Influência da velocidade de deslizamento e pressão de contato sobre o desempenho tribológico de aços ferramenta para estampagem a frio

Abstract

The use of high-strength metal sheets in the mechanical forming process results in an increase in tool wear rate. This wear is especially significant in deep-drawing tools, presenting a significant economic impact on the costs of manufactured products. To design a wear-resistant tool that is not unnecessarily oversized, it is important to predict the tribological behavior during operation, as a result of the adopted operational parameters. This way, it is possible to ensure that it is sufficiently resistant while avoiding unnecessary oversizing that results in higher production costs. In this context, this research aimed to investigate the influence of contact pressure and sliding speed on the tribological behavior of two steels used in the manufacture of tools for cold sheet metal forming (here named, steels A and B). To achieve this objective, the strip drawing test was adopted, seeking to approximate the experimental conditions to the real scenario of the deep drawing process. Based on the test results, the tribological behavior of steels A and B was evaluated in terms of friction and wear coefficients, as well as the active wear mechanisms. The characterization of worn tracks was performed using scanning electron microscopy, energy dispersive X-ray spectroscopy, and confocal laser scanning microscopy, the latter also used to determine the 3D roughness parameters of their surfaces. The drawn sheets were also characterized after testing, using macrographic analyses and 2D roughness parameter measurements. To support the discussion of the tribological test results, steels A and B were preliminarily characterized in terms of its microstructural and topographical characteristics. The microstructural characterization showed that both steels have a microstructure formed by carbides surrounded by a martensitic matrix, with equivalent hardness. The preliminary topographical characterization showed that the tool manufacturing operations produce distinct surface characteristics on each steel. The results of the tribological characterization indicated that steel A has a lower friction and wear coefficients rate than steel B, with the former being more prone to abrasion wear mechanism and the latter to adhesion. It was found that the variation of the relative sliding speed of the sheet produces changes in the friction coefficient only under conditions of high contact pressure, decreasing for steel A and increasing for steel B. It was also shown that increasing the contact pressure increases the values of the friction and wear coefficients rate in both steels. In these cases, abrasion wear mechanism predominates in steel A under low contact pressures, and adhesion wear prevails under high pressures. For steel B, on the other hand, adhesion wear predominates under both high and low contact pressures. The topographical and 3D roughness characterization of the tested tools showed that the increase in contact load and sliding speed has a direct relationship with some roughness parameters and an inverse relationship with others, with this behavior being dependent on the predominant wear mechanism. Finally, the results of the analysis of variance confirmed that contact pressure has a greater influence on the tribological behavior of both steels.