Microstructure and Properties of HAZ in Low-Carbon Bainite E550 Steel during Double-Pass Welding Thermal Cycle

Abstract

Investigations on the microstructure and properties of the Coarse-Grained Heat-Affected Zone (CGHAZ) and intercritical reheated Coarse-Grained Heat-Affected Zone (ICCGHAZ) of a low-carbon bainite E550 steel were carried out using thermal simulation technology in this paper.Double-pass welding thermal cycle were performed on Gleeble-3800 thermal simulator, tempering heat treatment of the critical coarse crystal zone carried out in a box resistance furnace, low impact energies at -40 °C and Vickers hardness determined, and the microstructure were observed. The experimental results show that the microstructure of CGHAZ (Tp1 is 1320 °C) was dominated by coarse granular bainite and Lath bainite Ferrite, the impact toughness of CGHAZ was poor. The toughness of the CGHAZ was improved after second welding heat cycle except intercritical two-phase heating. When the peak temperature of the second thermal cycle(Tp2) was 650 °C, martensite-austenite (M-A) constituent of original CGHAZ wasdecomposed and refined, impact toughness and hardness were all higher than that of CGHAZ; When Tp2 is 750 °C, there was a ” necklace” distribution of massive M-A constituent in this ICCGHAZ, the impact energy at -40 °C prominently decreased and Hardness went up; When Tp2 was in the temperature range of 850 °C ~1100 °C, the microstructure was mainly finer granular bainite, the toughness of CGHAZ could be effectively improved; When Tp2 was over 1100 °C, M-A constituents become coarse, the toughness declined slightly . The changing of hardness was the opposite of toughness but the hardness fluctuation was comparatively small. After tempering at different temperature (520 °C~640 °C) , the grain boundary "necklace" structure of ICCGHAZ was still obvious, some of the M-A constituent were decomposed, the hardness decreased, the lowest hardness was obtained in 610 °C.