Patent ID: 8758527

Claim:
A surface hardening method comprising the step of: determining a diffusivity of a metal using an equation, D=D o exp(−Q/RT), where D is the diffusivity, D o is a diffusion constant for the metal, Q is an activation energy, R is the Universal gas constant, T is the selected temperature, the metal having a composition that includes about 0.21-0.25 wt % carbon, about 2.9-3.3 wt % chromium, about 11-12 wt % nickel, about 13-14 wt % cobalt, about 1.1-1.3 wt % molybdenum, and a balance of iron; carburizing the metal at a selected temperature for a selected amount of time in a carbonaceous atmosphere, including controlling the temperature, time and carbon potential of the carbonaceous atmosphere during carburization with respect to the diffusivity of the carbon in the metal such that the carbon dissolved into the metal from the carbonaceous atmosphere during the carburization does not exceed a solubility limit of the carbon in the metal, the carburizing increasing a surface carbon concentration of the metal from the 0.21-0.25 wt % carbon to about 0.5-0.65 wt % carbon; and quenching and aging to precipitate a distribution of carbides within grains of the metal, thus increasing an initial surface hardness of about 51-55 HR C of the metal to a second hardness of about 58-62 HR C .