Chabak Y. The enhancement of heat treatment modes for high-chromium cast irons in purpose to improve their machinability

Рurpose - the creating of necessary microstructural state in high chromium complex-alloyed cast irons which allows to improve their machinability by means of heat treatment modes improving. Object - the processes of phase-structural transformations in alloyed cast irons. The subject - the regularities of influence of heat treatment on microstructure and properties of high chromium cast iron with a high content of austenite-forming elements. Methods - an optical and scanning electron microscopy, energy dispersive analysis, X-ray difraction, thermal analysis, dilatometric analysis; hardness and microhardness measurement; wear tests and tests for machinability; computer simulation, mathematical statistics. Novelty - basing on constructed isothermal diagrams of decomposition of primary austenite it was shown that the precipitation of secondary carbides in cast irons containing 13-20% Cr and totally 3-6% of Mn, Ni, Cu, Mo, proceeds with a maximum rate at 950 °C and complete within 2-2.5 h of soaking; the processes occurring in the surface layers of iron 270Х15Г2Н1МФТ at 1100-1150 °C in an oxidizing atmosphere is studied; the peculiarities of the mechanism of dissolution of eutectic carbides during decarburization are described, it is shown that they consist of the development of channel-shaped cavities inside eutectic carbides which are oriented along the long axis of carbides; basing on constructed isothermal diagrams of decomposition of primary and destabilized austenite at temperatures below the A1, the effect of slowing the eutectoid transformation in the complex-alloyed high chrome cast irons due to the precipitation of secondary carbides at high temperature destabilizing soaking was found; the effect of increasing by 23 % of abrasive wear resistance in high chrome cast irons was revealed; this effect occurs in the case of preliminary (softening) heat treatment followed by quenching from lower at 50 °C temperature with reduced exposure by 4-5 times that leads to form of austenitic-martensitic-carbide structure with increased wear resistance.