Fatigue of nitrided En41B steel: effect of internal-stress distribution

Abstract

The axial fatigue properties of nitrided En41B steel specimens of differing diameters but with constant case depth have been studied. The tensile residual stress in the specimen core is increased as the case/core area ratio rises. No relaxation of these stresses was detected after stressing close to the fatigue limit, although at higher stress levels a 10% reduction was measured after 50% of the expected fatigue life. Significant improvements in fatigue properties were observed after nitriding; close to the fatigue limit the greatest improvement occurred in larger (5 mm dia.) specimens, but the smallest specimens (3 mm dia.) showed the greatest increase in fatigue strength at lives below 10⁵ cycles. The mechanisms of failure were identified metallographically in each case; Yielding of the core occurs when the sum of the applied tensile stress and the axial core residual stress exceeds the yield stress of the non-nitrided material, and this leads to a redistribution of stresses between case and core. When the stress redistribution is taken into account, the fatigue-limit data for all specimen sizes tend to lie on a common curve between the Goodman line and Gerber parabola.