Improving tool life under cyclic loadings is of great economic interest, since forming tools often fail due to fatigue fracture initiated at volume or surface defects. Increasing the fatigue strength of tool steels can be achieved through improved steel making or properly chosen case hardening. Diffusion layer plasma-nitriding has never been investigated statistically or microstructure-dependently, especially for carbide-rich tool steels. This study provides a complete, statistically evaluated comparison of plasma-nitriding effects on the fatigue strength, additionally considering microstructural characteristics.
Rotating bending tests of plasma-nitrided specimens from AISI D2 (X155CrVMo12-1) and AISI M2 (HS6-5-2/-3) were performed. Thereby powder-metallurgical (PM) and melt-metallurgical (MM) production via ingot casting with forging in longitudinal/transversal direction were distinguished to furthermore consider manufacturing-related differences in purity, carbide and defect sizes. Nitriding leads to greater surface hardness and reduced case toughness due to precipitation hardening, but no negative effects can be statistically demonstrated. For MM of AISI M2 in longitudinal/transverse direction no positive effect occurs. Plasma-nitriding results in an increase of fatigue strengths for MM of AISI D2. The effects of plasma-nitriding not only depend on defect sizes but also on the initial matrix hardness and are thus depending on a combination of the primary and secondary influencing factor.
In contrast to MM, nitriding results in a remarkable increase of fatigue strength for both PM steels. Defect sizes and fatigue mechanisms do not change by nitriding, since nitrides are significantly smaller compared to carbides and thus not relevant for crack initiation. In PM steels with comparatively high purity and much smaller defects, the generated compressive residual stress leads to retardation and delay of crack initiation at small oxide inclusions. Since defect sizes in MM are much larger, nitriding has less or no positive effects on fatigue strengths, that strongly depend on the matrix hardness set during heat treatment.