Fracture Properties of Mn-Mo-Ni Low Alloy Steels and Their Simulated Heat Affected Zones

摘要

This study is concerned with the effect of alloying elements on mechanical and fracture properties of Mn-Mo-Ni low alloy steels and their heat affected zones (HAZ's). Three kinds of steels whose compositions were varied from the composition specification of SA 508 steel (Grade 3) were fabricated by vacuum-induction melting and heat-treatment, and their mechanical and fracture properties were examined. ASTM E1921 standard test method was employed to measure fracture toughness in the transition temperature region of the steels. As C content decreased and Mo and Ni contents increased, the amount of fine M_2C carbides was greatly increased, while that of coarse cementites was decreased, thereby leading to the improvement of tensile and fracture properties. Brittle martensite-austenite constituents were also formed in these steels during cooling, but did not deteriorate fracture toughness because they were decomposed to ferrite and fine carbides after tempering. Their simulated heat affected zones also had sufficient impact toughness after post-weld heat-treatment. These findings suggested that the low-alloy steels with high strength and toughness could be designed by decreasing C and Mn contents and by increasing Mo and Ni contents.