TITANIUM TUBING AND FITTINGS FOR

DEEP SUBMERGENCE HYDRAULIC SYSTEMS

Marsh K. Eckhardt Design Specialist

Lockheed Missiles & Space Company, Inc. Sunnyvale, California

Introduction

Titanium possesses strength, weight, and corrosion resistant prop­erties ideally suited for deep submergence hydraulic systems. With this in mind the U.S. Navy instituted a study to evaluate titanium tubing and fittings; the results of that study (1) are summarized in this paper.

Deep ocean vehicles are weight and space critical because of limi­tations imposed by handling and transportation requirements on land, air and sea. The advantages of titanium tubing and fittings in deep ocean vehicles are high strength to weight ratios, space saving, weight saving and general corrosion and pit resistance (2). An additional advantage is an overall reduction in cost due to reduction of buoyancy material required. An example of overall cost saving despite the more costly titanium material can be shown by the case of a 20, 000 foot depth vehicle weighing 150, 000 lbs. requiring 50, 000 lbs. of buoyancy material. The weight saved would be about 400 lbs. for tubing and fittings plus 520 lbs. of buoyancy material for a total of 920 lbs. An overall cost savings up to $30, 000 would accrue due mainly to the reduced cost of buoyancy material. An ancilliary benefit would be the saving of about 15 cu. ft. of volume.

The requirements of a 3, 000 psi submersible main hydraulic oil system are similar to an aircraft system except for the submersible re­quirement. The system operating pressures remain at 3, 000 psi at any depth by virtue of pressure compensation. There are other smaller hydraulic systems in submersibles which contain salt water and which operate over a range of plus and minus 9, 000 psi (for 20, 000 ft. depth). As a matter of fact, the first installation of titanium tubing on a U.S. Navy submersible is being planned for such a salt water system on the ALVIN. The only disadvantages of titanium tubing on a submersible are possibly galvanic corrosion and fouling, both effects are controllable

131

132 M. K. ECKHARDT

however, i.e. , consummable anodes and cleansing of salt water systems with fresh water and chemicals (2).

Material Selection

The selection of tube materials and fittings was based on studies made by aircraft companies, commercial availability, and experiences and reports from operating deep submersibles.

Tube Selection

Over a period of about 10 years the aircraft industry has developed titanium alloys which have good formability, good strength and are practical to manufacture at a reasonable cost. Recent efforts were directed primarily towards Ti -6Al-4V and Ti -3Al-2. 5V. Although stronger, the T i-6Al-4V did not meet all their requirements as well as Ti-3Al-2. 5V. As a result of this continuing search there are at least three alloys in development in addition to the three (6Al-4V, 3Al-2. 5V, commercially pure) which are generally available today; see Table I for a list and principal characteristics of these six alloys.

TABLE I. TITANIUM TUBE MATERIALS

Material Tu(psi) Ty(psi) Exl06 Elong. (%)

3Al-2. 5V Annealed 90,000 75,000 15 14 C.W.S.R. 125,000 105,000

6Al-4V Annealed 130,000 120,000 16.5 10 C.W.S.R. 170,000 138,000

(3-3, 11. 5Mo- Annealed 122,000 107' 000 10 20 4. 5Sn-6Zr Aged 190,000 179,000 15 6

Commercially Grade I 40,000 30,000 14.9 24 Pure Grade II 50,000 40,000 14.9 20

Grade III 60,000 50,000 15 8

{3-C, 3Al-8V- Aged 177,000 173,000 14 15 6Cr-4Mo-4Zr

Transage 129 Annealed 122, 000 60,000 10 20 2Al-11. 5V-2Sn- Aged 195,000 180,000 14. 5 6 11. 3Zr

Ti -3Al -2. 5V tubing was chosen in this program as the prime can­didate because of its success in aircraft and its excellent corrosion

\

TITANIUM TUBING AND FITTINGS FOR HYDRAULIC SYSTEMS 133

resistance properties. Samples of BET A-3, Ti -6Al -4V and various grades of commercially pure were also obtained for bend and flare test comparisons with Ti-3Al -2. 5V.

As shown in Table I, the minimum guaranteed properties of Ti -3Al -2. 5V are 125, 000 psi ultimate tensile strength and 105, 000 psi yield tensile in the cold worked stress-relieved condition. The alpha phase of this alloy is strengthened by the addition of aluminum, and the beta phase is stabilized to room temperature by the addition of vanadium. Ti -3Al -2. 5V resists general attack, pitting, intergranular and crevice corrosion. It possesses excellent cavitation and erosion resistance. It has stress corrosion cracking characteristics equal to or better than Ti-6Al -4V because of lower aluminum content and therefore less Ti Al (which tends to make Ti-Al alloys prone to stress corrosion crackin/).

The only other commercially available titanium tubing is commer­cially pure in grades l, 2, and 3 with tensile yields of 30, 40, and 50, 000 psi. It can be purchased in higher strengths by special order but would be susceptible to stress corrosion cracking (3).

Fitting Selection

One purpose of this study was to evaluate (by testing) fittings for submersibles. Both permanent and separate joints were represented in tests. The permanent joints will be discussed first because two of their joining methods were also used to install the separable fittings.

Permanent Joints

The four following basic permanent joining methods are in use today: welded, mechanically swaged, heat shrinkable, and brazed. See Figure 1 for sketches of these types of fittings and materials from which they are made for use in titanium systems. Submersibles generally use brazing and welding whereas aircraft use all four. Three of the four methods were chosen for testing: welding, swaged type, and heat shrinkable type.

The method of welding chosen was an automatic TIG process with a butt joint. The equipment used was Astro Arc head plus Dyna Surge power supply.

The selected swaged type fitting (Permaswage, made by the Deutsch Co., Los Angeles, Calif.) was made of commercially pure AMS-4921 titanium. The fitting was mechanically swaged on the tube externally by a radially swaging tool. The primary sealing interfaces were elastomer rings with further sealing from imbedded high spots around each end which also prevented rotation from torsion.

The heat shrinkable fitting (Cryofit, made by Raychem Corp. , of

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BUTT WELDED WITHOUT INSERT (Ti-3Al-2 .5V)

SWAG ED (COMMERCIALLY PURE AMS492 l)

M. K. ECKHARDT

BRAZED (Ti-6Al-4V BRAZE 48Z-48 Ti-4Be)

HEAT-SHRINKABLE (nitinol)

Fig. 1 Examples of Permanent Tube Joining Methods

Fig. 2 Field Installation of Permanent Heat Shrinkable Fitting

TITANIUM TUBING AND FITTINGS FOR HYDRAULIC SYSTEMS 135

Menlo Park, Calif.) was chosen because of its successful uses on the F-14 Navy fighter aircraft on Ti -3Al -2. 5 titanium tubing hydraulic systems. This is a lightweight, low profile, nickel titanium alloy fitting which shrinks on the tubing when allowed to warm from cryogenic to room temperature. When shrunk on the tubing, rings machined in the inside diameter grasp the tubing, causing a local deformation in the exterior tube wall. Both ends of the fitting are tapered to minimize the stress concentration. During shrinking on the tubing, the tubing ends, which lie underneath the thick section of the fitting, are plastically deformed. At the thin ends of the taper, the fitting itself is plastically deformed. The fitting shrinks on with forces that can exceed 100, 000 psi. The fitting may either be stored in liquid nitrogen, or be cooled and expanded by the user just prior to installation. The method of installation is shown in Fig. 2.

The nickel titanium material used in these fittings is basically similar to the original Nitinol 55 material which was developed at the Naval Ordnance Laboratory. It possesses the characteristics of thermal recovery after deformation, i.e., a deformed part will return to its original shape upon heating. When the metal is cooled below its trans -formation temperature the austenitic phase transforms to a martensitic phase which exhibits physical properties such as low modulus, easy deformation and high damping. In this state it can be reversibly loaded indefinitely without fatigue within a limit of 10% strain. When the fitting is allowed to warm through its transition temperature (-160° F in this case), it reverts to its austenitic state and recovers to its original shape. In the austenitic state, the material has a typical non-ferrous stress strain curve with tensile yield around 70, 000 psi. This material is corrosion resistant.

Separable Joints

A survey of separable fittings used on aircraft and submersibles was made. Submersibles have been usingO-ring, flared and swaged type 316L stainless steel fittings. Aircraft with titanium systems have been using several types of swaged fittings. Another fitting recently tested by air­craft companies is a separable version of the heat shrinkable fitting described above. The survey resulted in six candidates, consisting of four types (Figure 3). A comparison was then made based on the following criteria; Insertion depth, type of seal, type of tube connection, number of parts, weight, size and shape, cost, corrosion resistance, inspection method, seal surface protection, assembly and disassembly, experience and test data. Three were chosen for testing as the most promising for a submersible, Le. , flared, 0-ring and heat shrinkable. The methods selected for attaching these fittings were flaring, heat shrinking and in the case of the 0-ring type, both welding and the permanent heat shrink fittings.

The flared fitting chosen, Fig. 4a, was a standard 37-deg flare with sleeve and "B" nut made of Ti -6Al -4V and treated with moly­disulfide lubricant anti-galling coating. The sleeve is free floating,

136 M. K. ECKHARDT

HEAT-SHRINKABLE (nitinol + 6 Al-4Y)~=----"""

Fig. 3 Examples of Separable Fittings-a) Flared, b) 0-Ring, c) Heat Shrink

• a

c Fig. 4 Titanium Separable Fittings

TITANIUM TUBING AND FITIINGS FOR HYDRAULIC SYSTEMS

aligns the flare, acts as a lock washer, supports the tube and resists twisting of the flare. The flaring of the tubing was done on a Lakeland Orbital Flarer, Model 216.

137

The 0-ring fitting chosen (Min-0-Seal, made by Combination Pump Valve, Philadelphia, Pa.), Figure 4b, is comprised of an 0-ring plus three metal parts. One end part is a flat faced fitting machined out of Ti -6Al -4V round stock so that one end is tube size for butt welding. The other end part is flat faced also but has an 0-ring groove and threads on the outside diameter while its opposite end is machined to tube size for butt welding. A union nut slips on to a shoulder on the flat faced end and joins the two ends together by screwing onto the threads on the opposing piece. The 0-ring is thereby compressed creating an initial ·seal between the static metal surfaces. With internal pressures the 0-ring is pressed against the outer wall of the groove for a final seal. For this program it was decided to join the fittings to the tube both by butt welding and permanent heat shrinkable fittings.

The heat shrinkable fitting included in the program was a three­piece assembly, Figure 4c, consisting of a nitinol heat shrink flange, a Ti-6Al -4V captured shouldered nut and a Ti -6Al -4V threaded adapter fitting. For connection to a component, the fitting may be used with an 0-ring or proprietary metal seal. One end of this fitting has a shrink-on nitinol sealing ring which mates with a standard boss and the other end has an integral flexible metal seal which mates with a similar seal on the flange. Both seal surfaces are protected. The flange part of the fitting is installed on the tube similarly to the permanent heat shrinkable fitting described earlier.

Test Program

Test Program Definition

The test program was planned to provide basic information on fit­tings and T1-3Al -2. 5V tubing in 1/2-inch size. The first tests were burst, collapse, flexure fatigue and tension. Collapse ·tests were peculiar to this program, the others were more or less standard tests run by the aircraft industry. The objectives of these tests were to destroy the specimens and compare the results with each other and with theoretical values which were calculable, i.e., collapse and burst. Tubing of 1/2-inch diameter and . 024-inch wall thickness was used in all tests, this material being readily available.

Other tests were bending and flaring of the four alloys (Beta-3, Ti-3Al-2. 5V, Ti-6Al--4V and commercially pure) in sizes readily available from 1/2 to 1-1/ 4 inch diameter. The tests were deliberately conducted in a production plumbing shop to see if personnel with no prior titanium experience could satisfactorily fabricate titanium tubing and to determine if there were any notable differences among the workability of the five alloys.

138 M. K. ECKHARDT

Additional tests were included in the program as opportunities pre­sented themselves. One consisted of testing 1-1/4 inch Ti -3A1 -2. 5V tubing and heat shrinkable fittings by burst, tensile and collapse tests plus actual installation on an operating mock up of a submersible's hydraulic control system. The opportunity also presented itself to sub­merge tubing and a heat shrinkable fitting, already having passed a standard corrosion test, in tidal waters at the Naval Civil Engineering Laboratory at Port Hueneme, California.

Test Procedures

The equipment and procedures used in burst, tensile and collapse tests were conventional and will not be described in detail. However the flexure fatigue test methods and equipment (subject to controversy even amongst the specialists) will be discussed and may be of particular interest to those with testing experience. The procedure used is one described in Aeronautical Recommended Practices (ARP) 1185 (not published yet) of the Society of Automotive Engineers, i.e., fixed beam planar flexure. It is a revision of procedures for flexure and impulse testing included in MIL-F-182800, Ref. (4). In this procedure the fitting is held in a fixed block and the opposite end is deflected in a plane. The tube is filled with liquid at 3, 000 psig and is flexed until failure, indicated by a leak. The amount of deflection is determined by the bend­ing stress measured by strain gauges located 3/16-inch from the end of the fitting. See Figure 5 for sketches of the test configurations. The stress (S) levels versus the numbers of cycles (N) are then recorded to produce S/N curves.

Results and Discussion

Burst Tests

The results of the burst tests are summarized in Table II which shows failure in the weld area at 14, 500 psi and in the tubes with fittings at about 16, 000 psi. The fittings did not fail. For comparison, the cal­culated tube failure value is slightly lower, 14, 700 psi. For the normal hydraulic operating pressure of 3, 000 psi the requirement is burst pressure >4 x 3, 000 or 12, 000 which is well satisfied.

Tensile Tests

The results of the tensile tests are summarized in Table III which shows that the unjoined tube meets the guaranteed minimum ultimate tensile of 125, 000 psi, and that the joined tubes exceeds the requirement of 2,000 lbs. from MIL-F-21467D, (5), or the higher general criteria of 1/2 the ultimate tensile strength, about 2, 500 lbs. for this tube.

STRAIN GAGE SEPARABLE FITTINGS

Fig. 5 Planar Flexural Fatigue Fitting Test Configuration.

PLANAR FLEXURE

..... ::j > z c ~ ..... c °' z G'> > z 0 ..,, ::j ..... z G'> Cl> ..,, 0 "" :c -< 0

"" > c !:: ()

Cl> -< Cl> ..... m ~ Cl>

140 M. K. ECKHARDT

TABLE II. BURST TEST RESULTS, Ti-3Al-2. 5V TUBES

Pressure Size at Failure

Fitting (in.) (psi) Type of Failure

Welded 1/2 14, 500 Weld failed

Welded 0-Ring 1/2 14, 500 Weld failed

Heat Shrink (P) 1/2 16,000 Tube failed

Swaged 1/2 15,000 Tube failed

Heat Shrink (S) 1/2 16, 500 Tube failed

Flared 1/2 16,300 Tube failed

Heat Shrink (P) 1-1/4 15,000 No failure

TABLE III. TENSILE TEST RESULTS, Ti-3Al-2. 5V TUBE

Diameter Tensile Strength Specimen and Wall Failure

(in.) lb Ultimate psi

Tube 1/2 x o. 024 5,300 141,000 In tube

Welded 1/2 x o. 024 4,990 130,000 At weld

Heat Shrink (P) 1/2 x o. 024 3,360 89,600 Pulled out

1/2 x o. 024 of fitting

Swaged 3,480 92,800 " Heat Shrink (P) 1-1/4 x o. 095 31,800 96,000 " Tube 1-1/4 x o. 095 43,500 131,000 In tube

Welded 1 x o. 048 17,500 121,000 At weld

Tube 1 x o. 048 18,000 125,000 In tube

Collapse Test

Failure by collapse of the 1/2-inch Ti-3Al-2. 5V tube as joined by the various methods averaged 5, 000 psi. This compares to a calculated value of 3, 700 psi by formulas (6). Calculation of the effect of maximum allowable ovality of . 004 inch reduces this value to 2, 500 psi. In the case of the 1-1/ 4-inch speciments, failure was about 17, 000 psig. Critical buckling was calculated to be 14, 700 without ovality and 10, 000 with . 002 ovality. The high test results are not readily explainable except that the calculations might be conservative.

TITANIUM TUBING AND FITIINGS FOR HYDRAULIC SYSTEMS

Flexure Tests

A total of 35 planar flexure tests was made. The results of these tests are plotted in Figure 6. (The results on swaged Vttings are omitted as inconclusive). A modulus of elasticity of 15 x 10 was used

141

in converting strain gage readings to bending stress. These test results disclose more information about the various methods of joining than the previous tests because the interaction of the fitting and tubing is critic­ally examined and the fracture fatigue characteristics of the tube are measured. Fretting corrosion and/or fatigue were the failure mechanisms of the specimens as outlined below:

Unjoined Tube Welded Tube Heat Shrinkable Permanent 0-ring Fitting Welded 0-ring +Heat Shrinkable Perm. Flared · Heat Shrinkable Separable

Fatigue

x x x x x x x

Fretting

X (at clamp) X(at fittinro

x (at fittinro

Failure

Tube Weld Tube Weld 0-ring fitting Flare Tube

Calculations (using Ref. 7) showed for the tubing an approximate value of the fracture toughness parameter, ~ = 20 ksi in. and Klc/crys = . 20, well within the range of brittle c behavior.

From the results of these tests a general conclusion can be drawn that the fittings and tubing tested are acceptable for deep submersible hydraulic systems. Differences were found among joining methods, Fig. 6. In the permanent joints, the heat shrinkable fittings were better than welding. In the separables there was a crossover of 0-ring type attached with heat shrinkable fittings and the heat shrinkable fitting by itself. . Redesign of the sharp fillet on the 0-ring fitting could make them comparable to the separ­able heat shrinkable fitting. Flared fittings were next and lastly, the welded 0-ring type. The tubing itself exhibited relatively good resistance to fatigue failure.

It is important to point out that this tubing was cold worked, stress re­lieved (CWSR), belt sanded and shot peened. Recent tests run by the author on CWSR chem milled tubing indicate much poorer performance. Two other facts which might effect performance are 1) manufacturing anomalies which are not detectable by ordinary non-destructive test methods, and 2) impuri­ties from shot peening in areas where welds are to be made. Even so, the particular tubing flexure tested in this program performed very well.

Corrosion Tests

Several corrosion tests were conducted which, while not rigorous by any means, are considered significant enough to report. In one case, a 1/2-inch heat shrink fitting on open ended Ti-3Al-2. 5V tubing became available. It had already passed a standard corrosion test (4). It was sent to the Naval Civil Engineering Lab where 1) it was placed in tidal waters for

142

80,000

w 70,000 0

~ z ~ E. V)

~ V) V) w QI::

60,000

t;; 50,000 0 z iS z w dl

40,000

30,000

\ \

\ \

\ \

10,000

' \ ' ' ', .,

' ..........

• WELDED l

0 HEAT SHRINK (P) l

e 0-RING WELDED l

D FLARED 2

M. K. ECKHARDT

0 0-RING 2 + HEAT SHRINK FITTING (P)

!::,. HEAT SHRINK (S) l

• UNJOINED TUBE l

............. ....

• TUBE CRACKED FITTING CRACKED P • PERMANENT FITTING S • SEPARABLE FITTING

........... .... -.... 100,000

NUMBER OF CYCLES

1,000,000

Fig. 6 Planar Flexure Test Results (S/N Curves) of 1/2-inch C. W. S. R. Shot Peened Ti-3Al-2. 5V Tube at 1, 800 cpm With 3, 000 psi Internal Pressure

TITANIUM TUBING AND FITIINGS FOR HYDRAULIC SYSTEMS 143

3 months, 2) cycled 24 times. to 12, 000 psig with 60 hour dwells, and 3) exposed to 24 .hours humid salt air and finally disected. The results were a clean specimen showing that nickel-titanium and Ti-3Al-2. 5V are galvanic­ally compatible, highly resistant to corrosion and that no water had gotten between the fitting and tubing.

Other efforts included tests for indications of corrosion of Ti-3Al-2. 5V, such as bent tubes with 316L stainless steel fittings in tidal water for 5 months and heat treatment and soaking of welded tubing in trichloroethylene. Corrosion was not evident in these tests.

Fabrication

Results of shop fabrication (bending and flaring) on three alloys and four different diameters showed that there were no unusual problems with forming any size or alloy tested. Several preliminary observations on bending are: more power is required to form titanium than stainless steel tubing; the mandrel must be at least 1/8 inch behind the bend-target line and springback is greater but less predictable than with other metals such as stainless steel. Flaring did not present any unforeseen problems. As a result of these and other data a preliminary specification for fabrication of Ti-3Al-2. 5V tubing by bending, flaring, deburring, and cutting was prepared.

Installation

Four heat shrink fittings were installed on 1-1/ 4 inch . 095 inch wall Ti-3Al-2. 5V tubing. This was part of an operating mock up of a deep submersible 3, 000 psi hydraulic control system which ran for six months as a practical demonstration of the use of these components. No leaks or failures of any kind were noted.

Conclusions

It is concluded that titanium tubing and fittings are ideally suited for deep submergence hydraulic systems. Of the four tube alloys examined the one recommended is CWSR glass/shot peened Ti-3Al-2. 5V. Of the many joining methods examined, there are four that are recommended for deep submersible systems. These methods include welding, heat shrink, flared and 0-ring fittings.

Finally, the results of this study suggest that titanium piping may be feasible for shipboard saltwater systems. It is therefore recommended that the marine industry evaluate the advantages of titanium for piping systems especially where corrosion resistance, reliability, maintainability, and weight are important. -

144 M. K. ECKHARDT

References

1. Eckhardt, M. K. "Titanium Tubing", DSSV Engineering Report, Lock­Missile and Space Co., report no. LMSC-Dl77821-6, Navy contract N00024-70-0254, 30 June 1971.

2. Williams, W. L. "Marine Applications of Titanium", The Science, Technology and Application of Titanium, Editors Dr. R. Jaffee and N. Promisel, Pergamon Press, New York 1970.

3. Litvin, D. A. and Smith, D. E., "Titanium for Marine Applications", Naval Engineers Journal, Vol. 83, No. 5, October 1971, PP. 37-44.,

4. Government Specification MIL-F-18280C, Fittings, Flareless Tube, Fluid Connections, 27 June 1969.

5. Government Specification MIL-F-21467D (SHIPS), Fittings, Flare­less, Fluid Connections, 22 April 1969.

6. Timoshenko, S. P., Theory of Elastic Stability, 2nd Edition, New York, McGraw Hill Book Co., 1961

7. Pellini, W. S., "Advances In Fracture Toughness Characterization Procedures And In Quantitative Interpretation To Fracture-Safe Design For Structural Steels", Naval Research Laboratory Report 6713, April 3, 1968.