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The ASME SA-542/SA-542M steel is currently used for various large size, heavy thicknesscomponents 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 aconsequence, a Post Weld Heat Treatment is required to stress relieve both the weld metal and the heat affectedzone, and to decrease their hardness. This PWHT unavoidably results in a further tempering treatment also ofthe base metal whose hardness and mechanical strength are further diminished. In addition, in field PWHT arenot 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 varyfrom zone to zone, and even under a single panel differences of +/- 5° C can be recorded. Furthermore, K typethermocouples that are usually used have an allowable max. error at the PWHT temperature of 0.5%.Furthermore, also in the case of furnace treated components, considering the large size of the furnaces, theposition of the heating burners, the fluido-dynamics of the heating fumes, etc., it cannot be excluded that somezones may experience maximum temperatures somewhat higher than the prescribed one.Under these premises, tempering of base metal at temperatures higher than the prescribed level cannot beexcluded; over tempering not only produces a solution-reprecipitation of carbides, which increase their size andare increasingly located at grain boundaries, but may also promote an increase of the ferrite grain size. All thesemicrostructural modifications may impair the toughness properties of the base metal, because of theirdetrimental effect on the fracture behavior at low temperatures.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 to assess the effect of treatment temperatures higherthan the maximum level for PWHT on either microstructure and toughness.