The unlubricated reciprocating sliding wear of a martensitic stainless steel in air and CO 2 between 20 and 300 °C

Abstract

A limited programme of work has been undertaken to investigate the self-wear behaviour of a high strength martensitic stainless steel. This material is used as boiler tube supports in advanced gas-cooled reactors. Some tests were carried out in air at room temperature to permit comparison with data sets for other materials but most of the tests were in CO 2 in the temperature range 20–300 °C. Wear rates were measured by specimen weight loss and by continuous collection and intermittent weighing of wear debris. At room temperature, both in air and CO 2, specific wear rates were initially high, i.e. of the order of 10 −12 m 3 N −1 m −1. The severe wear rate however decreased with increasing temperature in CO 2 reaching a value of about 1 × 10 −13 m 3 N −1 m −1 at 300°C. Wear transitions were observed at all temperatures except 200 °C and resulted in a decrease in wear rate by between a factor of two and two orders of magnitude. The extent of the decrease in rate, however, showed no consistent trend with either temperature or load. Associated with the wear transition was a change in friction behaviour from widely fluctuating during the severe wear stage to smoother sliding in the post-transition stage. The duration of the severe wear stage at 20°C in CO 2 appeared to be proportional to load. A correlation was found between the initial wear kinetics and the mode of wear surface deformation. The highest wear rates occurred when the surface contact interactions resulted in gall formation; galling being defined as the displacement of material from one part of a surface to another region by a ploughing mechanism. The severe wear rate progressively decreased as the surface deformation mechanism changed to one of transfer from one surface to the other with the subsequent formation of features called prows. There was clear metallographic evidence that wear debris was generated by breakdown of these surface features.