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摘要
The use of thermochemical treatments to enhance fatigue and wear resistance performances of highly stressed mechanical components is well known and established [1,2]. The application of treatments like nitriding and nitrocarburizing to construction steels results in a hardened layer with limited distortion and dimensional variation. However, nitriding and nitrocarburizing can alter not only the steels mechanical properties, but also their corrosion resistance. Thus, trade-off solutions are usually required when the project requirements involve both wear and corrosion resistance. Post-treatments, like oxidation and oil impregnation, can help overcome this dichotomy, leading to high-performance engineered surfaces. Aim of the work is the characterization and optimisation of post-treatments on nitrided and nitrocarburized 41CrAlMo7 steel, in order to improve corrosion resistance and to reduce wear during dry sliding against counterparts. in this study, two kinds of post-treatments have been performed after gaseous nitriding and nitrocarburizing: a) oxidation, b) oxidation and oil impregnation using two different oil in water emulsions (10wt% and 30wt% oil). The morphological features of the post-treated surface have been characterized using optical profilometry and atomic force microscopy,in order to evaluate the surface morphology influence on wear and corrosion resistance. In can be concluded that, in the applied experimental conditions,:
nitrocarburizing and nitriding originate surfaces with very different morphology: nitrided surface is smooth, regular with a low porosity, while after nitrocarburizing the surface presents higher roughness parameters and abundant open microporosity;
nitriding and nitrocarburizing promote an significant improvement of wear resistance. Nitrided 41CrAlMo7 presents the lowest wear rate because of its surface hardness. Oxidation treatments causes a moderate surface hardness decrease, but an increase of total and effective hardening depth;
oxidation forms Fe3O4 on both nitrided and nitrocarburized samples. The oxide surface morphology is influenced by the substrate chemical composition and mainly by the surface morphology of the treated substrate: on the nitrocarburized sample the oxide is smooth, non porous and well adhered to the substrate; on nitrided substrate the oxide presents higher roughness value and a high porosity content. As a consequence, oxidation promotes an improvement of wear and corrosion resistance when performed on nitrocarburized substrate, but it has a detrimental effect on corrosion and wear resistance when it is performed after nitriding;
oil impregnation effect on wear resistance is moderate, but it is decisive on corrosion resistance, because the oil acts as an insulating barrier.
- 出版日期2010-5
