Surface hot-shortness of 1045 forging steel with residual copper

Hot-shortness is brittleness in metals during high temperature deformation. In this study, surface hot-shortness of 1045 steels with residual copper was investigated. Eight 1045 steels with differing copper contents, from 0.09 to 0.39% (by weight), were tested. High strain rate compression tests of pre-bulged samples were used to simulate forging deformation conditions. It was found that oxidation time and temperature are critical parameters for the control of hot-shortness. The testing was divided into two stages: stage one, in which the steels were oxidized at different temperatures from 1100 to 1200 °C for 10 and 30 min and subsequently deformed to determine the critical temperature where the surface cracking becomes severe; and stage two, in which the steels were oxidized for 1, 3, 5, and 7 min at their critical temperature, and then deformed. In the stage one testing, all steels oxidized for 10 min and subsequently deformed exhibited a critical temperature. The critical temperature decreased with decreasing copper content, from 1160 °C for the steel with the highest copper content to 1110 °C for the steel with the lowest copper content. A simple model based on copper enrichment and depletion along surface grain boundaries is presented to explain these observations. Steels oxidized for 30 min and subsequently deformed did not exhibit severe cracking at any temperature, but the steel with the highest copper content that was deformed at 1140 °C exhibited cracking. In stage two testing, the results were less consistent. The steels with high copper content (0.39-0.32%) exhibited maximum cracking at shorter times, while for the steels with medium copper content (0.30-0.21%) the maximum cracking occurred at longer times. No steel exhibited cracking when oxidized at 1200 °C and subsequently deformed. The study shows that steels have a critical temperature at which cracking is severe. Steels oxidized and deformed above the critical temperature did not exhibit hot-shortness surface cracking. Hence, a forging practice that both maintains and deforms the steel above the critical temperature could reduce or eliminate surface hot-shortness due to residual copper.