Effect of increasing post weld heat treatment temperature on the fracture toughness of an ASME SA-542/SA-542M steel

The ASME SA-542/SA-542M steel is currently used for various large size, heavy thickness components that must withstand severe service conditions in the petrochemical industry. Frequently in field welding operations have to be carried out either for fabrication or repairing purposes; as a consequence, a Post Weld Heat Treatment is required to stress relieve both the weld metal and the heat affected zone, and to decrease their hardness. This PWHT unavoidably results in a further tempering treatment also of the base metal whose hardness and mechanical strength are further diminished. In addition, in field PWHT are not simple to carry out; heating panels are used, consisting of electrical resistances, embedded in ceramic tiles, under a proper insulation. Notwithstanding each panel is controlled by a thermocouple, temperature may vary from zone to zone, and even under a single panel differences of +/- 5° C can be recorded. Furthermore, K type thermocouples that are usually used have an allowable max. error at the PWHT temperature of 0,5%. Last, but not least, some degree of freedom is also left for the selection of the PWHT maximum temperature and time. Under these premises, tempering of base metal not only produces a redistribution of carbides, which increase their size and are increasingly located at grain boundaries, but may also result in some increase of the ferrite grain size. All these microstructural modification may produce an embrittlement of the base metal. In the present research work fracture toughness measurements have been carried out on an ASME SA-542/SA-542M steel subjected to different tempering conditions.