INFLUENCE OF THE TWO-STAGE SINTERING MODE ON STRUCTURE FORMATION AND PROPERTIES OF POWDERED ALL-PRESSED BIMETALLIC IRON-BASED MATERIALS

The paper justifies the urgency and efficiency of obtaining bimetallic iron-based materials by two-step isothermal sintering to enable forming the structure of the product bases at the first stage and activating diffusion processes in the wear-resistant layer only at the second stage to eliminate any high-porosity areas and brittle inclusions at interlayer boundaries typical for powder materials doped with carbides, nitrides and borides. The analysis of equation solutions for diffusion in two-component heterogeneous powder systems made it possible to propose an option for determining the time and temperature of homogenizing sintering of bimetallic materials taking into account grain-size distribution of powders, concentration and partial diffusion coefficients of components, charge bulk density, initial and final porosity of the products. Experiments proved that bimetallic materials containing 15– 20 wt.% of chromium carbide, 20–25 wt.% of ferrochromium and iron as the rest component in the wear-resistant layer charge have the best combination of hardness, wear resistance and radial compression strength after sintering in a chamber furnace in protective medium at 1150–1180 °C with a holding time of 1,5–2,0 hours at the first stage, and in an induction furnace at 1350– 1370 °C for 25–35 s with a heating rate of 450–470 °C/s at the second stage. Structure formation peculiarities of the interlayer boundaries and wear-resistant layer during two-step sintering of all-pressed bimetallic materials are shown. It is found that for high-temperature sintering by high-frequency (8 or 16 kHz) heating at the second stage, the depth of chromium diffusion from the wear-resistant layer to the matrix is 120–130 μm, and Cr concentration in various points of interlayer and interparticle boundaries varies between 1 and 30 wt.% thus allowing formation of a transition layer with a structure consisting of a ferritic-austenitic matrix with martensitic colonies and dispersed particles of (Cr,Fe)23C6, (Cr,Fe)7C3 and (Cr,Fe)3C2 ferrochromium carbides uniformly distributed over the volume.