Austenite formation during heat treatment of P92 power plant steel welds: dependence of A1 temperature on compositional changes

P92 steels are commonly used in advanced steam generating power plants with welding being a main method of component fabrication. Such welds are typically post-weld heat treated at temperatures up to 780 degrees C, which is designed to be as high as possible without the danger of the formation of any austenite during the heat treatment itself. Moreover, it is recognised that, in practice, there will be differences between the specified heat treatment temperature and that which is actually achieved. There are a variety of weld consumables available that have been designed for joining P92 steels; these consumables are heavily alloyed but have ranges over which the compositions of each element in the consumable are allowed to vary. While reaustenitisation of P92 base metals during post-weld heat treatment in the normal temperature range is not likely, it is known that weld metals have much lower A(1) temperatures than parent metals. As such, concerns have been raised that, although weld consumables may exist within their specifications, certain combinations of elemental variations may lead to a reduction in the A(1) temperature to below that at which the post-weld heat treatment takes place. This paper investigates the effect of variations in alloying element content on the A(1) temperature of P92 steels and appropriate weld consumables using a combination of modelling and experimental observations. An equation has been derived to predict the Ae(1) temperature as a function of chemical composition for weld consumables designed for use with P92 steels, which can be used to identify compositions that may be in danger of reaustenitisation during normal post-weld heat treatment.