MICROSTRUCTURE AND ABRASIVE WEAR PROPERTIES OF HI-C MEDIUM CHROMIUM WEAR RESISTANT ALLOY STEELS AFTER HEAT TREATMENT （3）
2.1 Heat-treated structure
Fig. 3 is a metallographic picture of each alloy steel (1.1% C) as-cast and corroded by 4% nitric alcohol after being annealed at 1000 °C, quenched at 980 °C, and tempered at 220 °C. As-cast, it has a large dendrite and a large amount of Carbide. XRD analysis (Fig. 4) shows that the annealed microstructure is mainly ferrite, and there are large M7C3 Carbide + other carbides. A metallographic microscope can observe the microstructure of quenched and tempered state. There are small-sized carbides with a uniform distribution that are obviously different from that of an annealed state. It is mainly composed of martensite and Carbide, martensite fine
Fig. 5 the microstructure of alloy steel quenched and tempered at different temperatures after 4% nitric acid alcohol corrosion (1 #, 1.1% C, annealed at 1000 °C, tempered at 220 °C). It can be seen from the diagram that with the increase of quenching temperature, the grain size of alloy steel becomes coarser gradually, and part of carbides dissolves, and the small dispersed M7C3 carbides become finer. The grain size of medium chromium alloy steel quenched at 860 °C is very small, the grain size is less than 5um, the grain size of alloy steel increases gradually with the increase of quenching temperature and the grain size of alloy steel quenched at 1020 °C is larger, reaching about 50um. The quenching at 860 °C and 940 °C has little effect on the size of M7C3 carbides, but the size of M7C3 carbides quenched at 980 °C has doubled. When the quenching temperature reaches 1020 °C, most of the small M7C3 carbides are dissolved.
Fig. 6 is an SEM image of 1 # medium chromium alloy steel in different heat treatment state after 4% nitric acid alcohol corrosion. In the as-cast structure of 1 # medium chromium alloy steel, the carbide is very big; according to the scanning energy spectrum, the microstructure has sulfide inclusion. The quenched microstructure at 980 °C is composed of retained austenite, dispersed small particles M7C3 carbide, and martensite. The martensite is a bamboo-leaf shape with non-parallel distribution and is typical lamellar martensite; after tempering at 220 °C, the martensite in the microstructure increased obviously.