Fatigue Properties of Materials Surface Hardened by Plasma Nitriding and Laser Beam

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Abstract
Carbon steel, standard bearing steel, grey cast iron, and structural steel, respectively, were used to determine the effect of laser transformation hardening and low temperature plasma nitriding on S-N (Wöhler's) curves. For all materials and treatment conditions examined, the homogeneity of the case hardened layers seems to be the most important factor that influences the fatigue crack nucleation, much more important than the case hardened layer thickness and total level of hardening. Contradictory results obtained with the grey cast iron show that a microstructural constituent (graphite) can weaker the influence of the relatively high but inhomogeneous surface layer hardening and can be without these negative effects in the case of very thin but homogeneous hardened surface layer. Application of discontinuous surface hardening as well as the two overlapping beads yield to the almost negative influence on fatigue life.

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