Heat treatment after welding of DP۹۸۰ steel: effect of heat treatment parameters onmicrostructure and mechanical properties of resistance spot welding

Effect of in-situ Post Weld Heat Treatment (PWHT)parameters on microstructure and mechanical properties ofresistance spot welded DP۹۸۰ steel was investigated.Samples were welded at ۸.۵ kA and ۴ levels of PWHT current(۳.۵ to ۵ kA, with ۰.۵ kA steps) at a constant PWHT time (۸۰cycles) and ۳ levels of PWHT time (۱۰, ۵۰ and ۹۹ cycles) ata constant PWHT current (۴ kA). Results showed that whileno noticeable martensite tempering occurred at ۳.۵ kA PWHTcurrent, PWHT at ۴ kA resulted in a fully temperedmicrostructure consisting mostly of a ferritic matrix withdispersed carbide particles. However, higher PWHT currentsincreased heat input and consequently temperature at thewelded region up to the austenite formation temperature,resulting in formation of new martensite phase upon cooling.Tensile test results showed that at PWHT current of ۴ kA, thejoint has the highest peak load (load bearing capacity) andfracture energy (۱۳.۳ kN and ۱۵.۱ J, respectively). For PWHTtime, both peak load and fracture energy increased withPWHT time (up to ۱۴.۳ kN and ۱۵.۷ J, respectively) andmicrostructure consisted mainly of ferrite and carbideparticles. To summarize, PWHT enhanced mechanicalproperties and reduced martensite phase in microstructure.However, PWHT did not change the fracture mode andinterfacial fracture occurred in the as-welded and all PWHTsamples.

کلیدواژه ها:

DP Steel ، Post Weld Heat Treatment ، ResistanceSpot Welding ، Martensite Tempering ، Interfacial Fracture