Improvements of Mechanical Characteristics of Gray Cast Iron by Heat Treatment and Justification of Its Parameters

Abstract

The article discusses the technology of heat treatment of gray cast irons in order to improve their mechanical characteristics by implementing a bainitic transformation of the metal matrix. It is proposed to use the environmentally safe technology of hardening “in two waters” to start the process of transformation of austenite of a gray cast iron matrix into bainite without using environmentally harmful baths with molten alloys or salts, and further complete or partial isothermal decomposition of austenite in air furnaces. This not only improves the environmental performance of the process, but also significantly reduces the cost of it. The definition of rational parameters of the process of heat treatment of gray cast irons was carried out using the method of planning and implementing a multifactor experiment. The adequacy of the results obtained as a result of measurements with the implementation of parallel experiments was checked using Student's criterion and satisfied the requirements, and also allowed to discard insignificant members of the regression equation. The regression model of the influence of process parameters on the hardness of thermally hardened cast iron was tested using the Fisher criterion, which showed its adequacy. Experimental studies to determine the factors that determine the strength of gray cast iron hardened to bainite made it possible to establish rational temporal parameters of heat treatment. It is shown that the exposure time in the furnace at 850 °C before cooling and the exposure time in the furnace with a temperature of 400 °C in the process of austenite decomposition should be limited to 30 minutes. The implementation of heat treatment technology with certain parameters to improve the quality of casting, mass produced at the plants of the region, allowed to increase the compressive strength of cast iron by 1.5 times, the surface hardness by 2 times, and the brand of cast iron to increase from SCH20 to SCH25—SCH30. At the same time, it was possible to reduce emissions of vapors of salts, metals and other products of the interaction of melts with the atmosphere.