Effect of PWHT on the Fracture Toughness and Burst Test Response of Grade 91 Tube Weldments

Abstract

Requirements for post-weld heat treatment (PWHT) of steels for pressure boundary component applications have been defined in terms of exceeding a minimum Charpy impact energy in an attempt to overcome the risk of brittle cracking. This approach can be excessively conservative since it does not properly assess resistance to cracking under the relatively static conditions encountered in pressure service. It is well established that under these conditions, cracking resistance, brittle or ductile, is more properly characterized using fracture toughness (KIc/JIc) type tests. The current paper describes room temperature fracture testing conducted via Charpy testing, fracture toughness testing, and tube (burst) pressurization of Grade 91 weldments after different PWHT conditions. The results obtained highlight (a) the excessive conservatism associated with use of Charpy data for assessing burst, (b) the value of fracture toughness testing to measure the sensitivity of fracture resistance to changes in PWHT (not seen in the Charpy data), and (c) the response of tube weldments with known flaws to pressurization. The observed burst pressure results are consistent with predictions made by analysis using the fracture toughness test results. Detailed analysis, including information from post-test examination of tube weldments and a fracture mechanics-based interpretation of test data for component flaw tolerance, provide a quantitative basis for specifying repair procedures and quality assurance methods for such welds. The findings illustrate the need to utilize long-established material fracture toughness testing methods and related properties to establish rules for construction and for post-construction repair. This is particularly true for the creep strength enhanced ferritic (CSEF) steels where the elevated temperature behavior may be compromised by potentially over-tempering weldments to meet requirements based simply on the results from inexpensive but inappropriate tests such as the Charpy test.