Investigations of rheological properties of nitrided steels using thermodilatometric method with temperature modulation

Abstract

In this work were developed issues concerning the use of thermodilatometric methods for the evaluation of rheological properties of nitrided and non-nitrided steel substrates and systems composed of steel substrate and physical vapour deposition (PVD) coating. Analysis of the rheological properties of these systems (e.g. viscosity, ability to dissipate the energy of deformation or stress relaxation) is essential from the point of view of designing nitrided substrates to the so-called duplex processes: nitriding + PVD, since the viscosity of the substrate affects the fracture toughness of the deposited coatings and their adhesion state. Analysed samples were made of 42CrMo4 steel and had shapes of a cylinder, and the nitrided layer consisted only of diffusion zone and had a thickness of 350 μm. The scope of research included analysis of the effect of thermal loads on rheological properties of the systems using created diagnostic curves (sequences of annealing at 290 °C lasting for 26 h). Experimental measurements of elongation and temperature of the systems were made using thermodilatometric method with temperature modulation. In particular, were analysed the changes in the time shift between the temperature of the sample and its elongation τ—corresponding to viscoelastic properties of the system. Was also determined the relationship between the changes of the introduced equivalent thermal expansion coefficient (corresponding to the thermomechanical interactions in the system) for the tested systems, and the stages of annealing cycles. Basing on obtained results, it was shown that the nitriding of steel substrates significantly enhances their thermal stability as compared to non-nitrided substrates. It was also shown that the nitrided substrate is less susceptible to heat-induced deformations. In addition, the used method enabled the identification of the influence of gas atmosphere in which annealing sequences were carried out, on rheological properties of substrates. In particular, for nitrided and non-nitrided substrates different discontinuity of changes in the α AC and τ parameters as a function of annealing time at the moment of change the gas atmosphere from argon to air was observed.