Directional solidification of high chromium cast iron Направленная кристаллизация высокохромовых чугунов Насочена кристализация на чугуни с високо хромово съдържание S. C. PANIGRAHI

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Cast irons in general are rated as poor so far as the mechanical properties are concerned although they have many other attractive properties. High chromium cast irons for example find good use wherever the corrosion and oxidation resistance properties are of prime importance. The mechanical properties are however poor. This work was aimed at obtaining high, strength in high chromium cast irons by suitable alignment of the carbides through directional solidification. Fe-30 per cent Cr-C alloys with carbon varying from 2.6 to 3.2 per cent were prepared from high purity constituents in a vacuum induction furnace. The alloys were directionally solidified at rates 3 to 7200 mm/hr in a moving sample type apparatus with a platinum wound furnace making use of gallium-indium eutectic as a coolant, so as to maintain a high and fixed thermal gradient at the liquid solid interface. The sample length was scanned for the regions of aligned growth. Quantitative metallographic analysis was done for these regions and the conditions of aligned growth with respect to the rate of growth and the composition determined. The strength of the directionally solidified samples were measured and related to the quantitative metallographic data. It is shown that it is possible to obtain aligned structures free from primary constituents with compositions in the neighborhood of the monovariant eutectic line in the Fe-Cr-C system. In alloys far removed from the eutectic line there was complete or partial segregation leading to an initial zone with primary constituents. Very high strength levels exceeding that of the super strength materials (3100 MPa) has been obtained with low rates of growth and near the monovariant eutectic line of the system by the above method. The strength levels obtained is very closely related to the structure at rates of growth up to 600 mm/hr. The presence of primary constituents reduce the strength to an appreciable extent. Volume percentages carbide, intercarbide spacings also affect the properties. At rates of growth comparable to that of continuous casting the strength obtained was around 2500 MPa. For compositions removed from the eutectic line and with primary constituents have a logarithmic relationship between strength and the rate of growth.