Influence of microstructure on the sliding wear behavior of nitrocarburized tool steels

The wear mechanisms during dry sliding of two representative tool steel grades, one of lower and one of higher percentage of carbide-forming elements-AISI H13 hot work and Cr-Mo-V cold work steels respectively-were investigated. Prior to testing, both grades were properly subjected to heat treatments aiming to their hardening up to final values of 40, 45 and 50 HRC. After heat treatments, a part of the specimens from each hardness level was subsequently surface treated via liquid nitrocarburizing (Tufftriding), under typical industrial processing conditions. The sliding friction behavior of all specimens was studied on a ball-on-disc apparatus, applying normal loads in the range of 1-10 N. The continuous recording of the friction coefficient, the measurement of the wear volume, together with the post-testing microscopic observation of the wear surfaces provided for the evaluation of material removal mechanisms and of the effects of prior heat and surface treatments on the response of the material under surface loading. It was found that the dry friction coefficient remained practically constant (0.80-0.95) for all specimens. The hardening heat treatment did not affect substantially the wear coefficient, in contrast to the surface treatment that resulted in its significant reduction. Finally, in the case of the grade with the higher percentage of carbide-forming elements, it was found that both the wear coefficient and related mechanisms are strongly affected by the presence of coarse chromium carbides within the metallic matrix.