High-temperature wear performance and mechanism of NM400/NM500 mining machinery steels

The friction and wear behavior of NM400 and NM500 steels in the temperature range from room temperature to 300℃ were investigated, including the formation of interface oxide, wear surface morphology, and microstructures. A high-temperature sliding friction tester was used to study the behavior of sliding friction between wear-resistant steel and Al₂O₃ ceramic balls under different loads of 200-300 N and speeds of 100-400 r·min^-1. A ball-disc friction pair containing mix-sintered Al₂O₃ ceramic balls with a diameter of 5 mm was mounted on the holding tool and steel plate. The friction coefficients of the two materials from room temperature to 300℃ are determined to be in a range of 0.27-0.40, whereas the average friction coefficients of NM400 and NM500 steels are found to decrease gradually from 0.337 to 0.296 and from 0.323 to 0.288. The generation of oxides is the primary reason for slight decrease in the friction coefficient at a high temperature of 300℃. The friction behavior is controlled by the abrasive wear mechanism, and then the phenomenon of pressure-into-peeling-oxidation of oxide gradually occurs at a higher temperature, which slightly reduces the wear rate. Larger amount of oxides are produced on the interface as the temperature increases, but this is not sufficient to form a continuous oxide layer. The main wear pattern at this time is still abrasive wear, although the wear rate and friction coefficient are affected by oxides. The main factors influencing the wear behavior are the hardness, oxide volume fraction, and oxidation activation energy of the wear-resistant steel, as found through the analysis of high-temperature frictional wear behavior and micro-oxidation model. In conclusion, the wear mechanisms of NM400 and NM500 steels from room temperature to 300℃ are influenced by the combined effect of abrasive wear, extrusion deformation wear, and trace oxide wear. NM500 steel exhibites better wear resistance than NM400 steel, and this can be mainly attributed to higher level of its hardness. A small amount of additional alloying elements in the high-strength microalloyed martensitic wear-resistant steel can reduce the wear rate to some extent, due to the formation of a certain amount of stable attached oxides that are produced during the high-temperature friction process.