Dissimilar metal friction welding of austenitic–ferritic stainless steels

Journal of Materials Processing Technology 160 (2005) 128–137

Dissimilar metal friction welding of austenitic–ferritic stainless steels V.V. Satyanarayana a , G. Madhusudhan Reddy b , T. Mohandas b,∗ b

a Vasavi College of Engineering, Hyderabad 500 031, India Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad 500 058, India

Received 30 June 2003; received in revised form 20 May 2004; accepted 20 May 2004

Abstract Continuous drive friction welding studies on austenitic–ferritic stainless steel combination has been attempted in this investigation. Parameter optimization, microstructure–mechanical property correlation and fracture behaviour is a major contribution of the study. Sound welds are obtained at certain weld parameter combinations only. The mechanical properties of dissimilar metal welds are comparable to those of ferritic stainless steel welds. Evaluation of the joints for resistance to pitting corrosion revealed that the dissimilar welds exhibit lower resistance to pitting corrosion compared to the ferritic and austenitic stainless steel welds. Interface on the austenitic stainless steel side exhibited higher residual stress possibly due to its higher flow stress and higher coefficient of thermal expansion. © 2004 Published by Elsevier B.V. Keywords: Dissimilar metal welding; Friction welding; Austenitic stainless steel; Ferritic stainless steel; Microstructure; Notch tensile strength; Hardness; Impact toughness; Pitting corrosion; Residual stresses

1. Introduction Several situations arise in industrial practice which call for joining of dissimilar materials. The materials employed are location dependent in the same structure for effective and economical utilization of the special properties of each material. The joining of dissimilar metals is generally more challenging than that of similar metals because of difference in the physical, mechanical and metallurgical properties of the parent metals to be joined. In order to take full advantage of the properties of different metals it is necessary to produce high quality joints between them. Only in this way can the designer use most suitable materials for each part of a given structure. The growing availability of new materials and higher requirements being placed on materials creates a greater need for joints of dissimilar metals. Joining of ferritic stainless steels are faced with the problem of coarse grains in the weld zone and heat affected zone of fusion welds and consequent low toughness and ductility due to the absence of phase transformation during which grain refinement can occur [1,2]. In general austenitic stainless steels are easily weldable. When austenitic stainless steel joints are employed in cryogenic and corrosive environment the quantity of ferrite in the welds must be minimized/controlled to avoid property degradation during

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0924-0136/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.jmatprotec.2004.05.017

service [3,4]. In addition these steels are prone to sensitization of their fusion welds. These problems have been addressed by solid state welding processes, such as friction welding [3–6]. Dissimilar metal combination between ferritic stainless steels and austenitic stainless steels is commonly employed in TiCl4 reduction retorts. This calls for welding of the combination. Such transition joints are necessary because austenitic stainless steels with superior creep strength and oxidation resistance are required in the higher temperature regions, while ferritic stainless steels to avoid the problem of nickel leaching by molten magnesium. Welding of ferritic to austenitic stainless steels is considered to be a major problem due to difference in coefficient of thermal expansion, which may lead to crack formation at the interface, formation of hard zone close to the weld interface, relatively soft regions adjacent to the hard zone; large hardness difference between the hard and soft zones and expected differences in microstructure may lead to failures in service [7–9]. Solid state welding is a possible solution for these problems. This paper reports on a study that has been taken up to develop an understanding on the friction welding characteristics of austenitic stainless steel–ferritic stainless steel dissimilar metal welds. Detailed microstructural examination in the different regions of the welds and a correlation between the microstructure and mechanical properties and corresponding fracture behaviour forms the goal of the study and therefore assumes special significance since such detailed studies are

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not hitherto reported. Parameter optimization also forms a major contribution of the study. 2. Experimental 2.1. Parent metals The parent metals employed in this study are AISI 304 austenitic stainless steel and AISI 430 ferritic stainless steel. The composition and mechanical properties of the starting parent metals are given in Tables 1 and 2, respectively. The austenitic stainless steel contains equiaxed grains of austenite with occasional twinning, while the ferritic stainless steel consists of coarse and elongated grains of ferrite (Fig. 1). For friction welding studies the material employed was in Table 1 Chemical composition of parent metals (wt.%) Composition

C

Si

Mn

Cr

S

P

Ni

AISI 430 AISI 304

0.06 0.06

0.4 0.32

0.4 1.38

17.0 18.4

0.03 0.28

0.04 0.4

– 8.17

Table 2 Mechanical properties of parent metals Property

Ultimate tensile strength (MPa)

Yield strength (MPa)

Elongation (%)

AISI 430 AISI 304

488 600

380 250

28 58

129

Table 3 Experimental design matrix Trial run

Friction force (kN) (X1 )

Forge force (kN) (X2 )

Burn-off (mm) (X3 )

AF1 AF2 AF3 AF4 AF5 AF6 AF7 AF8

4 6 4 4 6 6 4 6

8 8 12 8 12 8 12 12

3 3 3 5 3 5 5 5

the form of rods of 18 mm diameter obtained from the plate material. 2.2. Friction welding Welding was performed on a continuous drive friction welding machine at a speed of 1500 rpm in a continuously and step less variable speed machine of 15 kN capacity. In the continuous drive friction welding process a stationary member is pressed against a rotating member with an axial pressure. The relative motion generates frictional heat which causes the material to soften and plastically deform. After a preset displacement (known as burn-off) has occurred, the machine is rapidly braked, and the pressure is increased to generate a high quality solid state weld. During welding the primary parameters (friction force, forge force, rotational speed and displacement) were continuously monitored and recorded. To arrive at suitable welding parameters trials were carried out with the same parameters as employed for ferritic–ferritic stainless steel and austenitic–austenitic stainless steel [5,6]. Examination of the joints revealed defects like delaminations (Fig. 2). Few more trials were carried out with different parameters in order to get defect free welds. Based on this a 23 factorial design of experiments was adopted [10]. The experimental matrix is given in Table 3. The main parameters employed are friction force, forge force and burn-off (length loss during friction/forge stage). Trial welds were made by varying one parameter keeping other parameters constant to find the limits. 2.3. Metallography Low magnification stereo microscope of Leitz make was employed for observing the bead shape. To observe deformation and microstructural features Leitz optical microscope was employed. Fractographic examination was carried out under a Leo scanning electron microscope. 2.4. Mechanical testing

Fig. 1. Optical microstructure of parent metals: (a) austenitic stainless steel (AISI 304); (b) ferritic stainless steel (AISI 430).

The mechanical tests consist of Vickers hardness, notch tensile test and Charpy ‘V’ notch impact testing. Hardness measurements included hardness survey across the interface. In notch tensile and impact tests the notch was located at the

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Fig. 2. Typical micrograph showing delamination.

interface. The location of notch is positioned at the center of the weld as shown in Fig. 3. Standard specimen configurations were employed for notch tensile and impact toughness testing (Fig. 4). Plain tensile specimen was not included as failures occurred outside the weld (Fig. 5) and data would yield only the parent metal properties. 2.5. Residual stress measurement Residual stress measurements were carried out across the interface employing X-ray stress measurement technique. The equipment employed for the purpose was model X 2002 of American stress technologies. Before measurements the flash was machined and the surfaces were polished electrolytically. Residual stresses were measured in the weld bead center region using peak shift sin2 ψ technique with Cr K␣ radiation. 2.6. Corrosion testing The weld joints and the parent metals were tested for pitting corrosion in an electrolyte of 0.5 M H2 SO4 + 0.5 M

HCl. The electrochemical measurements were made using a potentiometer dedicated for the purpose. Steady state potential was recorded 10 min after immersion of the sample in to the electrolyte and the potential was raised anodically using scanning potentiostat at a scan rate 2 mV/s. The potential at which the current increases abruptly after the passive region was taken as pitting potential (Epit ) [11]. Specimens that exhibit more positive potential value were considered to be those having better pitting resistance. 2.7. Statistical analysis of the data The mechanical property data were subjected to statistical analysis to understand the influence of individual effects of the parameters and their interactive effects on the properties. ANOVA technique of Yate’s algorithm was employed to study the significance of coefficients [10]. Regression equations were obtained from this analysis. The regression equations enabled to understand the influence of the friction welding parameters on mechanical properties. Correlation coefficient was also obtained from the statistical analysis.

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Fig. 5. Appearance of friction welded plain tensile test specimen after test.

3. Results 3.1. Metallography and visual examination

Fig. 3. Schematic diagram showing location and orientation of: (a) notch tensile sample; (b) Charpy ‘V’ notch impact sample in bond zone.

A view of friction weld joints shown in Fig. 6 exhibits higher flash with increase in burn-off and forge pressure. The flash was observed to be from ferritic stainless steel and austenitic stainless steel did not participate in the flash formation suggesting deformation is mainly limited to ferritic stainless steel side. Typical cross-sectional view of the weld (AF7) and microstructure details at the center and periphery are presented in Fig. 7 shows that the deformation is mainly confined to ferritic stainless steel. The central region consists of fine grains while peripheral region consists of coarse grains. The microstructure in the central region of the welds shown in Fig. 8 indicate that all the welds consist of fine equiaxed grains of ferrite. The grain size is in general

Fig. 4. Configuration of: (a) notch tensile specimen; (b) Charpy ‘V’ notch impact specimen.

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shown in Fig. 9. The central region consists of equiaxed grains and is confined to ferritic stainless steel. Adjacent to this region bent and elongated grains are observed on the ferritic stainless steel side. The austenitic stainless steel side consists of parallel banded features adjacent to the central equiaxed grain structure at the interface. The interface is narrow and straight in high burn-off welds. At low friction and low burn-off parallel bands are observed in the austenitic stainless steel side adjacent to the interface (Fig. 10). Fig. 6. Visual view of friction welded ferritic–austenitic stainless steel joints.

coarser in the low forge pressure welds as well as in high friction pressure welds. It is to be noted that forge pressure aids in grain refinement while, friction pressure aids grain coarsening (Fig. 8). Typical microstructural features in various regions of the weld across the interface are

3.2. Mechanical properties Hardness, notch tensile strength (NTS) and Charpy ‘V’ notch impact toughness data of the welds are presented in Table 4. The hardness of the weld region is in the range 195 Hv (min) to 270 Hv (max). Highest hardness (265 Hv) and lowest hardness (195 Hv) were obtained at low burn-off

Fig. 7. Typical friction weld and its cross-sectional view (AF7).

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Table 4 Mechanical properties of AISI 430–AISI 304 friction welds Run

AF1 AF2 AF3 AF4 AF5 AF6 AF7 AF8

Parameter (X1 –X2 –X3 )

4–8–3 6–8–3 4–12–3 4–8–5 6–12–3 6–8–5 4–12–5 6–12–5

Notch tensile strength (MPa)

Impact toughness (J)

Hardness at center

Trial 1

Trial 2

Trial 1

Trial 2

Trial 1

Trial 2

668 647 610 663 640 600 616 689

630 635 629 607 669 628 619 637

15 18 16 18 21 14 25 22

17 19 17 20 18 17 28 25

247 208 214 222 270 208 242 231

236 195 210 230 265 215 235 229

Fig. 8. Microstructure of friction welds at center: (a) AF1; (b) AF2; (c) AF3.

Fig. 9. Micrographs of friction welds at center (4–12–5).

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Fig. 11. Microhardness traverse along the bond line of a typical weld (4–12–5).

coefficient is about 0.98 in respect of hardness, 0.84 in the case of impact toughness and 0.7 in respect of notch tensile strength. Fig. 10. Micrographs of the friction weld at the interface: (a) AF6 (6–8–5); (b) AF3 (4–12–3).

and at high and low forge pressures, respectively. Impact toughness was on the higher side at high burn-off while notch tensile strength followed a reverse trend. Notch tensile strength ranged from a minimum of 600 MPa to a maximum of 689 MPa while impact toughness was in the range 15–28 J. Typical hardness distribution across the weld (AF7) shown in Fig. 11 reveals that hardness is higher on austenitic stainless steel side of the interface. Regression analysis of the data in the form of regression equations is presented in Table 5. It is observed that individual parameters of friction welding do not have an influence on notch tensile strength and only friction and forge pressure exhibit an interactive effect. Burn-off has an influence on impact toughness while forge and burn-off have an interactive effect. Hardness is influenced by forge pressure and interactive effect of other parameters. The correlation

3.3. Fractography The fractographs for low and high notch tensile strength are presented in Fig. 12. At low notch tensile strength the fracture features are of cleavage while for high NTS the features of fracture contain ductile micro-voids. Similar trends were noted in the impact specimens. 3.4. Residual stresses Residual stress data at interface on the austenitic stainless steel side are tabulated in Table 6. The magnitude of the stresses range from a minimum of 180 MPa to a maximum value of 260 MPa. On an average the stresses are maximum at high burn-off (∼241 MPa average), while they were low (∼221 MPa) at low burn-off. A consistent trend is noticed although the differences are marginal. Typical stress distribution across a weld (AF7) shown in Fig. 13 reveals that the stresses are higher on the austenitic stainless steel side of the interface. Weld center also has a stress value almost equal to ferritic stainless steel side of the interface. The stress distribution trends follow hardness distribution across the weld (Fig. 11).

Table 5 Regression equations for response function and their coefficient of correlation S. no.

Response

Regression equation

Coefficient of correlation

1 2 3

Notch tensile strength Impact toughness Hardness

Y = 636.6 + 13.68X1 X2 Y = 18.75 + 2.38X3 + 2.388X2 X3 Y = 228.56 + 8.44X2 + 12.69X1 X2 − 4.81X1 X3 − 11.89X1 X2 X3

0.707 0.84 0.98

X1 —friction force; X2 —forge force; X3 —burn-off.

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Fig. 12. Fractographs of notch tensile samples: (a) AF6 (6–8–5)—low notch tensile strength; (b) and (c) AF8 (6–12–5)—high notch tensile strength.

Table 6 Residual stress at the interface of austenitic stainless steel and weld Run

Parameter (X1 –X2 –X3 )

Residual stress (MPa)

AF1 AF2 AF3 AF4 AF5 AF6 AF7 AF8

4–8–3 6–8–3 4–12–3 4–8–5 6–12–3 6–8–5 4–12–5 6–12–5

210 260 180 230 235 250 225 260

in Table 7. Pitting corrosion studies indicate that among the welds ferritic–ferritic stainless steel joints exhibit highest pitting resistance (Epit 1023 mV) while the dissimilar joints exhibit least resistance (Epit 931 mV). Similar metal welds exhibit marginally superior performance compared to the corresponding parent materials.

3.5. Pitting corrosion A typical polarization curve is shown in Fig. 14. The pitting potential (Epit ) was used as a measure of resistance to pitting. Epit values for various types of welds are presented Table 7 Pitting potential of friction weld joints and parent metals Type of joint

Pitting potential (mV)

Ferritic–ferritic Austenitic–austenitic Austenitic–ferritic Parent metal AISI 304 Parent metal AISI 430

1023 940 931 914 988

Fig. 13. Residual stress distribution traverse across the bond line of a typical weld (4–12–5).

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sequent lowering of flow stress in this region. The coarser grain structure observed in low forge pressure combination can be attributed to lower degree of working of the material than at high forge pressure that results in higher degree of working. The central region consists of fine grains, while the peripheral region consists of coarse grains (Fig. 7). The fine grain size at the central region is due to dynamic recrystallization. The temperature of the peripheral region would be higher [17] and therefore exhibits coarse grain size. Despite the coarse grain structure at the periphery, the overall bond region remains stronger than the parent material (Fig. 7), as evidenced by the plain tensile test results shown in Fig. 5. 4.2. Mechanical properties and regression analysis Fig. 14. Typical polarization behaviour dissimilar austenitic–ferritic stainless steel weld.

4. Discussion 4.1. Deformation and microstructure The different thermal and physical properties of the materials to be welded in dissimilar metal welding (heat capacity, thermal conductivity, relation between hardness and temperature) generally results asymmetrical deformation. Austenitic stainless steel have lower thermal conductivity and greater hardness at higher temperatures compared to ferritic stainless steels. For this reason austenitic stainless steel does not undergo extensive deformation while ferritic stainless steel specimen undergoes extensive deformation. The same phenomenon has been reported during friction welding of dissimilar welds namely aluminium to copper, titanium to steel, aluminium to steel, etc. [12–16]. The formation of upset collar (flash) on the ferritic stainless steel side only is due to low strength of the ferritic stainless steel. A narrow zone of deformation bands (Fig. 7) on the austenitic stainless steel suggests that this region undergoes deformation although it does not take part in the upset collar suggesting that the deformation is not extensive. The view that only ferritic stainless steel takes part in upset collar formation is substantiated by the shortening of the ferritic stainless steel rod only. The zone of deformation band width is wider at low burn-off than at high burn-off could be attributed to lower heat content and high flow stress of the region at low burn-off than when the burn-off is high that aids in the spread of heat resulting in increased heat content and con-

The mechanical and thermo-physical properties of dissimilar substrates will have a major influence on the properties of the dissimilar joints because the temperature attained by each substrate markedly depends on the thermo-physical properties of the two substrates and on the joining parameters selected. Consequently, the flow stress–temperature relations for each substrate will have an important influence on the joint properties produced during friction welding. In general high forge pressures resulted in high toughness and notch tensile strength. Fine grain structure exhibited high strength and low toughness while coarse grain microstructure exhibited a reverse trend. Fracture features of the notch tensile and impact specimens are as per expected trends, in that at low strength and toughness the fracture is predominantly cleavage as against high energy ductile micro-void fracture when the strength and toughness are high. It is opined that the grain size in the weld region dictates the mode of fracture in that fine grain gives raise to ductile fracture while, coarse grain promotes cleavage fracture. A comparison of properties of similar material combination welds with the dissimilar combination welds (Table 8) shows that the properties of austenitic stainless steel–ferritic stainless steel compare well with the ferritic stainless steel welds. This further substantiates that the deformation is confined to ferritic stainless steel and the interface properties are dictated by the properties of ferritic stainless steel alone. Analysis of mechanical property data suggest that hardness is dictated by forge pressure and an interactive effect of friction and forge pressure while toughness is controlled by the interaction of friction pressure and burn-off. This suggests that toughness is dictated by the heat content

Table 8 Comparison of notch tensile strength and impact toughness of similar and dissimilar combinations of austenitic and ferritic stainless steel Material

Parent metals Welds

Austenitic stainless steel

Ferritic stainless steel

Austenitic–ferritic stainless steel

Notch tensile strength (MPa)

Impact toughness (J)

Notch tensile strength (MPa)

Impact toughness (J)

Notch tensile strength (MPa)

Impact toughness (J)

830–896 693–753

213–214 71–148

547–590 660–784

6–7 8.5–30

– 600–697

– 16–28

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and spread of heat. Higher burn-off aids in spread of heat due to the availability of friction time. These conditions aid in stabilizing the microstructure as well and therefore favour toughness improvement at higher burn-off. Hardness and notch tensile strength are mostly influenced by an interactive effect of friction and forge pressures, which are a combination of heat input and degree of working. 4.3. Pitting corrosion The excellent improvement in the pitting corrosion resistance of the welds compared to parent metal could be due to the presence of dynamically recrystallized microstructure and possible composition uniformity resulting from faster cooling rates that do not favour elemental segregation. 4.4. Residual stresses Higher residual stresses at the austenitic stainless steel side of the interface can be attributed to the higher flow stress of the austenitic stainless steel aided by higher coefficient of thermal expansion of the austenitic stainless steel. Higher flow stress of austenitic stainless steel resulted in the deformation confined to a very narrow region consisting of deformation bands. These deformation bands are likely to consist of high density of dislocations and hence the observed higher residual stress peak is confined to this region. Penetration depth of the X-rays is of the order of 20 ␮m and the stresses measured are within ±15 MPa. 5. Conclusions (i) Continuous drive friction welding has been used to successfully join austenitic–ferritic stainless steel. (ii) In friction welding of austenitic–ferritic stainless steel, deformation is confined to ferritic stainless steel only. (iii) Higher forge pressure combinations exhibit fine grain size and increased friction pressure aids in grain coarsening. (iv) The mechanical properties of austenitic–ferritic stainless steel welds are similar to ferritic stainless steel welds. (v) The toughness and strength properties of dissimilar metal welds are better than ferritic stainless steel parent metal. (vi) Notch tensile strength, hardness and impact toughness can be expressed in terms of the process parameters by regression equation obtained by statistical analysis.

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Acknowledgements The authors express their gratitude to Defence Research and Development Organisation for the financial support to carry out this programme. The authors are thankful to Dr. D. Banerjee Director, DMRL for his continued encouragement. One of the authors (V.V. Satyanarayana) is thankful to the Principal and the management of Vasavi College of Engineering, Hyderabad for their continued support during this work.

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