Crack-initiation toughness and crack-arrest toughness in advanced 9 pct Ni steel welds containing local brittle zones

Abstract

The present study investigates the influence of local brittle zones (LBZs) on the fracture resistance of the heat-affected zones (HAZs) in quenched, lamellarized, and tempered (QLT) 9 pct Ni steel weld joints. The results of Charpy impact tests using simulated coarse-grained, heat-affected zone (CGHAZ) specimens show that the intercritically reheated (IC) CGHAZ and unaltered (UA) CGHAZ are the primary and secondary LBZs, respectively, of the steel at cryogenic temperature. Compact crack arrest (CCA) tests and crack-tip opening displacement (CTOD) tests were conducted at a liquefied natural gas (LNG) temperature to measure the variations in crack-arrest toughness and crack-initiation toughness along the distance from the fusion line (FL) within the actual HAZ. While CTOD tests show a decrease in toughness when approaching the FL, i.e., the regions containing LBZs, the crack-arrest-toughness values are found to be higher than those in the regions near the base materials. This is due to the fact that the crack-arrest toughness is governed by the fraction of microstructures surrounding LBZs instead of the LBZs themselves. By direct comparison of the brittle-crack-arrest toughness (Ka) with the brittle-crack-initiation toughness (Kc), this investigation has determined that, with regard to crack-arrest behavior, the LBZs of QLT-9 pct Ni steel do not limit the practical safety performance of the weld joints in LNG storage tanks.