Effects of heat input on microstructure and fracture toughness of simulated coarse-grained heat affected zone for HSLA steels

In order to clarify the relationships among welding heat input, microstructural evolution and fracture toughness of coarse-grained heat affected zone (CGHAZ) for HSLA steels, the microstructure of simulated CGHAZ with heat input of 10–100 kJ/cm was quantified and the crack tip opening displacement (CTOD) at −20 °C was evaluate. With the increase in heat input, the bainite transformation temperature and duration transformation time increase, which lead to the higher contents of granular bainite and M-A constituents. The fracture toughness of simulated CGHAZ is highly related with the morphologies of M-A constitute. During CTOD test, the island M-A constituents can arrest microcracks and increase the CTOD value, and the fine-stringer ones can deform without excessive damage to fracture toughness. However, the coarse-stringer and massive M-A constituent would crack and debond from the matrix, which seriously decreases the fracture toughness. When the heat input increases from 10 kJ/cm to 100 kJ/cm, the proportion of island M-A constituent decreases from 86% to 35%, while that of coarse-stringer and massive M-A constituent increases from 1% and 6% to 7% and 57%, respectively, which result in the significant deterioration of fracture toughness. The fracture toughness is optimal at heat input of 30 kJ/cm, because the packets of lath bainite are segmented smaller by granular bainite and effectively improved the cracking resistance.