Microstructure analysis and hardness prediction based on microstructure evolution theory in multi-pass welded joint

Abstract

During multi-pass butt welding of high-strength steel (HSS) thick plate, it is difficult to estimate the hardness performance of thick weld since multithermal cycles can drive complex microstructure evolution. In this study, butt-welded joint of 75 mm-thick Q690 HSS was prepared by multi-pass shielded metal arc welding (SMAW) in advance; meanwhile, experiments were carried out to characterize microstructure and measure hardness distribution in butt-welded joint. Then, the welding temperature history of butt-welded joint was numerically examined. Based on the irreversibility of Martensite and Bainite, the microstructure evolution model was modified to predict the complicated microstructure evolution in multi-pass welded joint. Compared with experimental data, the validity of modified microstructure evolution model was verified. With the application of simulated thermal cycles and material chemical compositions as input parameters, volume fraction distribution of each phase was obtained by modified microstructure evolution model and hardness value was calculated by hardenability algorithm. Both experimental and predicted results exhibited that microstructure in weld metal (WM) is mainly composed of granular Bainite with a small amount of Ferrite and Martensite, and heat-affected zone (HAZ) consists of predominantly lath Martensite. Hardness distribution in butt-welded joint obtained by the prediction method is in accordance with the measurement. Average hardness value in WM and BM is 290 VPN and 250 VPN. A sharp decrease in HAZ hardness can be found from the fusion line to base metal (BM). The maximal value in the HAZ close to fusion line and the corresponding prediction deviation is 6%.