Effects of Thermal Cycle on Microstructure and Cryogenic Toughness of Coarse-Grained Heat Affected Zone of 9Ni Steel Joint

Coarse-grained heat affected zone (CGHAZ) samples of 9Ni steel joint were prepared with Gleeble-3500 type thermal simulator at t_8/5 (cooling time from 800℃ to 500℃) of 6,10,30,60,100 s, respectively, and then cryogenic impact tests were carried out on the samples. The effects of t_8/5 on the microstructure and cryogenic toughness of CGHAZ were investigated. The results show that the microstructure of CGHAZ was mainly composed of lath martensite. The granular bainite appeared at t_8/5 of 30 s and the amount increased with increasing t_8/5. With increasing t_8/5, the size of martensite lath in CGHAZ increased slightly and then decreased; the ductile-brittle transition temperature rose and then decreased. The impact energy at -100℃ and -125℃ of CGHAZ prepared at different values of t_8/5 was higher than 200 J. With decreasing impact temperature, the impact fracture morphology changed from dimple to quasi-cleavage.