Characteristics and effects of austenite resulting from tempering of 13Cr–NiMo martensitic steel weld metals

Low-carbon 13Cr–NiMo martensitic steels are remarkable for their high strength and high resistance to brittle failure while retaining corrosion resistance together with weldability. These properties can be obtained when an intercritical tempering is applied as heat treatment or postweld heat treatment (PWHT); promoting the precipitation of finely distributed austenite that remains untransformed after cooling. The content and stability of this austenite in the weld metal accounts for the high toughness even under subzero conditions. Transmission and scanning electron microscopy (SEM), X-ray diffraction, and Mössbauer spectroscopy were used to study both the austenite resulting from intercritical tempering of these soft martensitic stainless steel weld metals and the austenite–fracture interactions. To recognize the effect of the austenite content on impact toughness, single- and two-stage tempering have been applied and evaluated through Charpy tests. The studies have shown the austenite to be thermally stable, mainly due to its substructure, but not mechanically stable, indicating that the toughening mechanism of the austenite particles is associated with transformation-induced plasticity (TRIP).