Effect of Heat Treatment on Fracture Properties of High-Chromium White Cast Iron

摘要

High-chromium white cast iron contains chromium carbides that provide excellent wear resistance, but also cause poor fracture properties. Heat treatments are applied in an attempt to decrease the angularity of the eutectic carbides and to improve the toughness of the matrix by intentionally creating a high fraction of retained austenite, while minimizing the formation of secondary carbides. It is found that the austenitising temperature hardly affects the carbide morphology. However a large effect is found on the shape and size of the secondary carbides formed and on the relative, fractions of austenite and martensite. Measurements are performed with an instrumented drop-weight impact tower on Charpy-like specimens containing electric-discharge-machined notches. Results show that austenitising at high temperature (1100°C) leads to a significantly larger total fracture energy compared to the as-cast condition. However, the energy absorbed at crack initiation and the K_(Ic) value seem not to be affected. SEM observations of the fracture surfaces clearly indicate that the crack propagates along the interfaces of or through the eutectic carbides. A ductile fracture appearance is found around secondary carbides.