The wear resistance of a steel shaft is to be improved by hardening its surface. This isto be accomplished by 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 0.50wt%. 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 appropriate heat treatments in terms of temperature and time for temperatures between 475°C and625°C. Calculate values for 500°C, 550°C, 600°C temperatures that lie within the range stipulated in the problem. The pre-exponential and activation energy for the diffusion of nitrogen in iron are 3 x 107 m²/s and 76150 J/mol, respectively, over this temperature range. (9 marks including 2 marks for each final answer in the table and 3 marks for the solution)