Effect of heat treatments on resistance spot weldability of TRIP 980 steel

Cold rolled TRIP 980 steel sheets were resistance spot welded by different welding procedures. The effects of welding current, preheating and postweld heat treatments on the resistance spot weldability were studied. The results show that the increasing welding current increases the nugget diameter and tensile shear strength of spot weld. However, when the welding current is large enough to cause spatter, the nugget diameter and tensile shear strength of spot weld decrease. The preheating process raises the spatter occurring current, which results in larger nugget diameter and tensile shear strength. Under the condition of adding postweld heat treatment to spot weld, when the cooling time between welding current and postweld heat treatment current was larger than 900 ms, the microstructure of weld nugget was improved significantly, which decreased the hardness of weld nugget and increased the tensile shear strength of spot weld.