Effects of Cooling Procedures on Postfire Mechanical Properties and Fracture Resistance of G20Mn5QT Cast Steel

Abstract

Postfire mechanical properties and fracture resistance of G20MnQT cast steels were investigated experimentally and effects of different cooling procedures were studied. G20Mn5QT cast steels were heated to 600°C, 700°C, 800°C, and 900°C and cooled to ambient temperature in air or in water. Metallographic examination found significant phase transformations only in G20Mn5QT cast steels cooled quickly in water from elevated temperatures of 800°C (800W) and 900°C (900W). Original ferrite and pearlite transformed to harder phases, such as martensite and bainite, in these two postfire materials. Vickers hardness tests, Charpy impact tests, and tension coupon tests were conducted on original and postfire G20Mn5QT cast steels. All test results confirmed the findings about phase transformation in metallographic examination. The martensite and bainite generated in phase transformation caused significant hardening of 800W and 900W. With no significant phase transformation, most mechanical properties of 600A, 600W, 700A, 700W, 800A, and 900A varied within ±15% compared with those of the original material, and the postfire Charpy impact energies increased moderately due to improvement of material homogeneity. Smooth notched tensile specimens were tested and material parameters in the void growth model were calibrated to evaluate the postfire fracture resistance of G20Mn5QT cast steels at high stress triaxialities. It was found that the fracture of 600A, 600W, 700A, 700W, 800A, and 900A was almost as ductile as that of the original G20Mn5QT cast steels, whereas 800W and 900W fractured brittlely, and had fracture resistance 87.6% and 78.4% lower than that of the original material.