Abstract: Thermal simulation and modern physical test means like optical microscopy, X-ray diffractometry and scanning electron microscopy are used to investigate the effects of factors like grain size, retained austenite and martensite-austenite (M-A) constituents on cryogenic toughness in heat affected zone (HAZ) of 9% Ni steel for liquefied natural gas storage tanks. Results show that the cryogenic toughness of coarse-grained HAZ is much lower than that of base metal after experiencing welding heat cycles. Inter-critically reheated coarse-grained HAZ (IRCGHAZ) is a primary local brittle zone (LBZ) of 9% Ni steel at cryogenic temperature. The change in the amount of retained austenite is not the governing factor of toughness change. Cryogenic toughness is severely reduced with the increase of grain size, and the larger the size and the higher thecarbon content of M-A constituents are, the more unfavorable to the material toughness will be. Analysis results indicate that low heat input and high interpass temperature should be applied for field welding to control grain size and microstructure characteristics, in order to improve cryogenic toughness of CGHAZ of 9%Ni steel and prevent brittle failure of the welding joint.

Key words: 9% Ni steel, Cryogenic toughness, Grain size, Martensite-austenite constituents, Thermal simulation