Question

Questions and Problems 167 Fick's Second Law-Nonsteady-State Diffusion 5.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 nitro- gen 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 main- tained at 0.45 wt%. For this treatment to be effective, a nitrogen content of 0.12 wt% must be established at a position 0.45 mm below the surface. Specify an ap- propriate heat treatment in terms of temperature and time for a temperature between 475°C and 625°C The preexponential and activation energy for the dif- fusion of nitrogen in iron are 5 × 10-7 m2/s and 77,000 J/mol, respectively, over this temperature range.

Answer 1

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