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I.

INTRODUCTION

AUSTENITIC welded steels may become sensitized in heat-affected zones (HAZ), which causes localized intergranular corrosion at the weld site, although the cooling rate of the metal is sufficiently high to avoid carbide precipitation.[1] Susceptibility to intergranular corrosion in austenitic stainless steels can be avoided by limiting their carbon contents or by adding such elements as V, Nb, Ti, Zr, Hf, and Ta, whose carbides are more stable than those of chromium. At temperatures above 1088 K (815 C), these stable MC-type carbides remove carbon from solid solution and prevent the precipitation of M23C6-type carbides, mainly Cr23C6.[1,2] In comparison with M23C6, MC carbide formation is associated with lower interfacial energy, and thus, it is less harmful for the mechanical properties, such as creep resistance. In austenitic stainless steels aged between 873 K and 1073 K (600 C and 800 C), the growth rate of MCs is lower than that of M23C6.[3] In addition, MCs serve as strengtheners,[4] although it is known that excessive alloying of Nb and Ti may lead to ductility loss.[5,6] Despite the addition of MC-forming

PAUL ROZENAK, formerly Director with the Hydrogen Energy Batteries, Ltd., POB 195, 84965 Omer, Israel, was also Researcher with the Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, POB 653, Israel. YAAKOV UNIGOVSKI, Research Professor, and RONI SHNECK, Professor, are with the Department of Materials Engineering, Ben-Gurion University of the Negev. Contact e-mails: [email protected], [email protected] Paul Rozenak—deceased. Manuscript submitted March 29, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A

elements, complete suppression of M23C6 might be unachievable, as pointed out by Padilha and Rio.[7] Post-heating of stainless steels is performed for a complete solution annealing (>1273 K (>1000 C)), if a rapid cooling during a part of the cooling cycle is required to avoid unacceptable precipitation. However, it should be noted that this will create new stresses of considerable magnitude. In such circumstances, a lower temperature treatment (at around 723 K (500 C)) giving a partial stress relief without the precipitation of second phases may be preferable. The need for a stress-relief treatment in welded austenitic stainless steels has been over-emphasized in the past. Shielding gas protects the weld pool from adverse reactions with atmospheric environment and stabilizes the arc in the welding process. Besides, it affects the mechanical properties of the weld area, such as strength, toughness, hardness, as well as the corrosion resistance of the metal. Shielding gas affects the formation of weld defects, e.g., pores, which can be initiation points in crack formation and propagation and considerably decrease the lifetime of welded structure, for example, under a cyclic load.[8] It is well known that austenite in stainless steels partially transforms into a¢ (bcc) and e (hcp) martensite during plastic deformation[9–11] or under the conditions of h