Toughness and austenite stability of modified 9Cr–1Mo welds after tempering

The influence of tempering treatments on toughness and austenite stability was conducted on modified 9Cr–1Mo welds. Deteriorated impact toughness was noticed for the welds tempered between 450 and 610°C for 1 h, in which the peak hardness and the lowest impact energy occurred at 540°C. The impact toughness of the welds increased rapidly at tempering temperatures above 680°C, and the associated fracture surface exhibited mainly dimple fractures for 750°C/1 h tempered welds, in contrast to quasi-cleavage for 540°C/1 h welds. Retained austenite at martensite interlath boundaries was found to be mechanically stabilized by transformation stresses in the welds tempered at lower temperatures. It could also be transformed to untempered martensite upon cooling if the stresses were relieved to a certain degree by proper selection of temperature and time combinations in tempering. For welds tempered at higher temperatures, e.g. 750°C, the elimination of transformation stresses and the isothermal transformation of retained austenite to ferrite proceeded simultaneously. As a result, no martensitic transformation upon cooling could be found for such welds. The transformation of austenite to untempered martensite would certainly contribute to the minimum toughness of the weld after being tempered at 540°C/1 h.