The hardened region appears

Hardening improves

Carbon diffusion in iron is 

The presence of carbon atoms makes iron (steel)

Non-hardened region appears 

The rate of diffusion increases when temperature decreases

Diffusion is a time-independent process

 Ficvk's first law of diffusion

When the gradient of concentration equals 0 There is not net flux (J)

The rate of diffusion in a steady-state situation is independent of time

Small, interstitial atoms move from one interstitial position to an adjacent one

Vacancy diffusion is more rapid than interstitial diffusion

Atoms and vacancies exchange positions

The diffusion rate depends on the number of vacancies and the activation energy to exchange.