Studies on optimisation of cryogenic treatment to maximise the wear resistance of piston ring materials

Abstract

Deep Cryogenic Treatment (DCT) is a one-time permanent treatment process, carried out on steel components in such a way, that the material is slowly cooled down to the cryogenic temperature, after which it is held at that temperature for a specified period of time, and is heated back to room temperature at a slow rate followed by low temperature tempering. In this study, the Taguchi method has been used to optimise the process parameters of the DCT for three commercial piston rings; the SR34 made of 18 % Cr martensitic stainless steel, the SR35 made of 14 % Cr martensitic stainless steel, and the SR10 made of chrome silica spring steel, to obtain the maximum wear resistance. The DCT parameters considered for optimisation are: the cooling rate, the soaking temperature, the soaking time, the tempering temperature and the tempering time. Wear test was conducted on a Reciprocatory Friction and Wear Monitor (RFWM) by the weight loss method. The relative importance of the controlling parameters of the DCT and their interactions for enhancing the wear resistance of the three materials, were evaluated in terms of their percentage contributions, using the Analysis of Variance (ANOVA). A confirmation test was conducted subsequently for each material, and the results were found to be within the confidence interval. Subsequent to this, a wear characterisation study was conducted for the three materials, treated as per the optimised cryogenic treatment conditions, and the improvement in the wear resistance was found to be 41.4 %, 36.4 % and 49.8 % respectively in the SR34, SR35 and SR10 steels. The micro-hardness results reveal that there is only a marginal increase in the hardness of the three materials. Thus, an optimised DCT cycle has been evolved for the three materials using Taguchi technique and its effect on the wear resistance, microstructure, coefficient of thermal expansion and specific heat capacity are also studied. newline newline newline