Effect of Si content on microstructure and cryogenic toughness of heat affected zone of low nickel steel

Compared with 9%Ni steel, low nickel steel has great advantages in cost and weldability. However, cryogenic toughness of the heat affected zone (HAZ) of conventional low nickel steel deteriorates, which limits its application in the construction of liquefied natural gas tanks. In this work, two low nickel steels with different Si contents were designed, and the relationship between Si content and HAZ cryogenic toughness of these steel samples was investigated. After reducing Si content, the required Charpy impact absorbed energy was obtained in the HAZ at fusion line and 1 mm from fusion line. Microstructural characterization revealed that the reduction in Si content promoted auto-tempering of coarse-grained HAZ (CGHAZ), leading to improved plastic deformation ability of the matrix. In intercritically reheated CGHAZ (ICCGHAZ), martensite-austenite (M-A) constituents were formed, which consisted of twin martensite and austenite. The decrease in Si content reduced the amount and size of M-A constituents and changed their elongated morphology to blocky morphology, which is also believed to be beneficial for ICCGHAZ cryogenic toughness. This work provides a novel way to soften the HAZ microstructure for improving its cryogenic toughness.