Studying the influence of heat treatment modes on the properties of medium-carbon steel

Abstract

Problem. Parts of modern machines and structures operate under high dynamic loads, stress concentrations, and low or high temperatures. All of this contributes to brittle fracture and reduces the reliability of machines. Therefore, structural steels must, in addition to high mechanical properties, have high structural strength, which is manifested in the conditions of its actual use in the form of parts, structures, etc. [1] A properly selected heat treatment mode allows to obtain an optimal combination of properties in a part, thereby ensuring its reliability and operational durability.
 Goal. The aim of the study is to determine the effect of heat treatment parameters on the hardness, impact strength, and structure of 40 and 40X medium-carbon steels. Method. In the course of the work, a set of mechanical studies was carried out on samples of 40 and 40X steels in the initial state and after various heat treatment modes. Results. Based on the analysis of mechanical studies, the influence of heat treatment parameters on the mechanical properties and structure of medium-carbon steels 40 and 40X was studied. On the basis of mechanical and microstructural studies, the relationship between the effect of economical alloying of medium-carbon steel and the main indicators of mechanical properties in the range of heating temperatures was established. The experiments conducted in this work allowed us to obtain a comparative characterization of the effect of various technological parameters of heat treatment on the structure and properties of medium-carbon steels 40 and 40X, which are being improved. The factors that may affect the quality of parts made from these steel grades were also identified. Summarizing the results of metallographic studies and mechanical properties, the following conclusions can be drawn: 1. Steel 40, which has low supercooled austenite stability, must be cooled sharply (cold water, 8-12% aqueous solutions of NaOH or NaCl, etc.) to obtain continuous hardenability of parts. 2. The addition of 1 % chromium increases the stability of supercooled austenite, i.e. increases the hardenability of steel, so parts made of economical 40X alloy steel can be cooled in oil. However, if the part is large, a higher cooling rate should be used – in water. 3. Normalizing can be recommended for 40 and 40X steels as a work hardening heat treatment that increases ductility and impact strength. However, if the hardness level of the normalized alloy steel is high, then full or isothermal annealing should be used. 4. To prevent decarburization of steel at high temperatures and long holding times, protective atmospheres must be used. In the case of processing parts in furnaces with an oxidizing atmosphere, the parts must be filled with graphite, because cast iron chips do not provide reliable protection against oxidation. 5. In the range of tempering temperatures of improving steels recommended by the reference literature (400-600 °C), a monotonic decrease in hardness of both steels occurs, but in carbon steels, the hardening is more intense than in alloy steels. 6. To prevent the development of tempering brittleness of the second kind, to which 40X steel is prone, cooling during tempering should be accelerated (in oil, water), especially if the tempering temperature exceeds 550 °C.