J-integral analysis of ductile fracture margin in piping weld overlays

Cracking has been observed in the heat-affected-zones (HAZ) of girth welds in power plant piping made of austenitic stainless steels such as Types 304 and 316. The origin and the significance of these cracks were extensively studied and it has been shown that they were due to Intergranular Stress Corrosion Cracking (IGSCC) of the material sensitized by the welding process [ 1 ]. It is generally agreed that a combination of sensitized material, high tensile stress (generally due to residual stress), and oxygenated high temperature water leads to IGSCC. When a crack is discovered in a piping component during in-service inspection, an evaluation is necessary to assure that the safety margins intended in the original design code of the piping are still maintained. Evaluation methods [2, 3, 4] have been developed by General Electric under the sponsorship of Electric Power Research Institute (EPRI), which involve the calculation of expected crack growth till next inspection and comparison of the final flaw size with the acceptable flaw size. These methods also formed the basis of the flaw evaluation procedures given in Paragraph IWB-3640 of Section XI, ASME Code [ 5 ].