Fracture Toughness of Pressure Boundary Steels With Higher Yield Strength

Abstract

ASME Section III Appendix G provides a fracture mechanics methodology for evaluating the fracture resistance of pressure vessel materials based on comparing the applied stress intensity from service induced loadings on the assumed flaw to a material fracture toughness (KIR) curve. The applicable fracture toughness curve described in Appendix G is defined as a lower bound curve for static, dynamic, and crack arrest fracture toughness tests for a database that includes a number of pressure vessel materials having specified minimum yield strengths of 50 ksi or less. For materials which have specified minimum room temperature yield strength greater than 50.0 ksi but not exceeding 90.0 ksi, the same fracture toughness curve may be used provided fracture mechanics data points are obtained on at least three heats of the material on a sufficient number of specimens to cover the temperature range of interest, including the weld metal and heat affected zone, and provided that the data points are equal to or above that of the fracture toughness curve of Appendix G of the ASME Code (Fig. G-2210-1). At present, the pressure boundary components of steam generators and pressurizers typically use SA-508 Grade 3 Class 2 forgings and SA-533 Type B Class 2 plate material with minimum yield strength of 65 ksi and 70 ksi respectively. The fracture toughness for these materials is not readily available. This technical paper is compilation of fracture toughness of forging and plate material, weld metal, and heat affected zone of such higher strength forgings and plate materials. The paper includes toughness data of weld metal and heat affected zone resulting from high heat input process and includes the toughness test data with long term postweld heat treatment. All dynamic fracture toughness values for the ASME forgings with a minimum yield strength of 65 ksi and plate material with a minimum yield strength of 70 ksi, heat affected zone, and weld metals exceed the ASME specified minimum KIR given in Article G-2000 of the ASME Boiler and Pressure Vessel Code. This data concludes that the ASME specified minimum reference KIR curve can be used conservatively for the forging and plate materials with minimum yield strength of 65 ksi and 70 ksi that are currently specified for construction of steam generators and other Section III, Class 1 components.