Rolling and Rolling-Sliding Contact Fatigue Failure Mechanisms in 32 CrMoV 13 Nitrided Steel—An Experimental Study

The aim of this work is to characterize the rolling and rolling-sliding contact fatigue failure mechanisms on the 32CrMoV13 nitrided steel. During rolling contact fatigue tests (RCF), two general features were observed: specimens presenting short lives and rough and sharpened spalling damage and specimens presenting long lives and only microspalling marks. It was possible to determine a contact fatigue limit of 3 GPa. During rolling-sliding contact fatigue tests (RSCF), a clearly different behaviour between the two specimens in contact has been observed: the driver shows circumferential and inclined cracks and only inclined cracks appear in the follower. This behaviour can be understood if the effect of the residual stress state in near-surface layers is considered. Before RCF tests, the residual stresses are compressive in all near-surface layers. After RCF tests, strong residual stress relaxation and even reversing behaviour was observed in the axial direction, which facilitates the surface crack initiation in the circumferential direction at rolling track borders.

Effects of Carbon Content Change on Growth of Compound Layer in Surface of Nitrided Steel

When steel is nitrided, a compound layer mainly composed of iron nitrides, ε-Fe2~3N and the γ’-Fe4N phase, is formed on the steel surface. It is an extremely important industrial issue to clarify factors governing the formation of the compound layer during nitriding and to establish unified views on the mechanism of compound layer formation. Therefore, in order to clarify the effect of change in carbon concentration on the growth of the ε phase and the γ’ phase in the compound layer on nitrided steel, we evaluated the change over time in the concentration of the alloy elements in the surface layer, and the phases of the compound layer on nitrided steels containing various amount of carbon in the matrix. The results were that the change over time in the carbon concentration in the compound layer was mainly responsible for the change over time in the phases of the compound layer. Furthermore, it was discovered that the change over time in the carbon concentration distribution occurred because both increasing of carbon from the matrix to the compound layer, and decreasing of carbon from the surface of compound layer to the atmosphere. That caused the gradient change of chemical potential of carbon in the through-thickness direction of compound layer, and the phases of the compound layer were changed with the treatment time.

Influence of layer substrate hardening on nitrided carbon steels on their tribological properties

Nitriding of carbon steels does not allow for adequate hardening of the substrate of layers and core required in some applications. Such hardening can be achieved by using further heat treatment. As a result of this heat treatment, the zone of nitrides vanishes and a nitro-carbon martensite structure is formed, additionally hardened by ageing. The carried out tribological tests have shown that subjecting nitrided carbon steel to further hardening treatment significantly improves its wear resistance in comparison to nitrided steel, and the zone of good wear resistance goes deeper.