The significant impact of introducing nanosize precipitates and decreased effective grain size on retention of high toughness of simulated heat affected zone (HAZ)

Abstract

Abstract The study of toughening mechanism associated with the heat-affected zone (HAZ) continues to be of significant interest for improving the low temperature (−40 °C) toughness. In this study, the effect of different heat input on microstructure, hardness and low temperature impact toughness of coarse-grained HAZ (CGHAZ) in a high-strength low-alloy (HSLA) plate steel was explored by using welding thermal simulations. After different welding thermal cycles with heat input of 60, 90, 120, and 180 kJ cm−1, average effective grain size of 5.7, 5.9, 6.2, and 6.7 ± 0.2 μm, were respectively obtained. Although the effective grain size marginally increased with the increase of heat input, the values were remarkably smaller than the prior austenite grain size of 54, 93, 104, and 118 μm. Compared to the as-processed HSLA steel, impact toughness of steel welded with a heat input of 60 kJ cm−1 decreased dramatically from 318 ± 20 to 31 ± 8J·cm−2. But interestingly, impact toughness was increased to a peak value of 325 ± 20 J cm−2 when the heat input was increased to 180 kJ cm−1. Toughness was enhanced by grain refinement (D = 6.7 ± 0.2 μm) and through the nano-size precipitates (d = 10 nm), which were induced by medium heat input of 180 kJ cm−1. The values of precipitation strengthening were 203 MPa and 90 MPa while the critical stress values of crack propagation were 32 GPa and 10 GPa for the steels welded at 180 and 270 kJ cm−1, respectively. A critical analysis of fracture toughness and yield strength in terms of theoretical predictions is presented.