Fatigue Crack Growth Characteristics of 9% Ni Steel Welded Joint for LNG Storage Tank at Low Temperature

The fatigue crack growth characteristics of base metal and weld joint of 9% Ni steel for LNG storage tank was carried out using CT specimen at room temperature and $-162^{\circ}C$. Fatigue crack growth rate of base and weld metals at RT and $-162^{\circ}C$ was coincided with a single line independent of the change of stress ratio and temperature. In the region of lower stress intensity factor range, fatigue crack growth rate at $-162^{\circ}C$ was slower than that at RT, and the slop of fatigue crack growth rate at $-162^{\circ}C$ increased sharply with propagating of fatigue crack, fatigue crack growth rate at RT and $-162^{\circ}C$ was intersected near the region of $2{\times}10-4\;mm$/cycle, and after the intersection region, fatigue crack growth rate at $-162^{\circ}C$ was faster than that at RT. The micro-fracture mechanism using SEM shows the ductile striation in the stable crack growth region. Also the defects of weld specimen after fatigue testing were detected using the A scan of ultrasonic apparatus.

LNG 저장탱크용 9% Ni강 용접부의 저온피로균열진전 특성