Abstract
The martensitic transformation interval of some hypoeutectoid, all eutectoid and all hypereutectoid steels covers to a large extent the region of negative temperatures. Due to the fact that the plasma hardening operation is carried out in workshops where the minimum temperature is +20 °С, the surface temperature of the part after plasma heating cannot reach negative values. Because of this, the temperature range of the martensitic transformation is not fully used and in the hardened structure there is a certain amount of austenite, which has not undergone martensitic transformation. This circumstance reduces the hardness of the hardened layer and often low tempering is required to convert residual austenite to tempered martensite, which lengthens and makes the heat treatment more expensive. Complete or almost complete martensitic transformation is possible if the surface heated by the plasma beam is immediately cooled to a negative temperature.It is shown that local cooling of the hardened surface to a temperature of –40 °C can be carried out by air using the Ranque-Hilsch tube, which significantly expands the possibilities of full hardening for eutectoid and hypereutectoid steels. The studies consisted in heating the surface with a plasma stream to a temperature of 750 °C and 900 °C. The temperature was changed by the plasma torch current and by changing the velocity of the plasma flow spot moving along the sample surface. The experiments were carried out on steels 45 (0.45 % C), U8 (0.8 % C) and U10 (1 % C). The study of the structures was carried out on a MIM-7 microscope with a video camera and with the image displayed on the screen. The approximate quantitative composition of austenite, martensite, and associated structures was determined by the areas on the screen.During plasma hardening of steel 45 from a temperature of 900 °C using the Ranque-Hilsch tube, there is practically no residual austenite in the structure. When hardening U8 steel, residual austenite is detected in a small amount. When hardening U10 steel, the amount of residual austenite is approximately 15 %. Local surface cooling allows high-quality hardening of steels of most grades, regardless of the carbon content.
Highlights
In modern construction, a large number of machines and mechanisms are used, the parts of which work under significant loads, which often leads to significant wear of their working surfaces and failure of parts
The cooling temperature during hardening of eutectoid and hypereutectoid steels either completely passes the martensitic temperature range, or most of it. This circumstance makes it possible to obtain a minimum amount of residual austenite during plasma hardening of these steels and makes low tempering unnecessary for the decomposition of residual austenite (Fig. 8, 9)
It was found that with local cooling of the surface of the part during plasma hardening, almost complete austenite-martensite transformation is observed in eutectoid steels
Summary
A large number of machines and mechanisms are used, the parts of which work under significant loads, which often leads to significant wear of their working surfaces and failure of parts. Hardening of steels by heating with concentrated energy flows, in particular, plasma hardening, by analogy with other types of hardening, consists in the formation of an austenitic structure during heating and its further transformation into martensite at the cooling stage [1, 2]. In this case, the thermal energy is greater than the energy necessary for the restructuring of the crystal lattice, and the restructuring itself occurs at a certain finite rate. The transformation is carried out in the temperature range from Ас1 to Ас3, that is, the end of the austenitic transformation shifts to the region of higher temperatures (Fig. 1)
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