Effect of Different Chromium Additions on the Microstructure and Mechanical Properties of Multipass Weld Joint of Duplex
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stainless steels (DSS) are widely used in many environments and operating conditions, such as in the marine, chemical, petrochemical, nuclear, fertilizer, and food industries, because of their superior performance in comparison to traditional austenitic stainless steel. DSSs have higher strengths than austenitic stainless steels, higher toughness levels than ferritic stainless steels, good weldability, and high resistance to stress corrosion cracking. These good properties depend on the two-phase microstructure that consists of approximately equal volume fractions of c-austenite and d-ferrite phases.[1–4] Since the development of first-generation DSSs in the 1930s, continuing research effort have been exerted to improve both mechanical and corrosion properties, particularly by controlling alloying elements of Cr, Mo, N, W, etc.[5–8] Currently, however, most of the research on DSS has been performed on the wrought products, and only a limited number of studies has been reported on the weld products of DSSs, despite their DONG HOON KANG, Master Degree, and HAE WOO LEE, Professor, are with the Department of Materials Science and Engineering, Dong-A University, Saha-gu, Busan 604-714, Republic of Korea. Contact e-mails: [email protected]; [email protected] Manuscript submitted December 18, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS A
significance in the industry. Large industrial applications of duplex stainless steels utilize welding as a manufacturing process. Therefore, it is also very important to maintain an optimum phase balance in weld metal after welding. However, welding of DSS results in a disturbed phase balance, and this problem is more significant in flux-cored arc welding (FCAW) processes because of their low heat input and very fast cooling rates.[9] The resultant weld metals contain higher ferrite levels and have been reported to have inferior properties.[10] In order to restore the toughness of welded connections, weld filler materials are usually overalloyed with 2 to 4 pct more Ni than in the base material.[11] During recent years, the authors studied the mechanical behaviors of DSS welds with varying alloying elements and reported that the mechanical properties were strongly influenced by the change in the shape and volume fraction of austenitic phase with different amounts of additions. For example, Muthupandi et al.[12] examined the effect of nickel and nitrogen addition on the microstructure and mechanical properties of DSS weld metals, and they found that not only does the introduction of Ni or N not influence the hardness of resultant weld metals, but also these elements improve the impact toughness substantially at subzero temperature. Park and Lee[13] studied the effect of nitrogen and heat treatment on the microstructure
and tensile properties of 25Cr-7Ni-1.5Mo-3W-xN DSS castings, and consequently, it was revealed that the increase of ferrite volume fraction decreases the tensile strength and the elongation, and it increases the yield strength of the casting linearly. Some authors[14] have reported
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