Vyatka State University (Kirov, Russia)

E. A. Marinin, Cand. Eng., Associate Prof., Dept. of Information Technologies in Mechanical Engineering, e.marrini@gmail.com
A. A. Slyudova, Senior Lecturer, Dept. of Information Technologies in Mechanical Engineering
Fu Yifang, Master’s Student, Dept. of Information Technologies in Mechanical Engineering

The aim of this paper is to study the patterns of formation of the mechanical properties and structure of medium-carbon structural steels using Steel 45 as an example with various Q&P heat treatment modes. Modern methods of hardening treatment, particularly Q&P (Quenching and Partitioning), can increase the resource of widely used medium-carbon steels with limited mechanical properties. This technology opens up new opportunities in industry, solving two main tasks: reducing the weight of structures and increasing their safety and durability. The microstructure of steel after Q&P heat treatment consists of martensite, residual austenite and, in some cases, bainite. Martensite has a high dislocation density which increases the strength of the experimental steel; the residual austenite belongs to the soft phase, so plasticity induced by martensitic transformation (TRIP) is formed during deformation. The method of the experiment involves a temperature change of the finishing operation of Q&P treatment from 200 to 600 °C. Polymethylsiloxane (PMS-200) was used to maintain the temperature of the quenching medium between the beginning and the end of the martensitic transformation (230 °C). To get objective information, the study of the microstructure and mechanical properties (impact strength) were also made for normalized and improved samples. It is shown that when the temperature of the final Q&P heat treatment operation changes from 200 to 500 °C, there is a significant increase of impact strength (from 12.25 J/cm² to 22 J/cm²) and there is a slight decrease of hardness. When the temperature of the final Q&P heat treatment operation increases to 600 °C, there is a grain growth; it decreases its impact strength (12.75 J/cm²), that is, performance characteristics deteriorate. The evolution of the microstructure was found with a change of the temperature of the finishing operation of Q&P heat treatment from 200 to 600 °C. The structural mixture of martensite and residual austenite transforms into highly dispersed perlite with visualized carbide inclusions and a small amount of residual austenite.

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