Abstract
An investigation has been carried out into the fatigue behaviour of gas-nitrocarburized low-carbon steel which has been artificially aged at 170°C. Gas-nitrocarburized and quenched steel shows up to 140% improvement in fatigue strength relative to annealed material whereas material aged at 170°C shows an improvement of 110%. Ageing at 170°C increases the resistance of nitrocarburized material to cyclic plasticity, but unlike naturally aged nitrocarburized material cyclic hardening is almost absent. Residual stresses in artificially aged material fade in the early stages of fatigue and do not contribute significantly to the fatigue strength. The surface microstrain in material aged at 170°C is substantially reduced by fatigue. This is believed to be due to the production of dislocations around the originally coherent Fe16N2 particles relieving the strains caused by mismatch of the particles with the matrix.
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