· The carbon content of the surface is controlled by selecting a diffusion temperature and time: {c(x,t)_T - c₀}= {c_s- c₀}[ 1 - erf(x/ {4Dt}^0.5)] , 
where D = D₀exp(-Q/kT)
· The maximum penetration depth corresponds to an error function argument of 2: erf(2) = 0.995; [1 - erf(2)] = 0; c(x,t)_T = c₀
· Penetration Depth = 2(4Dt)^0.5 = 4(Dt)^0.5