An Experimental Study for Fatigue Performance of 7% Nickel Steels for Type B LNG Carriers

The most important issue associated with liquefied natural gas storage systems, such as LNG carriers, LNG FPSO and FLNG, is the structural safety. One of the most common materials for the LNG storage systems has been 9% nickel steel over the last 50 years as it has excellent mechanical properties under cryogenic temperature. Recently, the consideration for lowering the nickel content becomes necessary due to the increase of the nickel price and the high price of nickel based welding consumables. In this respect, 7% nickel steels are developed for cryogenic applications. Nippon Steel and Sumitomo Metals Corporation have developed 7% nickel steels with improved toughness comparable to that of 9% nickel steels by TMCP and micro-alloying technology. The major objective of this study is to evaluate the fatigue performance of 7% nickel steels with a special attention to Type B LNG carrier applications. Cyclic fatigue and fatigue crack growth rate (FCGR) tests for 7% nickel steels were conducted at room and cryogenic temperature. Fatigue tests were carried out with three types of specimens such as base metal, butt weld and fillet weld to characterize the fatigue properties. Also FCGR tests were carried out using compact tension specimens. The difference of FCGR characteristics among base, weld and HAZ (Heat Affected Zone) are investigated for three types of specimens. The results of 7% nickel steels are evaluated and compared with those of 9% nickel steels.