Cu-precipitation strengthening in ultrahigh-strength carburizing steels

Ultrahigh hardness levels greater than 700 VHN can be obtained in secondary hardening carburizing steels but depend on costly Co alloying additions to maximize hardness achieved through M ₂C-type carbide precipitation strengthening. This study aims to incorporate nanometer-scale bcc Cu precipitates to both provide strength as well as catalyze M ₂C nucleation in the absence of or with reduced Co. Cu additions of 1.0 and 3.7 wt pct were investigated, using a series of mechanistic models coupled with thermodynamic computational tools to derive final compositions. Thirty-pound experimental heats were cast of each designed alloy, samples of which were carburized and tempered to determine their hardness response. Characterization revealed the successful incorporation of Cu alloying additions into this family of steels, demonstrating a secondary hardening response even in the absence of Co. Matrix strength levels were close to those predicted by design models; however, all four alloys demonstrated a hardness deficit of approximately 200 VHN at the carburized surface, suggesting recalibration of the M ₂C precipitation strengthening model may be required in these alloys.