The wear resistance of a steel shaft is to be improved by hardening its surfaceby increasing the nitrogen content within an outer surface layer as a result of nitrogen diffusion into the steel; the nitrogen is to be supplied from an external nitrogen-rich gas at an elevated and constant temperature. The initial nitrogen content of the steel is 0.002 wt%, whereas the surface concentration is to be maintained at 0.50 wt%. For this treatment to be effective, a nitrogen content of 0.10 wt% must be established at a position 0.40 mm below the surface. Specify an appropriate heat treatment in terms of temperature and time for a temperature between 475∘C and 625∘C. The preexponential and activation energy for the diffusion of nitrogen in iron are 5×10−7m2/s and 77,000 J/mol, respectively, over this temperature range.