Effect of welding structure and delta-ferrite on fatigue properties for TIG welded austenitic stainless steels at cryogenic temperatures

High-cycle and low-cycle fatigue properties of base and weld metals for SUS 304L and SUS316L and the effects of welding structure and delta -ferrite on fatigue properties were investigated at cryogenic temperatures in order to evaluate the long-life reliability of the structural materials to be used in liquid hydrogen supertankers and storage tanks and to develop a welding process for these applications. The S-N curves of the base and weld metals shifted towards higher levels, i.e., the longer life side, with decreasing test temperatures. High-cycle fatigue tests demonstrated the ratios of fati gue strength at 10(6) cycles to tensile strength of the weld metals to be 0 .35-0.7, falling below those of base metals with decreasing test temperatur es. Fatigue crack initiation sites in SUS304L weld metals were mostly at bl owholes with diameters of 200-700 mum, and those of SUS316L weld metals wer e at weld pass interface boundaries. Low-cycle fatigue tests revealed the f atigue lives of the weld metals to be somewhat lower than those of the base metals. Although delta -ferrite reduces the toughness of austenitic stainl ess steels at cryogenic temperatures, the effects of delta -ferrite on high -cycle and low-cycle fatigue properties are not clear or significant. (C) 2 000 Elsevier Science Ltd. All rights reserved.