Physical modeling of heat treatment modes for large-sized forgings of Cr – Ni – Mo – V composition steel under conditions of long-term isothermal annealing using dilatometry

The study examined 25KhN2МFА steel of Cr – Ni – Mo – V composition, which is characterized by an increased tendency to structural heredity in large-section forgings. Physical modeling of heat treatment of a large-sized forging made of the steel under study was carried out using dilatometry in order to establish the modes of diffusion transformation of ferrite/pearlite and obtain a homogeneous fine-grained structure after re-quenching. Quantitative microstructural studies of the model samples after prolonged isothermal annealing were conducted using optical microscopy. It was found that after annealing in the temperature range from 650 to 700 ℃ for 30 – 50 h, significant decarburization of the surface layer of small-sized model samples occurs. Depletion of the surface in carbon reduces the stability of austenite, and the phase transformation begins at a higher temperature in the region of ferrite formation, which significantly affects the quantitative assessment of phase transformations given by dilation. To minimize this process, which does not allow the results obtained on small samples to be transferred to large-sized steel parts, it is necessary to find barrier coatings that suppress the diffusion of carbon from the surface layers under long-term annealing conditions. The authors tested palladium, high-temperature varnish and sealant, nickel, and aluminum coatings. The protective properties of diffusion barriers were investigated by determining the depth of the decarburized layer and carbon content both in the surface layer and in the entire volume of a small sample. The best protective properties are provided by an aluminum coating applied using the iodine transport method.