Activation of surfaces prior to gaseous nitriding of a 3wt.% Cr carbon iron-based alloy

Abstract

Gaseous nitriding is a well-established thermochemical treatment of carbon iron-based alloys that considerably enhances corrosion, wear and fatigue resistances of critical mechanical parts such as crank shafts and bearings. It is based on the diffusion of nitrogen atoms from the catalytic decomposition of ammonia at the outer surface. The process has generally been relying on the fast development of a compound layer that mainly serves as a nitrogen reserve during the treatment. The growth of the compound layer directly depends on the adsorption of nitrogen atoms at the surface of treated parts. Any barriers to the nitrogen adsorption may induce detrimental kinetics of nitriding, and so a lack of mechanical properties. So-called surface preparations prior to nitriding thus fundamentally assist with (i) nitriding of superior quality (i.e. with the required properties such as the effective depth and mechanical properties), (ii) uniform nitriding of the whole treated part (i.e. no mechanical heterogeneities induced by the process) and (iii) repeatable nitriding from batch to batch. The present work deals with three in-situ surface activation solutions prior to nitriding that are oxidation, urea and ammonium chloride. Pre-treatments are compared based on their efficiency on uncleaned samples, defined by different degrees of residues of water dissolved machining oil, to be nitrided. The first few hours of nitriding are reported on a 3wt.%Cr carbon iron-base alloy.