Influence of In Situ Thermal Processing Strategies on the Weldability of Martensitic Stainless Steel Resistance Spot Wel
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I.
INTRODUCTION
MARTENSITIC stainless steels (MSSs) are a cost-effective candidate for applications that require a combination of high strength and corrosion resistance under moderate conditions.[1] For instance, martensitic stainless steels are recently considered as an alternative materials solution to existing C-Mn boron hot-stamp steels for automotive applications.[2–5] However, their applications can be limited due to their weldability issues. It is well known that martensitic stainless steel welds are notorious for their susceptibility to low-energy failure due to the formation of brittle martensitic structure in the fusion zone and the heat affected zone.[1,6,7] The post-weld tempering is a key approach
HAMIDREZA AGHAJANI and MAJID POURANVARI are with the Department of Materials Science and Engineering, Sharif University of Technology, Tehran 11365-9466, Iran. Contact email: [email protected] Manuscript submitted June 3, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
for improving the mechanical performance of the MSS weldments via tempering of the martensite.[1] Tempering heat treatment in steels is generally carried out at temperatures between 150 C and 700 C.[8] The several overlapped stages involving segregation of carbon to lattice defects and the precipitation of carbides, the decomposition of retained austenite, and the recovery and recrystallization of the martensitic structure occur during tempering of steels.[9] MSSs exhibit complex physical metallurgy. Therefore, inappropriate tempering heat treatment can lead to a loss of its properties. It has been reported[10–15] that tempering of MSSs in the temperature range of 450 to 600 C can cause intergranular fracture with a drastic drop in impact notch toughness. This phenomenon, known as the tempered embrittlement (TE), has been attributed to multiple mechanisms including grain-boundary segregations of impurities (e.g., S and P), carbide precipitation, and intragranular network of delta ferrite at prior austenitic grain boundaries.[10] Resistance spot welding (RSW) is the primary process in joining of sheets in the automotive industry.[16–22] Due to ultra-fast cooling rates in RSW process, the
pre-designed microstructure of the steels is destroyed after spot welding. Therefore, the quality and mechanical behavior of spot welds significantly affect the reliability of the vehicles under crash conditions.[23,24] In recent years, resistance spot welding of MSSs has been widely studied.[25–29] Previous works confirmed that MSSs exhibited inferior weldability in terms of peak load and energy absorption during both the tensile-shear (TS) and the cross-tension (CT) loading due to the formation of hard and brittle martensite. Therefore, in order to implementation of MSSs in the automotive industry, there is a need to find a proper approach to improve the mechanical properties of their resistance spot welds. It has been established that the cross-tension properties of spot welds with high hardness values in the weld nugget (e.g., in the case of AISI420
