The weldability of molybdenum containing ultralow carbon bainitic steels.

摘要

Ultra-low-carbon bainitic (ULCB) steels, which can achieve a 550-MPa (80-ksi) yield strength and excellent low-temperature toughness through controlled rolling, microalloying additions, and a low-carbon bainitic structure, are designed to be competitive with HY and HSLA steels for welded fabrication. The low carbon content of these steels results in a relatively soft heat-affected zone (HAZ) structure which is resistant to hydrogen-induced cracking.;A series of ultra-low carbon, Ni-Mo steels was examined in order to determine the optimum composition for base-metal and HAZ properties. Based on these results, an experimental heat of 3Ni-1.5Mo-0.02C ULCB steel was melted and controlled rolled to 13-mm (0.5-in.) thick plate. The weldability of this steel was evaluated using weld simulation and direct weldability tests. The HAZ transformation characteristics of this steel were also evaluated using optical and scanning-electron metallography and hardness testing. The results of weldability tests were compared to similar results for HY and HSLA steels.;The maximum simulated HAZ hardness of the ULCB steel, HV 244, was only slightly greater than the base-metal hardness of HV 220. As a result, no HAZ cracking was observed for augmented strain cracking tests of the ULCB steel when welded without preheat. The coarse-grained HAZ toughness of the ULCB steel was superior to that of HY-80 at impact test temperatures of ;The results of stress-relief cracking and hot-ductility tests indicated that the weldability of the experimental ULCB steel should be equivalent to, and in some cases, better than, that of production heats of HY-80. The results of HAZ transformation studies indicated that the ULCB steel undergoes a primarily bainitic transformation for cooling-time values of t