For some applications, it is necessary to harden the surface of a steel (or iron-carbon alloy) above that of its interior. One way this may be accomplished is by increasing the surface concentration...

Extracted text: For some applications, it is necessary to harden the surface of a steel (or iron-carbon alloy) above that of its interior. One way this may be accomplished is by increasing the surface concentration of carbon in a process termed carburizing; the steel piece is exposed, at an elevated temperature, to an atomsphere rich in a hydrocarbon gas, such as methane (CH4). Consider one such alloy that initially has a uniform carbon concentration of 0.25 wt% and is to be treated at 950 °C (1750°F). If the concentration of carbon at the surface is suddenly brought to and mainted at 1.20 wt%, how long will it take to achieve a carbon content of 0.80 wt% at a position 0.5 mm below the surface? The diffusion coefficient for carbon in iron at this temperature is 1.6 x 10-11 m²/s.