초저온용 강 용접열영향부의 저온 파괴특성에 관한 금속조직학적 분석 (1) - 열재현된 CGHAZ 시험편을 이용한 국부취화현상의 평가 -

The present study was focused on the correlation of the microstructure with the cryogenic fracture characteristics in the heat-affected zones (HAZs) of multi-pass welded cryogenic steel, i.e., QLT (quenching, lamellarizing and tempering)-processed 9% Ni steel used for LNG (liquefied natural gas) storage tanks in Korea due to the advanced cryogenic fracture toughness. The fracture behaviors of the various micro-zones within HAZ including LBZ (local brittle zone) were investigated using simulated HAZ specimens. From the Charpy tests using various synthetic CGHAZ specimens, the primary LBZ in this steel was found to be inter-critically reheated coarse grained HAZ (IC CGHAZ) like other structural steels. However, unlike other cases, IC CGHAZ in this cryogenic steel showed LBZ phenomena only at cryogenic temperature, not at room temperature. Micro-mechanical processes were identified by in situ observation within SEM (scanning electron microscopy) using miniaturized SENT (single edge notched tension) specimens. Based on the test results, the possible causes on cryogenic LBZ phenomena were analysed and discussed. These results suggest that the LBZ phenomena could be explained by total consideration of various metallurgical factors incluting carbon contents in M-A(martensite-austenite) constituents and their influences on fracture behaviors at cryogenic temperature.