120 160 200

Distance From Outer Surface, mils (0.001 inch)

Fig. 22—Distribution of elements and microhardness across sample PPHD Vs

40 80 120 160 200 240

Distance From Outer Surface, mils (0.001 inch)

Fig. 24—Distribution of elements and microhardness across sample SMALD Va

100

60

>-<

,1000

"Stellite overlay 304 SS

Iron Cobalt Nickel Chromium Hardness

Fusion line

500

0 20 40 60 80 100

Distance From Outer Surface, mils (0.001 inch)

Fig. 21—Distribution of elements and microhardness across sample PPLD 1/16

100 r

80 -

60 -

$ 4 0

42 X

20

—i

Fusion Line x Iron o Cobalt A Nickel f " " 3

D Chromium J r • Hardness \>—*

r V , 4 ~ - •§-—-a— - • $ — •©•— -a—-cr- 1

1 / ^ o _ O ^ — ^ o _ V - • ;

1 1 1 1 1 1 1 1 I A 1

1000

•500 8

0 20 40 60 80 100 120

Distance From Outer Surface, mils (0.001 inch)

Fig. 23—Distribution of elements and microhardness across sample PPHD 1/16

100 41 X

60

,1000 Stellite overlay • 304 SS

Iron Cobalt Nickel Chromium Hardness

Fusion l ine

500 g

0 20 40 K) 80 100

Distance From Outer Surface, mils (0.001 inch)

Fig. 25—Distribution of elements and microhardness across sample SMALD 1/16

Table 5—-Chemical Compositions of Microprobe Sample Surfaces

PPLD Element, % Va Iron 8.4 Nickel 1.2 Chromium 27 Cobalt 62

PPLD 1/16 14.6 2.7

26.3 51

PPHD PPHD Va 1/16

12.3 11.8 1.5 1.7

27 27 59.2 59.5

SMALD SMALD Va 1/16

10.2 23.4 1.2 2.3

26 24 62.6 50.3

SMAHD Vs

11.2 1.6

25.8 61.4

SMAHD 1/16 15.7 1.2

25 58.1

ECoCr-A*

5.0 3.0

25.0-32.0 Balance

*AWS 5.13, single values indicate maximum percentages, cobalt balance is not based on the four elements shown.

See Table 3 for complete composit ion.

tt

100

w 60

5 40

20

41.5 X

-\

Stellite overlay -,1000

•304 SS

Fusion line

i I I i .. i i i i _

5006

0 20 40 60 80 100 120 140

Distance From Outer Surface, mils (0.001 inch)

Fig. 26—Distribution of elements and microhardness across sample SMAHD Vs

IOO

80

47.5 X

Stellite overlay • -,1000 "304 SS

x iron o Cobalt Fusion line" A Nickel

20 40 60 80 100 120 140

Distance From Outer Surface, mils (0.001 inch)

Fig. 27—Distribution of elements and microhardness across sample SMAHD 1/16

f rom the two techniques, especially in the iron content. Because of the small area analyzed by the micro­probe, and the method used to de­termine the composit ion,* the results of Table 3 are considered to be more representative of the composition of the overlays.

The microhardness results, also plotted on Fig. 20 through 27, show that hard surfaces on the order of 1/1 6-in. thick are too th in to provide the necessary assurance that an adequate hardness level wi l l be ob­tained, or that reasonable wear could occur before the overlay would no longer function as intended.

Of particular interest in these f ig ­ures is the apparent relationship which exists between the cobalt con­tent and the microhardness. A quantitative relationship could not be developed on the limited data f rom this investigation; however, the trend indicates that one could be de­veloped w i th additional data. To the authors' knowledge, this relationship has not been reported previously.

S u m m a r y An investigation of the cyclic

thermal shock resistance of ECoCr type alloy surfacings on 304 stain­less steel has been completed, including study of several process parameters. The principal f indings are as fol lows:

1. Both the plasma-arc (trans­ferred arc) process and the shielded metal-arc process produced satis­factory surfacings under the condi­tions of this study. However, some preference for the plasma-arc pro­cess can be expressed based upon the possibility of better process con­trol and lower dilution.

•The 304 stainless steel was used as the standard for the iron, nickel, and chromium values The cobalt in Samples 1 and 2 was obtained using a pure cobalt standard and in Samples 3 through 8 by difference Since no corrections were applied for absorption or fluorescence effects, the compositions are only first order approximations.

2. The use of high dilution tech­niques, purported to impart better thermal shock resistance because of the gradient in composit ion and me-chancial properties, did not show any detectable advantage over the deposits made w i th standard tech­niques u t i l i z ing low d i l u t i o n . Whether this behavior would con­tinue for a greater number of cycles is unknown; however, there was no indication it would not. One distinct disadvantage of the high dilution technique, w i th either process but more pronounced w i th the shielded metal arc, is the difficulty in control l­ing composition and subsequent hardness. In almost all of the samples, the composition of the high dilution deposits was upset more than that of the low dilution deposits. The composition of the 1 /16- in . deposits was also upset more than those in the Vs-in. de­posits; however, this is to be ex­pected because they were machined closer to the base metal.

3. Of the two deposit thicknesses studied, 1 /16- in . and Vs-in., one can conclude that the overlay thickness of Vs-in. is preferable. Overlays w i th thicknesses on the order of 1 /16- in . are more likely to show greater deviations from the expected hard­ness and composition and would be too hard to control in production. Also, the possibility would be too great of having a surface wi th non-wear resistant ha rdnesses or compositions.

4. The microstructures of the over­lays exhibited absence of bond failure or excessive porosity. Some random porosity was found in the shielded metal arc deposits. This was attributed to operator technique rather than a shortcoming of the fil ler metal.

5. Surface hardness was not extremely sensitive to composition variations and all but two of the 1/16- in. deposits met the min imum

of Rockwell " C " 38 hardness estab­lished by Reactor Development Tech­nology (using an average of the first four microhardness readings). This would also indicate that the Reactor Development Technology limit of 6% iron is too restrictive because three of the shielded metal-arc deposits met the hardness requirement wh i le exceeding the iron limit. An ele­mentary calculation using 67% iron in the base metal and 3% iron in the filler metal showed that 6% iron re­quires less than 5% dilution, wh ich is very difficult to do w i th any process except oxyacetylene we ld­ing. From our experiments, an iron content of 12% max appears to be more realistic provided the carbon and tungsten are also kept above the limits previously described. The composition of the deposit should be controlled to give a min imum carbon content of 1.20% and a min imum tungsten content of 4.5%. Mainta in­ing these elements at the recom­mended levels wi l l insure adequate hardness for iron contents up to 12%.

References 1. Jackson, C B, Editor, Liquid Metals Handbook.

Sodium-Nak Supp., Atomic Energy Commission, July 1, 1955.

2. Tidball, Robert A.."Liquid-Metal Heat Exchangers," Power, February. 1960, pp. 82-85.

3 Wolf, J , Editor. Aerospace Structural Metals Hand­book, Vol 1 and 3, Air Force Materials Information Center, Wright-Patterson Air Force Base, Ohio, 1 970,

Curve A — Furman, D E , "Thermal Expansion Char­acteristics of Stainless Steels Between -300 and 1000 F," Trans AIME, Vol. 188, April, 1950. p. 688. Curve B — Allegheny Ludlum Steel Corp., Stainless Steel Fabrication, 1958, p. 273 Curve C — The Timken Roller Bearing Co., Steel and Tube D ivision. Digest ol Steels, 1957, pp. 56-57. Universal Cyclops Steel Corp., High Temperature Metals Uniloy 1 8-8s, 1 959, p. 22.

4. Funk, W, H., "Stresses In Clad Steels In High-Temperature Corrosive Environments," Proceedings American Petroleum Institute, Vol. 39. 1 959. pp. 65-73.

5. Phillips, L. E , et al, "Conceptual Design of Large Sodium Valves," Allis-Chalmers Mfg. Co (ASNP-655791, March, 1966

6. Personal Conversation with John Thorel, Tech­nical Consultant, LMEC, Canoga Park. Caifornia.

7 Douty, R A , and Rodgers, W. T , "Evaluation of Plasma Arc Surfacing for Valve Parts," Rockwell Mfg. Co., Materials Engineering Report No. 1151, January, 1969

8 Strabel, G R., and McCall, J L. "Evaluation of Stellite No 6 Hard Facing Overlays on Type 304 Stain­less Steel." Battelle Memorial Institute. November 24, 1970, Figures 43-51.

416-s I A U G U S T 1 9 7 2