Effect of M₇C₃→M₂₃C₆ transformation on fracture behaviour of cast ferritic stainless steels

Abstract

The fracture behaviour of three 29 wt-%Cr ferritic steels, two containing zirconium and titanium respectively, has been investigated in the as cast condition and after annealing at 660°C for different times up to 2210 h. The fracture energy and the mode of fracture depend on both the morphology and the nature of the eutectic, which consists of carbides and ferrite. In the as cast condition, fracture is predominantly transgranular cleavage and it can be associated with the discontinuous morphology of the M₇C₃ carbides present in the eutectic as coarse particles surrounded by the eutectic ferrite. After prolonged heating, the ambient fracture energy decreases and the interdendritic mode of fracture is enhanced. This change in fracture mechanism is associated with transformation of the M₇C₃ to M₂₃ C₆ carbides. The M₂₃ C₆ carbides, unlike the coarse M₇C₃ carbides, form a continuous network within the eutectic mixture and constitute an easy path for crack propagation. The zirconium and titanium additions result in a more massive morphology of the carbides in the eutectic mixture and accelerate the M₇C₃ to M₂₃C₆ transformation during the heat treatments, enhancing the interdendritic mode of fracture both in the as cast and in the annealed condition.

MST/1734