Microsturcture and Mechanical Properties of Resistance Spot Welded Joint of Q&P980 Ultra High Strength Steel
Taking the 1mm thick Q&P980 steel plate as the object,the appearance,microstructure and mechanical properties of the resistance spot welded joint were investigated.The results show that when the welding current is 12.5 kA,welding time is 10 cyc,the electrode pressure is 0.3 MPa,the appearance of the spot welded specimen is good without external splash,and it only has slight internal spatter.The microstructure of the fusion zone is coarse lath martensite,and the microstructure of the coarse grain and fine grain heat affected zones is lath martensite,a small quantity of ferrite,and the lath martensite size is smaller than that of the fusion zone.The microhardness of the above zones is obviously higher than that of the base metal.The intercritical heat affected zone has a small amount of tempered martensite,which results in slight softening.The tensile fracture mode of the welded specimen is button fracture,the fracture is divided to crack initiation zone,crack extension zone and transient crack zone.Mixed fracture occurs in the crack initiation area,and ductile fracture occurs in crack extension zone and transient crack zone.
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