Strengthening steel 38Kh2MYuA by nitriding in various media

Abstract

At the present time critical parts of various machines and equipment operating under conditions of wear under cyclic loads are often produced from low-alloy steel 38Kh2MYuA. Its operating properties are usually improved by surface strengthening by the method of nitriding. A wide range of nitrogen-saturating atmospheres has been developed for this purpose, with each diffusion medium characterized by special process properties. The present paper is an analysis of data on nitriding structural steel 38K.h2MYuA in gas, liquid, ionic, vacuum, and powder nitrogen-bearing media. Recommendations are given for practical use of nitriding of the studied steel so as to obtain the requisite operating properties.Gas nitriding is often conducted in dissociated ammonia NH 3 (25-60%) at 520-560~ with the formation of nitrogen ions that are adsorbed by the surface and diffuse into the depth of the metal. For steel 38Kh2MYuA (0.35-0.42% C, 1.35-1.65% Cr, 0.15-0.25% Mo, 0.7-1.1% AI, 0.2-0.45% Si, 0.3-0.6% Mn) the duration of the process depends on the requisite thickness 'of the nitrided layer and amounts to 24 -60 h.Another variant consists in nitriding at 570~ for 5 -10 h in an atmosphere containing 50% endogenic gas and 50% ammonia. As a result of this treatment a thin carbonitride layer (Fe, M)2_s(N, C) with a hardness of 600-1000 HV that possesses insignificant brittleness and high wear resistance is formed on the surface of the steel. This treatment increases considerably the endurance limit of the parts [1].One of the methods for intensifying the nitriding process consists in treating the steel in ammonia with an additive of 6 vol.% oxygen [2]. In this case the diffusion layer is characterized by high physical and mechanical properties. The saturation rate in the medium of ammonia and oxygen is about twice that in conventional nitriding, but the method is dangerously explosive, which limits is use.Another promising technique for nitriding steel 38Kh2MYuA consists in using a mixture of ammonia and air. The diffusion layer formed consists of a zone of internal nitriding and a zone of chemical compounds having the form of a composite layered structure. The activating effect of the air Pskov Polytechnic Institute of St. Petersburg State Engineering University, Pskov, Russia. in the saturating atmosphere is caused not only by the lowering of the partial pressure of ammonia but also by the fact that the air oxygen promotes the formation of a large number of active centers on the surface through which nitrogen penetrates the metal. This promotes the formation of a nitrided layer that is uniform over the thickness and possesses a high corrosion resistance [2].A widely used controlled nitriding process consists in changing the nitrogen potential of the atmosphere by diluting the ammonia with ammonia-dissociated nitrogen (25% N + 75% H 2), air, and carbon-containing gases. By controlling the nitrogen potential of the atmosphere we can obtain a diffusion layer with a developed nitride zone (e or y ' ), which is important for parts ope...