EFFECT OF M7C3-]M23C6 TRANSFORMATION ON FRACTURE-BEHAVIOR OF CAST FERRITIC STAINLESS-STEELS

The fracture behaviour of three 29 wt-%Cr ferritic steels, two contain ing zirconium and titanium respectively, has been investigated in the as cast condition and after annealing at 660-degrees-C for different t imes 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 predom inantly transgranular cleavage and it can be associated with the disco ntinuous morphology of the M7C3 carbides present in the eutectic as co arse particles surrounded by the eutectic ferrite. After prolonged hea ting, the ambient fracture energy decreases and the interdendritic mod e of fracture is enhanced. This change in fracture mechanism is associ ated with transformation of the M7C3 to M23C6 carbides. The M23C6 carb ides, unlike the coarse M7C3 carbides, form a continuous network withi n the eutectic mixture and constitute an easy path for crack propagati on. The zirconium and titanium additions result in a more massive morp hology of the carbides in the eutectic mixture and accelerate the M7C3 to M23C6 transformation during the heat treatments, enhancing the int erdendritic mode of fracture both in the as cast and in the annealed c ondition.