Nizhny Novgorod State Technical University named after R. E. Alekseev, Nizhny Novgorod, Russia

M. N. Cheerova, Cand. Eng., Associate Prof., Dept. of Materials Science, Materials Technologies and Heat Treatment of Metals, e-mail: gavrita@yandex.ru
T. V. Nuzhdina, Cand. Eng., Associate Prof., Dept. of Materials Science, Materials Technologies and Heat Treatment of Metals, e-mail: vegas.80@mail.ru

Using the example of the technology for manufacturing spring wire from steel 65G, which includes alternating operations of cold plastic deformation (drawing) and heat treatment, the structural transformations occurring in the material during this process are considered, leading to the obtaining of different characteristics of the austenite grain and affecting both the properties of the semi-finished product and for finished products in general. The influence of strain aging, occurring during cold plastic deformation, on the processes of formation of actual austenite grains when heating a spring wire for hardening has been studied. The dependences of the characteristics of austenite grains (grain size, degree of grain size, degree of inhomogeneity and dislocation density) on heating temperature for different initial states of the ferrite-cementite matrix of 65G steel were obtained. Differences in the initial states are determined depending on the type of dislocation substructure formed after preliminary cold plastic deformation, and the completeness of the processes occurring at the first stage of heating for hardening (strain aging, polygonization, recrystallization in ferrite, spheroidization of cementite). It has been established that by excluding the influence of random factors accompanying dynamic strain aging, the characteristics of the austenite grain are significantly improved: the resulting austenite grain is finer, more uniform, resistant to growth, while maintaining a sufficient dislocation density. By calculating the generalized indicator, it was revealed that the best set of austenite grain parameters, leading to an improvement in the structural homogeneity of 65G steel, was achieved after low-temperature artificial aging before final heat treatment.

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