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SynopsisWith a view mainly to safety considerations of LPG storage tanks, Vnotch Charpy, three point bending COD and wide plate tension tests were undertaken on four plate materials and thirteen weld metals welded by shielded metal arc, submerged arc and automatic TIG welding processes. In respect of fracture toughness of the weld metals, some correlation was seen between V-notch Charpy absorbed energy and three point bending COD value, both these values being found to be particularly high for Ni free 1.4Mn-Ti-B type weld metal. For the base metal, Ti-added Al-killed steel showed good toughness in the heat-affected zone by both V-notch Charpy and through-thickness notched three point bending COD tests. This latter test applied to specimens with surface notch, however, yielded a low COD value within 400 pm of fusion line for all the metals tested and for all values of heat input. Nonetheless, tensile tests using wide plate welded specimens with artificial defects located in the region of lowest toughness proved that, from considerations of fracture mechanics, all the joints possessed ample strength against brittle fracture initiation. It should thus appear that evaluation of welded joints may not always be valid when based on local three-point bending COD test, and that it should be more practical to assess the safety of such welded joints by wide-plate tests.

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Major requirements to low temperature service steels are the resistance to fracture initiation and crack arrestability which, naturally, are required for LPG storage tanks. Low temperature service steels are classified into Al-killed steel, 2.5 % Ni steel, 3.5 % Ni steel and so on. A welded joint is generally not uniform but contains partially embrittled region, which often is unignorable and makes it difficult to evaluate the toughness of welded joint. In this paper, the welded joints containing embrittled regions were prepared artificially and its influence on the fracture behavior was studied. In the three point bending COD test, the small-sized specimen showed an indication of the fracture initiation when the crack tip is located in an embrittled region. However, since the probability of finding a crack tip in the embrittled region is small and the force of restraint is large when a whole welded joint is considered, fractures by a low stress seldom occur. Even under the most unfavorable condition, there is a tendency of a multi-step fracture rather than a single-step fracture.