Effect of preheating on microstructure and low-temperature toughness for coarse-grained heat-affected zone of 5% Ni steel joint made by laser welding

Five percent Ni steel plates were welded by fiber laser after preheating. Preheating temperature was 200 degrees C, 350 degrees C, and 450 degrees C to adjust cooling rate of coarse-grained heat-affected zone (CGHAZ). Thermal simulation was used with Gleeble machine to investigate the microstructure evolution and low-temperature impact toughness of CGHAZ. With the increment of preheating temperature, the microstructure of the CGHAZ changed from lath martensite (LM) to the mixture of lath martensite and lath bainite (LM/LB), and finally to granular bainite (GB). The volume fractions of retained austenite in all CGHAZs were less than 0.2% no matter preheating was used or not. Impact toughness test was carried out at temperature from - 90 to - 170 degrees C. Among the preheated and non-preheated specimens, the CGHAZ of specimens preheated at 350 degrees C had best impact toughness. The optimal cooling rate for the CGHAZ of 5% Ni steel was about 15 degrees C s(-1). It was found that the low-temperature impact toughness could be improved by the high density of high misorientation packet and block boundaries in lath bainite/lath martensite. The crack propagation could be depressed by large-angle grain boundaries to some extent.