Technological solutions for producing quality castings from high chromium iron

Abstract

The article presents the results of research conducted to check the effect of alloying with manganese and molybdenum and modification of high-chromium cast iron with niobium on the formation of its structure during heat treatment. The effect of heat treatment in the temperature range of 450...900°C and normalization at a temperature of 1000...1200°C on structural transformations, hardness and wear resistance of complex alloyed high chromium cast irons was studied. The research material was samples of cast iron (1.8...2.3% C and 12...20% Cr), doped with manganese, molybdenum (2...4% Mn, 0.6...1.2% Mo) and modified with niobium, it was checked microstructures of samples in the cast state and when heated to temperatures of 650 °C and 780 °C with the following content of elements: С – 2.07 %, Cr – 19.7 %, Mn – 3.55 %, Mo – 1.02 %, Nb – 0.3%, Si – 0.6%. Experiments were carried out in the temperature ranges: 450...500 °C; 500...550 °C; 600...650 °C; 700...900°C; 1000...1200 °C with varying holding time from four to twenty hours. The hardness of cast iron is 40-42 HRC. The microhardness of austenite is 340-420 Nμ. The influence of the degree of alloying and the number of individual alloying elements in the alloy on the beginning of the austenite transformation was established. When heated to 500...550 °C with an increase in the chromium content in the alloys, the time to the beginning of the transformation increases. In alloys with a chromium content of 17.7%, the beginning of transformation was observed after 4 hours, while in alloys with a chromium content of about 22%, the beginning of transformation was detected after 9...10 hours of exposure. During normalization (1050...1100°C), the Me7C3 → Me23C6 transition process occurs faster in chromium cast irons with molybdenum than in alloys with tungsten. With a molybdenum content of more than 0.6%, Me7C3 carbide is completely transformed into Me23C6, which in the range of 0.6...1.2% is the only carbide phase. It was found that the amount of martensite depends on both the heating temperature and the cooling rate. When cooling at a rate of 30 °C/h, less martensite is formed in the cast iron structure than when cooling at a faster rate of 70 °C/h. It was found that in the cast state, the structure of high-chromium cast irons consists of austenite and austenite-chromium carbide eutectic, which contains trigonal carbide (Cr,Fe)7С3. As a result of heating cast iron in the temperature range of 450...900°C and normalization at a temperature of 1000...1200°C, austenite has a different ability to disintegrate, and this affects the phase composition of cast irons, their structure and properties. The transformation into austenite begins with the release of a dispersed carbide phase (Cr,Fe)23С6 at a temperature of 650°C. It was determined that the experimental complex-alloyed high-chromium cast irons in the cast state had a hardness of 40...47 HRC, and after normalization at a temperature of 1050...1100°C with a holding time of 1 h - 55...62 HRC. At the same time, about 30% of austenite, martensite and carbide phase remain in cast iron. It was determined that normalization at a temperature of 1050...1100°C with a holding time of 1 hour contributes to the formation of a metastable structure with high resistance to destruction with the content of excess austenite, martensite and the carbide phase (Cr,Fe)23С6. Keywords: alloying elements, cast state, high-chromium cast iron, heat treatment, phase-structural state.