Plastic deformation of 25CrMo4 steel during wear: Effect of the temperature, the normal force, the sliding velocity and the structural state
Résumé
This article follows a previous study on friction and wear of 25CrMo4 steel [N. Khanafi-Benghalem, K. Loucif, E. Felder, F. Delamare, Influence de la température sur les mécanismes de frottement et d'usure des aciers X12NiCrMoSi25-20 et 25CrMo4 glissant sur du carbure de tungstène, Matériaux et techniques 93 (2005) 347-362]. The aim of our work is to study in more details the process of plastic deformation and the wear rate of this steel in lubricated sliding against cemented tungsten carbide, process observed in the previous work. The considered parameters are the temperature T (from 20 to 200 °C), the normal force P (from 500 to 1500 N), the steel structure (normalised HV 220 and quenched/tempered HV 480 states) and the sliding velocity v (from 0.05 to 0.3 m/s). We measured the friction coefficient and the sample total volume loss. A displacement sensor follows the volume loss evolution during the test; this follow-up is approximate because of the sample plastic flow which leads to the formation of peripheral burrs. All the tests conditions generate a significant plastic deformation of the sample steel, even in the quenched/tempered state: it produces a marked increase of the surface hardness, the work hardened layer being much finer for the quenched/tempered state (15 μm) than for the normalised state (40 μm at 20 °C). For temperatures T ≤ 100 °C in normalised state, the wear follows the Archard's law with an increasing rate with temperature. For T ≥ 120 °C, the wear rate decreases during the test, the global volume of wear being a decreasing function of T. For the quenched/tempered state, the wear rate decreases with the increase of the normal force, this decrease is less than 30% of the normalised state value. The material heating during the wear tests is well correlated with the friction dissipated power, but remains small, except in extreme cases (v maximum, great friction at high temperatures). These results suggest the existence of two wear mechanisms: abrasion by sample debris and burrs emission by plastic flow. The abrasion is probably the dominating mechanism for the tests carried out at the lowest temperatures. The plastic flow becomes a significant component at the highest temperatures. Using a contact model, we discuss to what extent the influence of the temperature and the strain rate on the steel hardness and ductility could explain the temperature and the sliding velocity effect on wear. Other phenomena are probably present: the influence of the steel microstructure and the lubricant on the size and/or the number of particles responsible for abrasion.