Materials Transactions, Vol. 52, No. 8 (2011) pp. 1709 to 1712 #2011 The Japan Institute of Metals RAPID PUBLICATION Interface Characteristic and Properties of Stainless Steel/HSLA Steel Clad Plate by Vacuum Rolling Cladding Guangming Xie* , Zongan Luo, Guanglei Wang, Liang Li and Guodong Wang The State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, P. R. China 304 austenite stainless steel plate and HSLA steel plate were subjected to vacuum rolling cladding (VRC). The microstructure characteristic and properties of the clad interface were investigated in detail. During the VRC process, the clad interface was held at a high vacuum all the time, resulting in significantly decreased oxidation at the interface. A small amount of very fine oxide particles (diameter < 0:5 mm) can be found at the clad interface, and no obvious defects were observed. The remarkable diffusion of Ni and Cr elements led to the formation of a transition zone with high hardness. The shear strength of the clad interface was as high as 487 MPa, and the fracture occurred at the HSLA steel near the transition zone. No cracks were found at the clad interface after the 180 bending tests with both outside and inside manners. [doi:10.2320/matertrans.M2011127] (Received April 25, 2011; Accepted May 30, 2011; Published July 13, 2011) Keywords: vacuum rolling cladding, clad plate, interface, microstructure, mechanical properties 1. Introduction Stainless steel clad plates are widely used in chemical and nuclear industries in order to take advantage of the corrosionresistant and reduce the cost. The stainless steel clad plate was fabricated by cladding a stainless steel on the surface of the high strength low alloy (HSLA) steel.1) Recently, explosive cladding,2,3) rolling cladding4,5) and weld overlay cladding6–9) have been used to produce the clad plates. Explosive cladding is a widely applied technique to produce the clad plates, especially for the metals with dissimilar melting points. However, the explosive clad plate has bad flatness, and a hot rolling process is usually needed to improve the flatness quality.2,3) The clad plate with good flatness can be fabricated by rolling cladding, but it is difficult to control the interface oxidation, resulting in the significantly reduced bonding strength. Vacuum rolling cladding (VRC) was invented at Japan’s Kawasaki Heavy Ind. Ltd. in 1981.10) Figure 1 shows the concept of VRC. Two metal plates are surface-treated and made up into a built-up slab and the electron beam welding (EBW) was performed at the four sides (Figs. 1(a) and (b)). The built-up slab is hot-rolled to produce a clad plate (Fig. 1(c)). The surface treatment removes the scale and other foreign matter, and the EBW ensures the high vacuum in the built-up slab and avoids interface oxidation during heating and hot rolling. During the VRC, by severe rolling deformation at the high vacuum and high temperature, sound metallurgical bonding of the two metals was achieved at the clad interface. Recently, another rolling cladding method under a vacuum atmosphere was reported widely.11,12) Compared to the VRC, both heating and rolling processes of two metal plates were conducted in a vacuum chamber with the high vacuum. However, the large clad plate is difficult to produce by this method and the rolling capability of rolling mill, i.e., the rolling reduction rate, is also limited due to the small chamber volume. The Ti/stainless steel clad plate and aluminum alloy/stainless *Corresponding author, E-mail: xiegm@ral.neu.edu.cn Fig. 1 Schematic presentation of the VRC process for stainless steel/ HSLA steel clad plate: (a) Surface treatment and stacking, (b) EBW and heating, and (c) hot rolling. steel clad plate were obtained by this method,11,12) but no studies on the HSLA steel/stainless steel clad plate were reported. Currently, there are few investigations on the VRC. In 2004, JFE Steel Corporation reported the commercial production of the stainless steel clad plate by VRC, but no detailed microstructure characteristic of the clad interface was investigated.13,14) In our previous work, clad plate of two same carbon steel plates was achieved by VRC and exhibited sound mechanical properties.15) In this study, we investigated on the correlation between the interface microstructure and the mechanical properties of the clad plate of 304 stainless steel and HSLA steel by VRC. 2. Experimental Procedure 304 austenite stainless steel and HSLA steel plates with dimension of 200 mm 100 mm 10 mm and 200 mm 100 mm 50 mm, respectively, were clad by VRC. The chemical compositions of the 304 stainless steel and the HSLA steel are given in Table 1. The surfaces of stainless steel and HSLA steel were machined by the milling machine to reveal the fresh metal surface. EBW was performed at a welding current of 30 mA, a welding voltage of 80 kV and a welding speed of 200 mm/min with vacuum degree of 1 10 2 Pa. Hot rolling was carried out after soaking the built-up slab at 1200 C for 60 min. The thickness of the clad plate was reduced from 60 mm (10+50) down to 12 mm (2+10) after 4 passes hot rolling with a reduction ratio of 24% in each pass. 1710 G. Xie, Z. Luo, G. Wang, L. Li and G. Wang Table 1 Elements C Si Mn Chemical composition of 304 stainless steel and HSLA steel (mass%). P S Cr Ni Al Cu Ti Nb V Fe Stainless steel 0.06 0.26 1.73 0.003 0.003 19.2 9.8 — — — — — Bal. HSLA steel 0.05 0.36 1.45 0.005 0.004 0.15 0.21 0.029 0.29 0.017 0.037 0.0026 Bal. The clad interface sample was etched with a solution of 5 mL nitric acid and 95 mL ethanol. Microstructural features were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), equipped with energy dispersive spectroscopy (EDS). Element distribution analysis was carried out on a JEOL-8530 electron probe microscopic analyzer (EPMA). The Vickers hardness tests were performed on a Future Tech-type Vickers hardness machine with a 100 g load for 13 s. The bend and shear specimens were machined perpendicular to the rolling direction with a gauge length of 150 mm, gauge width of 10 mm and gauge thickness of 8 mm (2+6). The shear tests were accorded with the ASTM standard.16) The bend and shear tests were carried out using an Instron-type universal testing machine at a strain rate of 1 10 3 s 1 . 3. Results and Discussion Figure 2(a) shows the cross-sectional structural photograph of the stainless steel/HSLA steel clad plate. Relatively flat clad interface was achieved in the clad plate, which was different from the wave-like explosive clad interface originated from the concentrated plastic deformation in the localized contact zone.2,3) From the magnified view of the clad interface shown in Fig. 2(b), a small amount of very fine particles (diameter < 0:5 mm) were distributed at the clad interface, and the typical TEM morphology of the fine particle was shown in Fig. 2(c). A transition zone about 5 mm in width was observed between the stainless steel and HSLA steel from the contrast differences, and no obvious defects were observed at the interface. By comparison, many distinct defects were observed at the interface of the clad plate by explosive cladding and weld overlay cladding, resulting in significantly decreased bonding properties.2,6) Rao et al.3) reported that the un-bonded zone exceeded 20 mm, and melt pocket (10 mm) were found at the interface of the explosive 304 stainless steel clad plate. For the weld overlay clad procedure, various defects such as blow, pin hole and entrapped slag were easily formed at the clad interface.6) Certainly, interface defects have a significant effect on the interface bonding of the clad plate. Figure 3(a) shows the EDS result by TEM of the fine particle shown in Figs. 2(b) and (c). Besides the matrix elements of Fe and Cr, very high content of Mn, Si and O elements were clearly detected on the fine particle. Therefore, the fine particles are more probably oxides according to the results of Masahiroe et al.17) They indicated that the Mn-Si-O oxide was easily formed on the surface of steel containing Si and Mn elements due to the selection oxidation. For the conventional hot rolling cladding technique, however, the clad metals significantly reacted with the atmospheric oxygen, resulting in the formation of an obvious oxide film at the clad interface during heating.4) Morizono et al.18) reported that the Fig. 2 Micrographs of the clad interface of stainless steel/HSLA steel clad plate: (a) SEM backscattered electron image of clad interface, (b) magnified view of clad interface, and (c) TEM image of fine particle near clad interface. Fig. 3 EDS results of (a) fine particle, and (b) transition zone. high temperature exposure led to obvious interface oxidation in explosive welded titanium/430 ferritic stainless steel clad plate. In this study, though the high vacuum technique was used during the VRC process, the micro-oxidation still existed at the clad interface. Firstly, few Fe-O oxides might be formed at the surface of plates during the surface-treating (Fig. 1(a)). Then, the easily oxidized elements of Mn, Si would react with the O element in the Fe-O oxides during the VRC, producing the Mn-Si-O oxide particles. Simultaneously, the severe plastic deformation by hot rolling made these oxide particles broken and dispersed near the clad interface shown in Fig. 2(b), reducing the size of the oxide particles. Therefore, the fine particles near the clad interface should be the Mn-Si-O oxides, but the influence of the oxide particles on the mechanical properties should be very little due to the fine size and the limited amount. Interface Characteristic and Properties of Stainless Steel/HSLA Steel Clad Plate by Vacuum Rolling Cladding 1711 Table 2 Vickers hardness of clad plate. Position Stainless steel HSLA steel Transition zone Vickers Hardness, Hv 204 135 381 Fig. 6 Macrographs of the clad plate after bending test: (a) stainless steel faces outward to bend, and (b) stainless steel faces inward to bend. Fig. 4 Element distribution maps of the clad interface by EPMA. Fig. 7 (a) Failure location and (b) fracture surface morphology of the clad interface after shear test. Fig. 5 Line analyses of Ni and Cr elements at the clad plates by EPMA (the analysis line is shown in Fig. 4(a)). According to the EDS result shown in Fig. 3(b), the transition zone consisted of Cr, Mn, Fe, Ni and C elements, and exhibited a transitional chemical composition between that of the stainless steel and HSLA steel shown in Table 1. A high concentration gradient of Cr and Ni elements existed at the interface zone between the stainless steel and HSLA steel during the VRC process, thereby resulting in the remarkable diffusion of Cr and Ni elements. Therefore, the formation of the transition zone with 5 mm width (Fig. 2(b)) was related with the element diffusion. Figures 4 and 5 show the map and line distributions of Ni and Cr elements characterized by EPMA. It is clear that Ni and Cr elements diffused from stainless steel to the HSLA steel, and this would lead to the movement of the CCT curve to the right side and decreasing of critical cooling rate of martensite formation. Therefore, some austenite phase transformed to martensite phase under the air cooling during VRC process. Li et al.19) indicated that martensite formed in the transition zone of arc welded 308 austenite stainless steel/HSLA steel due to the diffusion of Cr and Ni elements. Similarly, fine martensite phase was also found by Tosto et al.7) in the interface of the weld overlay clad 316 stainless steel and mild steel. In this study, the transition zone exhibited a high Vicker’s hardness value (381 Hv), which was much higher than that of the stainless steel and HSLA steel (Table 2). Therefore, the martensite structure should very probably form at the transition zone during the VRC process according to the similar microstructure and hardness value to those reported by Li et al.19) Detailed microstructural examinations and discussion about the martensite structure in the transition zone will be presented elsewhere. From Figs. 4 and 5, it was observed that Cr and Ni elements diffused significantly from the stainless steel side to the HSLA steel side with the diffusion distances of 13 mm and 8 mm, respectively. Diffusion coefficients of Ni and Cr in -Fe at the rolling temperature of 1180 C were calculated to be approximately 5:7 10 11 and 4:6 10 11 cm2 /s, respectively.20) Although Ni has a little higher diffusion coefficient, the diffusion distance of Ni was lower than that of Cr. This should be attributed to the obviously different concentration gradients for Cr and Ni, which are 128 times and 47 times, respectively. Figure 6 shows the specimens of the clad plates after bending test. No crack was observed in the clad interface after bended to 180 in both outside and inside bend tests, which was in accord with the ASTM standard.16) The shear strength of the clad interface reached to a relatively high value of 487 MPa, which was much higher than 140 MPa of stainless steel clad plate in the ASTM standard and exceeded the shear strength of the stainless steel clad plate (380 MPa) by VRC, reported by JFE Steel Corporation.14) The failure location and fractural surface morphology after shear tests were presented in Fig. 7. The fracture occurred in the HSLA steel and the fracture surface consisted of the predominant 1712 G. Xie, Z. Luo, G. Wang, L. Li and G. Wang fine dimple rupture. Though martensite phase is a brittle phase, we think that only few fine martensite phases formed at the transition zone according to the study of Tosto et al.7) In this case, the transition zone may be strengthened by the few fine martensite phase. Therefore, the ductile fracture showing fine dimples occurred at the HSLA steel side with the lowest hardness near the transition zone. Kacar et al.2) reported that during explosive clad of 316L stainless steel/ HSLA steel, the shear strength of 380 MPa was obtained due to the occurrence of the melt zone. Rao et al.3) indicated that the shear strength of the stainless steel/HSLA steel clad plate by explosive cladding and hot rolling was 384 MPa and some rubbed fracture features with a bamboo-like appearance were observed. The low shear strength was attributed to the interface oxidation during hot rolling. Therefore, the interface microstructure characteristic plays an important role in the mechanical properties of the clad plate. In this study, under the combination of high vacuum, high temperature and high rolling deformation, significantly reduced interface oxidation was achieved and no obvious defects were found at the clad interface, resulting in the sound metallurgical bonding. Anyhow, obtaining a defect-free clad interface is the key for obtaining the high-property clad plate in any cladding processes. 4. Conclusions The sound 304 austenite stainless steel/HSLA steel clad plate was successfully fabricated by vacuum rolling cladding. A small amount of fine oxide particles were found at the clad interface. The transition zone was formed by the diffusion of Ni and Cr. Excellent mechanical properties were achieved in the stainless steel clad plate. No crack was observed at the clad interface after bended to 180 with both inside and outside manners. Shear strength of the clad plate reached to 487 MPa, and the obvious ductile fracture occurred in the HSLA steel with lowest hardness near the transition zone. Acknowledgements This work was supported by the Fundamental Research Funds for the Chinese Central Universities N090307003. REFERENCES 1) I. F. Lancester: Metallurgy of Welding, (Chapman&Hall, 1984) pp. 286–289. 2) R. Kacar and M. Acarer: Mater. Sci. Eng. A 363 (2003) 290–296. 3) N. V. Rao, D. S. Sarma, S. Nagarjuna and G. M. Reddy: Mater. Sci. Techol. 25 (2009) 1387–1396. 4) H. Y. Wu, S. Lee and J. Y. Wang: J. Mater. Process. Technol. 75 (1998) 173–179. 5) X. K. Peng, G. Heness and W. Y. Yeung: J. Mater. Sci. 34 (1999) 277– 281. 6) N. V. Rao, G. M. Reddy and S. Nagarjuna: Mater. Design 32 (2011) 2496–2506. 7) S. Tosto, F. Nenci and H. Jiandong: J. Mater. Sci. 29 (1994) 5852– 5858. 8) P. K. Ghosh, P. C. Gupta, M. Breazu and P. K. Gupta: Trans. Japan Inst. Metals 28 (1987) 579–584. 9) T. Ishda: J. Mater. Sci. 26 (1991) 6431–6435. 10) Y. Toshio: Patent Abstracts of Japan, Publication Number: 56-011189. 11) K. Takayuki, M. Masakazu, N. Kazumasa, K. Mitsuaki and H. Masahiro: J. JWS. 22 (2004) 300–308 (in Japanese). 12) J. C. Yan, D. S. Zhao, C. W. Wang, L. Y. Wang, Y. Wang and S. Q. Yang: Mater. Sci. Technol. 25 (2009) 914–918. 13) S. Nishida, T. Matsuoka and T. Wada: JFE Technical Report 5 (2005) 1–9. 14) http://www.jfe-steel.co.jp/en/products/plate/05-koucyo.html. 15) G. M. Xie, Z. A. Luo, H. G. Wang, G. D. Wang and L. J. Wang: Adv. Mater. Res. 324 (2010) 97–101. 16) ASTM, Designation A 263-09. 17) N. Masahiro, H. Ikuro, K. Shinji, K. Manabu and O. Yoshinobu: Tetsu To Hagane 92 (2006) 378–384. 18) Y. Morizono, M. Nishida, A. Chiba and K. Imamura: J. Iron Steel Inst. Jpn. 85 (1999) 340–345. 19) G. F. Li, E. A. Charles and J. Congleton: Corros. Sci. 43 (2001) 1963– 1983. 20) K. E. Rhelning: Steel and Its Heat Treatment, 2nd ed, (London Butterworths, 1984) pp. 25–26.