High tribological performance of quenched–tempered 40Cr steel treated by plasma oxynitriding

Abstract

Alloy steel 40Cr was treated by plasma oxynitriding after quenching and tempering. The microstructures, mechanical properties, and tribological performances of plasma oxidized–nitrided layer at tempering temperatures of 500, 520, and 540 °C were investigated. The typical tempered sorbite structure was present when the 40Cr alloy steel was treated by quenching at 900 °C and tempering. A special structure of mixed oxide–nitride layer along with the veinlike nitride phase was formed after plasma oxynitriding. The surface hardness of the quenched–tempered 40Cr alloy steel was significantly improved. The quenched 40Cr alloy steel tempered at 520 °C exhibited the highest hardness of approximately 973 HV0.2 and the lowest wear rate of about 1.72 × 10−14 m3 N−1 m−1 than the samples tempered at 500 and 540 °C. The oxide and nitride phases inside the plasma oxidized–nitrided layer played important roles in the improvement of the surface mechanical properties and the tribological performance of the quenched–tempered alloy steel.