Sliding-vvear evaluation of nitrided austenitic alloys

Abstract

The nitriding behaviour of a number of austenitic alloys was investigated for the temperature range 550–850°C. The maximum response to nitriding, as recorded by weight gain and thickness measurements of the nitride layer, occurred between 660 and 750°C. After an initial, faster rate of nitriding during the first few hours of treatment, the reaction followed parabolic kinetics. With most alloys, the nitride formed a compact layer, essentially composed of M₄N, at temperatures up to and including 750°C, although at 850°C unreacted metal remained in the surface regions. An exception to this was a high-nickel (∼ 20%) alloy, in which substantial unreacted metal was found at all nitriding temperatures. The role of different alloying elements is discussed in relation to nitriding performance. A selected nitriding treatment is applied to each austenitic alloy, and the resulting unlubricated wear behaviour evaluated using a pin-on-disc wear machine. A transition from mild to severe wear is identified in treated and untreated alloys, and this is dependent upon both sliding speed and applied load. Mild wear is associated with the formation of an oxide film on the wear pin promoted by frictional heating, whether the pin is treated or not. In the severe wear regime, the oxide is removed by abrasion faster than it can be formed, and the characteristics of the underlying substrate now control wear behaviour. It is shown that the wear performance of an austenitic alloy containing a small concentration of titanium is, after nitriding, comparable with that of a commercial nitrided ferritic alloy.