Wear and corrosion resistance modification of nitrided and nitrocarburized steeels | [Modifica della resistenza a usura e a corrosione di acciai nitrurati e nitrocarburati]

Nitriding and nitrocarburizing induce surface chemical modification that are influenced by alloying element present in steels [1]. Nitriding treatment forms this layers sequence: a. the more external one, named white layer, formed by e-Fe2-3N e y'-Fe4N nitrides, b. the underlying diffusion layer in which nitrides of alloying element precipitates at grain boundary. The simultaneous presence of active carbon and nitrogen during nitrocarburizing treatment forms an external compound layer, composed by e-Fe2-3(N,C) and Y'-Fe4(N,C) carbonitrides laying on diffusion layer. The use of thermochemical treatments to enhance fatigue and wear resistance performances of highly stressed mechanical components is well known and established [2-5]. However it's technologically interesting post-nitriding and post-nitrocarburizing treatment development to guarantee good aesthetic and mechanical properties and high wear and corrosion resistance. Post-oxidation treatment, already studied in years [6-12], causes: a. nitrides and carbonitrides partial decomposition; b. Fe3O 4 growth on the surface; c. partial surface microporosity closure. Remaining open porosity, filled with lubricant oil (impregnation), could further increase wear and corrosion resistance. Aim of the work is the characterization and optimisation of post-treatments on nitrided and nitrocarburized 41CrAlMo7 and X37CrMoV5-1 steels, 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 scanning electron microscopy and surface roughness measurements, in order to evaluate the surface morphology influence on wear and corrosion resistance. In can be concluded that, in the applied experimental conditions: 1. in both steels, nitrocarburizing a compound layer 11-16 m thick forms, constant in whole material, while nitriding a white layer, 9-10 (xm thick; 2. in both steels, the nitrided diffusion layer is thicker than nitrocarburized one and therefore nitriding allows higher total and effective hardening depth; 3. after the treatments of Tab. II, 41CrAlMo7 shows higher diffusion layer, while X37CrMoV5-1 achieved higher surface hardness than other steel. For this reason X37CrMoV5-1 presents lower wear rate; 4. 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; 5. oxidation forms Fe 3O4 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; 6. nitriding and nitrocarburizing promote an significant improvement of wear resistance, and nitrided steels exhibits the highest wear resistance. All the nitriding post-treatments decrease wear resistance; 7. nitriding and nitrocarburizing promote a significant corrosion resistance increment, oxidation has positive effects only after nitriding, because of post-oxidized surface morphology; 8. oil impregnation effect on wear resistance is moderate, but it is decisive on corrosion resistance, because the oil acts as an insulating.