Prediction of Hardness in Heat Affected Zone of 9%Ni Steel

Abstract

Various methods have been introduced to predict postweld hardness of the heat affected zone (HAZ) for 9% Ni steel which is a primary steel adopted in the construction of liquefied natural gas (LNG) storage facilities. Two models were derived for the evaluation of the HAZ hardness, and then validated. The formulae developed in this investigation are sufficient to predict the hardness of the HAZ for 9% Ni steel . For the model using a rule of mixture, it is suggested that the morphology of martensite should be taken into consideration. Since the prediction of hardness depends on the calculation of the critical cooling time and hardness of microstructural constituents, a formula to estimate the hardness of martensite in HAZ was given. For empirical equation relating welding parameters, calculation results were found to give a fairly good description of the postweld HAZ hardness.

9% Nickel Steels and Their Welding Behavior

“…The microstructure of fused weld and the transformed microstructure in HAZ areas are determined by the temperature gradient formed during welding. The occurrence of fine grain size contributes to hardening in an HAZ band with decrease in notch toughness at À196 C (29,35). Research results (1,4) also indicate that beyond 700-900 C, due to the presence of grain refinement agents (such as Al) present in the HAZ, fine grains are developed as compared to the low temperature region.…”