THE EFFECT OF HIGH-RATE ELECTRIC HEATING ON MARTENSITIC TRANSFORMATIONS IN SINTERED MULTICOMPONENT STEELS

L.O. Andrushchik, S.O. Oschkaderow

Institute for Metal Physics 36, Vernadsky blvd UA-252680 Kiev 142, Ukraine

During sintering of powder steels the properties of material obtained are largely determined by its structure and phase content which, in turn, strongly depend on heating and cooling rate. The decisive role therewith is formation of martensite as a structural component and arising in this case microstrains. The present work is dedicated to the study of activated rapid electric contact sintering influence on the structure, phase content and properties of chrome-manganese and nickel-molybdenum powder steels. Steels with a carbon content of 0-0,6% have been investigated. For comparison the investigation of steel has been performed both after classic sintering in the furnace at 1473 K (heating rate was 16K/min) and after electric contact sintering at 1473, 1573, 1673 K with exposure from 1 up to 30 min (heating rate 1900 K/min, cooling rate 250 K/min). To investigate the structure the electron scanning microscopy and X-ray method were used. The structure defect was investigated by X-ray method with use of diffractometer DRON-3M by change of physical widening of lines (110) and (220) in the dependence of steel content, pressing, temperature and duration of sintering. Along with this, mechanical properties of sintered steels were investigated. It turned out that mechanical properties of steels, sintered by electric contact heating method, are 1,5-2 times as large as those at classic sintering. The structure of the samples sintered by electric contact method turned to be ferrite-bainite-martensite one with martensite predominant. Martensite is practically absent at classic sintering. It has been determined that level of microstrains is substantially changed in the rate heating process but is little affected during isothermal exposure. Quantity of available dislocations, their type been calculated through the magnitude of the microstrains. The higher the mechanical properties of the material, the more is the percent of chaotically spaced dislocations and the less of the dislocations, spaced in the form of walls.