Fatigue Strength and Failure Mechanisms of Nitrided Small Parts of a 30CrMoV9 Steel

Abstract

An investigation of nitriding effect on fatigue behavior of small-size specimens of a 30CrMoV9 (ČSN 15330) nitriding steel heat treated to high strength was carried out, and fatigue damage mechanisms were evaluated. Circular cross-section specimens either smooth or notched, with the diameter approximately 2 mm in gauge section were used. Axial fatigue tests were carried out at constant load amplitude, loading was of repeated type—load asymmetry R = 0. The program was carried out as a separate part of the complex evaluation of fatigue resistance of gears against failure. The nitriding process resulted in unforeseen, mostly negative effects—in a strong reduction of fatigue life and fatigue limit in most cases. Fatigue limit only was slightly increased, namely, by 10% in case of smooth specimens. However, further small increase of load amplitude resulted in strongly reduced fatigue strength in comparison with specimens without nitriding. Nitriding applied on notched specimens reduced their fatigue limit by more than 40%. Fractographical and metallographical analyses showed an extensive brittle cracking of nitriding layer particularly in the vicinity of fatigue crack initiation sites and an existence of multiple deep fatigue cracks besides the main crack. A likely reason of premature failure was a superposition of external stress, which were close to yield strength, with tensile residual stresses under the nitrided layer, resulting in a contribution of the small specimen size in a local subsurface plastic deformation and brittle cracking of the thin nitrided layer. Once brittle cracks in the layer arose, fast fatigue crack growth occurred.