Granular bainite in microstructures after welding and its effects on impact toughness in STE355 steels

The effects of technology parameters of stimulated welding thermal cycle (peak temperature T_max and cooling time t_8/5) on microstructures and impact toughness of HAZ in low carbon microalloying Ti Nb steel STE355 were studied.The microstructures were etched by special interference technology and granular bainite and M-A constitucents were shown clearly in optical microscope.The observations were proved by electron microscope.The results show that coarsening of microstructures and decrease of toughness must take place with raising T_max and increasing t_8/5(lowering cooling rate).Growing and thickening of ferrite in boundary and in grains and growing of M-A constituents are detrimental to toughness improvement.