The wear resistance of a steel shaft is to be improved by hardening its surface.
ID: 1818884 • Letter: T
Question
The wear resistance of a steel shaft is to be improved by hardening its surface. This is to 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 0.50 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 appropriate heat treatments in terms of temperature and time for temperatures between 475oC and 625oC. The preexponential and activation energy for the diffusion of nitrogen in iron are 3 x 10-7 m2/s and 76,150 J/mol, respectively, over this temperature range.
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