text: Diffusion hardening is performed by completely surrounding a metal part with the element to be diffused into it in either the solid, liquid, or gas phase depending on the type of diffusion process being performed.  The concentration of the diffusing element surrounding the part must be higher than the concentration of the element inside the part, or diffusion will not occur.  The metal and the surrounding element must then be heated to a temperature sufficiently high for diffusion to occur.  In the case of pack carburizing, the temperature must be 900 °C and the part must be allowed to sit for 12 to 72 hours for the correct amount of diffusion to occur.