科研之友
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摘要
The purpose of this paper is to research the effect of heat input on the microstructure and mechanical properties of DP780 and DP980 dissimilar steel welded joints by using laser welding and analyze the transformation mechanism from austenite to martensite in laser welding. The distribution of martensite volume fraction in each region was similar to the hardness value distribution. The hardness of the fusion zone was significantly improved by the laser welding, but the highest hardness value of the dissimilar steel welded joint appeared in the hardening zone on the DP980 side. At the same time, there was an obvious softening phenomenon at the side of DP980, but the minimal hardness value of DP780 and DP980 dissimilar steel welded joints appeared in the sub. critical HAZs on the side of DP780. This was why the DP780 and DP980 dissimilar welded joints were broken at the DP780 side of the sub-critical HAZ. Compared with the DP780 base metal and DP980 base metal, the plastic deformation and strength-ductility of DP780 and DP980 dissimilar welded joints were significantly reduced. The largest deformation zone of the dissimilar steel welded joints occurred in the heat-affected zone of the DP780 side. The elongation of the heat-affected zone of the DP780 side decreased with decreasing heat inputs. In addition, through the analysis of the transformation mechanism of austenite to martensite for DP780 and DP980 steels, when the peak temperature was lower than Ac-1, the tempered martensite led to DP steels softening. When the peak temperature was higher than Ac-1 but lower than Ac-3, the decrease of martensite content of samples resulted in DP steels softening. When the peak temperature was higher than Ac-3, the DP steels began to harden, and the microstructure was dominated by massive martensite and lath martensite.
