Welding 9% nickel steel for LNG storage tanks demands strict control over heat input and interpass temperature. The material retains excellent toughness at cryogenic temperatures, but its sensitivity to hot cracking and hydrogen embrittlement makes process discipline non-negotiable.
Lesson one: keep heat input below 1.5 kJ/mm. Higher values promote grain coarsening in the HAZ and reduce impact toughness at -196°C. Many fabricators rely on pulsed GMAW with a stringer bead technique to stay within this window.
Lesson two: control interpass temperature tightly at 100-150°C. Exceeding 150°C risks martensite formation in the HAZ, which can crack under restraint. Cooling between passes with compressed air is common practice in large tank yards.
Lesson three: use matching filler metal ERNiCrMo-3 (Inconel 625) or ERNiCrFe-7 (Inconel 52M). These fillers accommodate the thermal expansion mismatch between 9% Ni steel and the austenitic weld deposit, preventing strain-age cracking during PWHT.
A weld that passes RT and UT may still fail Charpy at -196°C if these parameters drift. The only way to confirm is a dedicated PQR with cryogenic CVN testing.
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