Abstract
<title>Abstract</title> <p>9Ni steel remains a widely used material for LNG tanks and other cryogenic welded structures, but failure often initiates in millimeter-scale sensitive regions within the heat-affected zone. English-language studies show that cryogenic fracture resistance is highly sensitive to weld thermal history and local microstructure, with CTOD differences of 21.3% at − 193°C between FCAW weld conditions and total Charpy energies at − 196°C ranging from 18 ± 6 J to 71 ± 35 J among simulated HAZ subzones. This review examines how HAZ subdivision, coarse martensite blocks, interface degradation, reversed-austenite instability, crack-tip constraint, and corrosive or hydrogen-containing environments interact to promote local embrittlement. Available evidence shows that reversed austenite can increase from about 10% to 20% after suitable heat treatment, whereas in welded HAZs local continuity and stability appear more important than average fraction alone. The review also discusses mitigation routes, including process adjustment, filler matching, and post-weld heat treatment, and argues that assessment should move from average-property qualification toward local risk identification in cryogenic welded joints.</p>