Fick's Second Law-Nonsteady-State Diffusion 6.D3 The wear resistance of a steel shaft is to be im proved by hardening its surface by increasing the nitrogen content within an outer surface layer as a result of nitrogen diffusion into the steel; the ni trogen is to be supplied from an external nitrogen- rich gas at an elevated and constant temperature. The initial nitrogen content of the steel is 0.0025 wt%, whereas the surface concentration is to be