Evaluation of impact energy and brittle transition temperature of Cr-Mo steel under carburizing service conditions

This study investigates the fracture toughness and ductile-brittle transition behaviour of 9Cr-1Mo steel exposed to service conditions in a refinery. The material was tested by an Instrumented Charpy Machine over a wide range of temperatures, from −180°C to 200°C. Samples were obtained from both the service-exposed and the unexposed material following ASTM specifications. The impact energy values of the samples were taken, and these showed big differences in terms of absorbed energy between the two different batches. This suggests that the ductility in the service-exposed material decreased due to carbon diffusion and carburization while in service because the Ductile-Brittle Transition Temperature was higher in this material, i.e., −15°C, while it was found to be −45°C for unexposed material. Fractured surfaces of this material at room temperature and DBTT as revealed through Scanning Electron Microscopy were further characterized. The results point out the importance of monitoring material degradation in critical applications, especially when varying temperature conditions are involved.