Microstructure and Properties of Welding HAZ in NM400 Wear-Resistant Steel with Different t8/5

The welding thermal simulation experiments for NM400 wear-resistant steel under different thermal cycle conditions were carried out on Gleeble-3500 thermal simulator to study the effect of t8/5 (cooling time from 800 ℃ to 500 ℃) on microstructure and properties of coarse grained heat-affected zone (CGHAZ) and intercritically coarse grained heat-affected zone (IC CGHAZ). The results show that with the increase of t8/5 in single-pass welding thermal cycle, the hardness decreased gradually, while the impact energy (-20 ℃) increased first and then decreased. CGHAZ had the highest impact energy (64 J) when the t8/5 was 10 s, because the structure was composed of a great quantity of lath martensite and a little lower bainite, and the martensite had self-tempered. With the increase of t8/5 in twice-pass welding thermal cycle, both the hardness and impact energy of IC CGHAZ decreased, and were lower than those of CGHAZ, because granular bainite and coarse upper bainite formed in the process of incomplete recrystallizations. The t8/5 for this steel should be controlled around 10 s during welding.