An Investigation of Diffusion Bonding of Titanium to Stainless Steel

AN INVESTIGATION OF DIFFUSION BONDING OF TITANIUM TO STAINLESS STEEL

C. C. Chen Wyman-Gordon Company, Worcester, Massachusetts 01613

INTRODUCTION

The increasing demand for superior corrosion resistance and lower fabri cation cost for process pipes and vessels of the chemical industries has brought about recent developments in joining technology for titanium-steel bimetal systems (1-11). However, the fusion welding of titanium to steel is difficult due to the formation of brittle.intermetallic compounds at the interfaces (1, 2). The solid state cladding of steel directly to titanium also gives an unsatisfactory product because of the incompatibility of physico-chemical properties of the two metals to be bonded. The interdif fusion of titanium and iron often yields a series of brittle internietallic compounds at the interfaces; such brittle layers cannot sustain the strain of some subsequent metalworking operations which result in. internal rupture of the bonds.

Recently, the explosion-bonding technique has been developed as a manu facturing method of making titanium-clad steel plates (5-11). However, the deformation process during explosive bonding is relatively mechanical in nature and the diffusion process is difficult to control. Localized melting due to shock-waves may occur at the interfaces and the control of intermetal lic layers in the bond zones-may become difficult. Also, the process has limited shape flexibility and may be very expensive if the single setup per piece is needed for the bonding operation. Therefore, it appears to be highly desirable if. a solid state diffusion process with controllable diffusion process, more shape flexibility, and lower fabrication cost could be applied to produce titanium-steel bimetal systems.

The purpose of this paper is to describe a method of.bonding titanium to stainless steel by means of a solid state diffusion technique - hot isostatic processing (HIP). It is to further characterize the metallurgical features of. the bond zones and the quality of metallurgical bonds. The experimental approach involves determination of optimum bonding parameters, examination of microstructural characteristics, measurement of diffusion layers, determina tion of bond strength, and chemical analysis of transition zones.

EXPERIMENTAL PROCEDURE

The alloys used in this investigation were commercial pure titanium (CP Ti) and stainless steel Type-304 (SS-304). A series of 2 inch x 2 inch (5 cm x 5 cm) square Ti-SS composite samples with a Ti to SS-304 thickness ratio of 1 to 5 were produced through HIP for the investigation (Figure 1). They were prepared from 4 inch x 4 inch (10 cm x 10 cm) square SS-304 plates (0.25 inch/6.35 mm) and sheets (0.02 inch/0.5 mm), and 2 inch x 2 inch x 0.05 inch (5 cm x 5 cm x 1.27 mm) CP-Ti sheets. The chemical composition (wt.%) of the Ti-sheet used (TMCA heat number N-8173) is 0.12% Fe, 0.018% C, 0.010% N, 0.13% 0 , and 0.004% H . Tensile properties of this Ti-sheet material are 55 ksi 2 d19 MPa) yield strength, 76 ksi (524 MPa) ultimate tensile strength, and 28% elongation. The chemical composition of the 0.25 inch (6.35 mm) backing SS- 2380 C. C. Chen

SS: 14.sin Tl 7 ~ ~"-?' I/2-- . ~ ,~. ···~ .... E.B.-WELOED ASSEMBLY

METALLOGRAPHIC & ROLLING SAMPLES

(A) EXPERIMENTAL FLAT SAMPLE ASSEMBLY T,l.G. WELDED ZONES Figure 1 sketches showing (a) experimental flat sample assembly, and (b) s1.:1b scale pipe assembly used for thLs investigation

304 plate used is 0.049% C, 0.024% P, 0.007% S, 1.45% Mn, 0.42% Mo, 18.52% Cr, 0.22% Cu, 8.53% Ni, and 0.11% Co; the tensile strength of this plate stock is estimated to be about 85 ksi (586 l1Pa). The chemical composition for the 0. 02 inch (0.5 mm) SS-304 sheet used for welding and capsulating purposes is 0.075% C, 0.024% P, 0.005% S, 1.40% Mn, 0.19% Mo, 18.85% Cr, 0.46% Cu, 8.30% Ni, and 0.15% Co.

The sample and surf ace preparations prior to the HIP-bonding include cutting, surface-polishing, chemical-cleaning, water-rinsing, and alcohol drying. The tr!~le ~~ple blanks were then assembled by electron-beam welding in vacuum at 10 ~10 terrs. Evidence shows that this preparation sequence is useful in providing clean surface for bonding characterization. All assemblies were leak-checked prior to HIP-bonding.

The HIP-bonding is accomplished by interdiffusion in the field of high temperature, long time, and relatively low pressure. The diffusion rate is closely controlled by temperature, time, and pressure, and only limited plastic deformation occurs by the process. Table I lists both the chemical cleaning solutions and the HIP-cycles used for this investigation. A minimum of 20 HIP temperature-time-pressure combinations were evaluated.

After the HIP operation, the bonded samples were sectioned into halves (2 inch x 1 inch/5 cm x 2.5 cm) and quarters (1 inch x 1 inch/2.5 cm x 2.5 cm) for bond integrity evaluations. The evaluations included metallographic examinations, microhardness testing, chemical analysis, and rolling testing. The metallographic samples were examined for both as-electropolished and DIFFUSION BONDING OF TITANIUM TO STAINLESS STEEL 2381 etched conditions; the etching solution used was 2-1/2 HF:2-l/2 HN0 :95 H o 2 (by volume) solution for about 20 seconds. Both optical and scanning3 electron (JSM Model-U3) microscopies were used to characterize the bond structures.

Table I Chemical Cleaning Solutions and HIP Variables Used for this Investigation

Chemical Cleaning Solution HIP-Variables

Titanium Stainless Steel Temp. Time Pressure 0 ~CP-Ti2 (T:z::2e 304-SS2 °F~ c2 (minutes) ksi(MPa)

10% HNO (1) 100 cc HN0 : 1400 (760) 5 ( 35) 3 3 10 + 8% HF + 300 cc HCL for 1500 (816) 20 10 ( 69) H 0 (Bal.) 30 seconds at 1600 (871) 30 15 (103) 2 for 2 min. 100°F (38°C) 1700 (927) 60 at 140°F (2) 10% HN0 + 1750 3 (954) 120 (60°c) 8% HF + H 0 1800 (982) 2 (Bal.) for 1900 (1038) 5"'10 min. at 2000 (1093) 140°F (60°C) 2100 (1149)

Rolling experiments for the 2 inch x 1 inch composite blanks were carried out at room temperature, 257°F (125°C), 482°F (250°C), 600°F (316°C), 800°F (427°C), 1000°F (538°C), and 1200F (649°C). The total rolling deformation was controlled to 30%, 60%, and 80% reduction in thickness, with a 10% reduction per each pass. The bond conditions after each rolling pass and after the total deformation were examined.

Based on the results of flat samples, several hollow bimetal pipes with Ti-liner and SS-304 jacket were furth'er prepared for HIP-bonding (see Figure 1). These subscale bi-metal pipes are 4 inch (10 cm) round I.D. x 6 inch (15 cm) round O.D. x 14-1/2 inches (37 cm) long. They were roll-extruded to determine their formability. The roll-extrusion process was one in which a short, heavy wall tube was converted into a long, thin wall tube with little ~~2yge in the nominal diameter by rolling either the inside or outside surf ace

The procedures involved to produce the subscale pipes were liner fabri cation (surface machining and TIG-welding), chemical cleaning, jacket/liner assembly, leak-check, HIP'ing, and metallographic examinations of the bonds. A sliding fit was used in each case between CP-Ti liner and SS jacket and each element was machined to 150 rms surface finish. Thin SS-304 sheet of 1/8 inch (3.2 mm) thick was used as internal sleeves. After the HIP-bonding, the internal sleeves were removed by machining. The bond quality.of these sub scale pipes were further determined by roll-extrusion experiments up to a total wall thickness reduction of about 80%.

Chemical composition (wt.%) of the SS-304 jacket materials (7 inch/ 17.8 cm round O.D.) used for this portion of the program was 0.05% C, 1.76% Mn, 0.016% P, 0.013% S, 0.74% Si, 18.79% Cr, 9.36% Ni, 0.14% Mo, and 0.08% Co. Tensile properties of the pipe stock were 84 ksi (579 MPa) tensile strength (min.), 34 ksi (234 MPa) yield strength (min.), 56% elongation, and 57% reduction of area. The chemical composition of the CP-Ti (Ti-50A) cladding materials (5 inch/12.7 cm) round bar stock, purchased from TMCA (heat number N-9181) was 0.15% Fe, 0.007% N, 0.119% o , and 0.002% H . The average tensile 2 2 properties of the billet bar were determined to be 53 ksi (365 MPa) yield strength, 68 ksi (469 MPa) ultimate tensile strength, 30% elongation, and 56% reduction of area. The (a+S)/S transus temperature was about 1700°F (927°C). 2382 C. C. Chen

The microstructure of bar stock was characterized by 95~100% transformed acicular-a.

RESULTS AND DISCUSSION

The results of this investigation showed that titanium can be directly bonded to stainless steel by means of the HIP'ing. During this investigation, the nature and the microstructure of the diffusion layers were characterized and the effects of temperature, time, and pressure on the diffusion distance were determined.

Diffusion Bonding Through HIP

Titanium can be diffusion bonded to stainless steels at temperatures of 1400 to 2100°F (760 to 1149°C) at pressures of 5 to 15 ksi (35 to 103 MPa) and for time periods of 10 minutes to 2 hours. Since HIP is the process by which the parts are joined by a combination of temperature, time, and pressure at solid state conditions, atomic diffusion can be closely controlled by these variables without melting. Among the HIP-variables investigated (Table I), the optimum bonding conditions for CP-Ti and SS-304 system were determined to be 1400-1600°F(760-871°C)/30-60 minutes/10-15 ksi(69-103 MPa).

Like other bimetal systems for petrochemical applications, the purpose of titanium-steel bimetal systems was to combine titanium's corrosion resistance with steel's high strength. However, the diff~5ence in the thermal expan~~on coefficients between the two metals (8~10 x 10 /°C for Ti and 14~19 x 10 /°C for steels) was quite large. Therefore, severe thermal stresses could be developed along the joint in cooling or heating. During'this investigation, it was observed that thermal stresses of fusion welded or inadequate solid state diffusion bonded Ti-SS plates developed by cooling or heating were sufficient to cause cracking when brittle phases were present continuously.

Since the HIP is a three-dimensional bonding process, the local bonding pressure is the same in the furnace so that the shape-limitation and com plicated tooling design could be minimized. The time and temperature for creep deformation and interface diffusion processes under bonding pressure could be closely controlled, the ease of, controlling interdiffusion of steel and titanium can be visualized. Also, adequate cooling rate can be controlled to reduce the the,rmal stress at interfaces during cooling.

Several solid state bonding processes have been developed for producing titanium-steel bimetal systems (1-11). However, some of these competitive bonding techniques led to large amounts of brittle inter metallic compounds at the interfaces and results in weak joints for subsequent working. In some cases, the processes required an operation in high vacuum resulting in greater shape limitations and operation difficulties.

Nature of Bond Zone Structure

Metallographic examination shows that the bond-zone structures (Figure 2) of the HIP'ed Ti-SS composites are characterized by S-stabilized zones and intermetallic compound layers are observable at interfaces. The microstruc ture of the bond zones and the penetration distance of diffusion layers are strongly dependent on bonding temperature and are also affected by the bonding time and pressure. Brittle intermetallic compounds are formed along and near the interface regions. The amount of intermetallic compounds increases as the bond temperature and/or time increases. Continuous, massive compounds are formed at the temperatures at and above 1900°F (1038°C). However, with ade- DIFFUSION BONDING OF TITANIUM TO STAINLESS STEEL 2383

Stain less Steel

2100"F (1149°C)/30 min./15 kai (103 HPa) I ·, ~/ 1'" '} ,~.,,-.-..--..... •""

2000"P (1093°C)/30 •in./15 kei (103 MPa) 1600"F ( 87l"C)/30 min./15 kai (103 MPa)

1900°P (1038°C)/30 min./15 kei (103 MPa) 1500'F ( 816"C)/30 min./15 kei (103 MPa)

Figure 2 Variation of bond zone structures with bonding temperature for titanium (CP) stainless steel (304} bimetal system. HIP-variables (temperature/time/pressure) are included.

lBOO'P ( 982°C)/30 •in./15 luli (103 !!Pa) quate control of bonding cycles to reduce or to minimize the continuity of intermetallic layers at the bond interfaces, a metallurgical bond capable of tolerating severe rolling operations (up to 80% thickness reduction) was achievable for Ti-SS bimetal systems. Figure 3 further illustrates a com parison of the details of microstructural features at two different bonding conditions. At optical magnifications, no porosity and/or cracking could be detected for optimized bonding cycles.

Figure 4 presents both titanium and iron diffusion concentrations as a function of penetration distance. it is seen that during the diffusion bond ing, interdiffusion proceeds at high temperature and significant amounts of iron, chromium, and nickel diffuses into the titanium. A sharp fall and then a smooth decrease in the concentration of iron in titanium are observable. The interfacial diffusion of the titanium into stainless steel is at a much lower rate. Microhardness testings were made along the interfaces to char- 2384 C. C. Chen

Stainless Steel

Titanium Stainless

Steel (a) optical micrograph

Steel (b) SEM ,,..,.,,.,._.,._ "- -<>. ~~-- ,;,.,.,.: "''"'"~~ • ....oi..~~;,i micrograph

4 # • I~~;~ f"'"V.' ~•t•""' ... ·

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(c)

~ \- ,\.1 ,~ • ...._ V _.., ~ SEM- ...... ____...... 1 ___ _~,.; ti > ~ ~i~--.- micrograph '•• • ' ' 1••'' I''' • I • . • . ' •' j .1l>•t • "'1Jfll&' .. -~·· ,,.•'11' f\11 '( I "4,._,

(1) isoo•p (816°C)/JO min./15 ksi (103 MPa) (2) I750°F (954.C)/60 min./15 ksi (103 MPa)

Figure 3 Microstructural features at diffusion-bonded interfaces for titanium-stainless steel bimetal plates at two different bonding conditions. Differences in B-stabilized zones and intermetallic particle sizes are observable. acterize the bond strength (Figure 5) and across the bond zones to determine the strength changes for various bonding samples (Figure 6). No interface cracking or delamination was observable along Knoop indenters (Figure 5); the indentations were made with the longitudinal axis of the indenter along interface and 1000 gm load was applied. It is seen from Figure 6 that the increases in the hardness and the diffusion penetration distances appeared to depend strongly on the bonding temperature. Vickers indenter was used and the tests were made at 200 gm load and with 15 seconds duration.

Further Evaluation of Bond Integrity

The results of rolling experiments showed that the workability of the bond line structure varies significantly with the HIP bonding conditions, but normally the bonds have limited cold-workability, even with the optimized bonding structures of minimal, discontinuous intermetallics along the inter faces. Large shear stress along interface, probably due to strength differ ence between Ti and SS-304, may be developed during cold-rolling and resulted in separation. DIFFUSION BONDING OF TITANIUM TO STAINLESS STEEL 2385

(,wn) 100 ----o------.0-.------1i 90

80 ~ ]'. 70 fo z Q 60 :z i 50 0 v 40

10 N>-

J; 0 -10 10 30 40 50 bO 70 BO 90 100

PENETRATION DISTANCE {µrn)

Figure 4 Interdiffusion of titanium and stainless steel across diffusion-bonded interface

llicrol.Dd9Dtat.lon •V•l~tion of t.h• bond atren9th for u uper1-ul uapler HIP-bond..S at uoo•r cnt•c1/ JO •iD-/lS ••i UOJ MP•I · A llnOOp indenur Wleb 1000 vr- lo.di w.11a uHd.

Rolling experiments showed failure generally occurred on the titanium side for improper bonding cycles, and the bond strength reduced as the size and layer of intermetallics increased. However, the bond zone was quite ductile when warm rolling was applied. It was determined that optimum tem peratures for warm rolling are at the 600 to 1000°F (316 to 538°C) temperature range; the bonds were able to withstand plastic straining up to 80% reduction in thickness. Owing to the more formable strength and ductility of the two metals at temperatures of 600-1000°F (316-538°C), the shear strength at the interface may be significantly reduced and provide a more uniform deformation of the composite. Hot rolling at temperatures above 1200°F (649°C) may be beneficial from the flow stress standpoint, but it may significantly increase the absorption of interstitials (oxygen, hydrogen, carbon, and nitrogen) and 2386 C. C. Chen

(mm)

BONDING CONQ!J!ONS TEMP. TIME PRESSURE (~) (min,) (k.i.)(MPa) 600 1500 Si6 60--15 !03 600 1750 9S4 bO----- IS 103 ------1900 1038 30- • - IS 103 2000 l093 30 -- 15 103 ---- ~. i 500

~ ~ i5 m ; 400

JOO 300

200

lOOQ~.,----~0~3:-----o!0.~2---~0~1---__JOL-.--~0~.1---_JO.L2 ____0L3---__J0_4 ___ _J05lOO

STAINLESS 'STEEL TITANIUM PENETRATION DISTANCE FROM INTERFA~E (mrn.) Figure 6 Microhardness versus penetration distance from diffusion-bonded interface for several HIP-bonding cycles

contaminate the alloys. The atomic mobility at hot-rolling condition may be very significant resulting in a thick layer of intermetallic compounds. In addition, two subscale pipes (Figure 2) of (CP-Ti)-(SS-304) were produced through HIP and roll-extruded successfully for about 80% reduction in wall thickness at 600-1000°F (316-538°C).

Other Bimetallic Systems

During this investigation, the HIP-bonding feasibility was also extended to tantalum (Ta), mild steel (MS), copper (Cu), zirconium (Zr), 90Cu:l0Ni, 70Cu:30Ni, aluminum (Al), brass, vanadium (V), and niobium (Nb). A series of bimetal systems examined in this investigation included Ta-MS, Ta-SS, Ta-Cu MS, Ta-Cu-SS, Zr-SS, Zr-MS, (90Cu:l0Ni)-MS, (70Cu:30Ni)-MS, Ti-MS, Ti-Al, Ti Brass, Ti-V, Zr-Al, Zr-Nb, Zr-Ta, SS-Brass, Cu-SS, Al-SS, Ta-SS, V-Brass, et al. In particular, subscale pipes of 6 inch (15 cm) diameter O.D) x'14-l/2 inch (37 cm) long were successfully produced and roll-extruded for Ta-MS, Ta Cu-MS, Zr-SS, and (90 Cu:lONi)-MS systems. Evaluation of bond integrity indicated metallurgical bonds were formed through HIP-bonding. Figure 7 illustrates examples of the metallurgical features of diffusion-bonded inter faces for (CP-Zr)-(SS-304), (90Cu:l0Ni)-MS, (CP-Ta)-Cu-MS, and (CP-Ta)-MS bimetal systems. The fine wavy interfaces for Zr-SS and (Cu:Ni)-MS systems may provide excellent mechanical locking for improved interface workability. Roll-extrusion experiments for the Ta-MS, Ta-Cu-MS and (Cu:Ni)-MS subscale pipes showed that these metallurgical bonds could withstand at least 80% reduction in wall-thickness. Scale-up technology for manufacturing bimetal pipe systems are currently underway jointly by Wyman-Gordon and Rollmet (12) to determine the production capability of the full-size pipes. DIFFUSION BONDING OF TITANIUM TO STAINLESS STEEL 2387

-- Stainless - steel ,.. '•l_Cu-Ni >;· ·. -· .

Mild Steel

CP-Zirconium-304 Stainless Steel System (90Cu-10Ni)-Mild Steel System

Stainless ,Stainless Steel Steel

Figure 7 Other examples of bimetal systems developed by HIP-diffusion bonding process

SUMMARY

The HIP-bonding process has produced good quality bonds of titanium stainless steel and other bimetal systems capable of withstanding subsequent forming operations. The process provides controlled bonding conditions and has excellent shape-flexibility and process-simplicity for the diffusion bonding. It should offer a viable manufacturing alternative for solid state diffusion bonding of various bimetal systems of different combinations and forms.

Titanium has limited solubility with iron and forms intermetallic com pounds even under a close control of solid state diffusion conditions. The key to the successful development of metallurgical bond between titanium and iron which can tolerate subsequent cold or warm working operations is the control to limit the interdiffusion of Fe and Ti through HIP-bonding. Optimum HIP-bonding conditions for bonding titanium-steel systems are: temperature = 1400-1600°F (760-871°C), time= 30 minutes - 1 hour, pressure= 10-15 ksi (69- 103 MPa). Proper temperature range for subsequent working operations in air are 600 to 1000°F (316-538°C) temperature range.

REFERENCES

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4. F. E. Dolzhenkov and Yu. I. Krivonosov: Izvest, VUZ-Chern, Met., No. 11, November 1964, 137. 5. o. R. Bergmann, et al: Trans. Met. Soc. AIME, 236, (1966), 646. 6. J. L. DeMaris and A. Pocalyko: Preprint AD66-113, Am. Soc. Tool & Manufacturing Eng., Dearborn, Mich., 1966. 7. W. Klein: Materialprufung, 10 (3), 1968, 73. 8. K. RUdinger: Z.Werkstofftechnik/J. of Materials Technology, 2 (4), 1971, 169. 9. S.Inomata, et al, "The Science, Technology, and Application of Titanium", p.1065, Pergamon Press, New York, 1970. 10. K. RUdinger: "Titanium Science and Technology", 4 (1973), Plenum Press, New York. 11. T. J. Enright, et al: Metal Progress, July 1970, 107. 12. D. L. Corn: Metal Progress, June 1977, 28.