Duplex treatment of 32CrMoV13 steel by ionic nitriding and triode sputtering: application to wood machining

The efficiency of hard coatings, such as Cr-x.,N-y,, in increasing abrasion resistance of cutting tools for peeling has been shown in previous studies [Surf. Coat. Technol. 74-75 (1995) 629-633; Meca. Indus. 3 (2002) 333-342]. Furthermore, it has also been demonstrated that this efficiency was increased by a previous thermal treatment of the steel cutting knife that means by a duplex treatment consisting in nitriding followed by CrN-sputtered coatings [Meca. Indus. 3 (2002) 333-342]. The aim of the study is to optimise the application of duplex treatments by modifying the nitriding gas mixture in high temperature process. The influence of gas mixture (composed of N-2, H-2, and CH4) on the mechanical and structural properties of 32CrMoV13 low alloy steel samples was studied. The composition and structure of the nitriding layer was determined by the EDS and the XRD, respectively. Vickers microhardness profiles were also performed to study the influence of the gas mixture. The morphology of the nitriding layers was observed by optical microscopy. Cubic CrxNy coatings have been realised by triode sputtering on both faces of the cutting knives. The EDS analyses permitted to verify the composition of the layers while their structure was determined by the XRD. Different kind of cutting knives were tested in peeling of non-defect beech wood: non-treated, nitrided, CrN-triode sputtered and CrN-duplex treated cutting knives. The friction coefficient between the cutting material and the wood was determined and the wear of the knive edge (reduction of the edge) was also measured. It was obvious that increasing the nitrogen contents from 20 to 80% in the nitriding gas mixture N-2 + H-2 or adding 5% of methane permit to increase the nitrided layer's thickness and hardness. The nitrided cutting knives and CrN-triode sputtered duplex based treated knives were tested and compared in peeling of beech. The duplex treated knives obtained with 80% of N-2 or 5% of CH4 performed best and allowed to increase almost twice the tools service life. (C) 2003 Elsevier Science B.V. All rights reserved.