Development of bainitic martensitic wear resistant cast steel

In the late 1920s, Robertson first discovered the medium temperature transformation product later named bainite in steel. Later, the researchers further found the microstructure of upper bainite, lower bainite, granular bainite, carbide free bainite, columnar bainite, abnormal bainite, massive bainite, low-carbon and low-alloy bainite and quasi bainite, forming a relatively complete bainite transformation theory. In recent decades, great progress has been made in the application and research of bainite theory. The research and development of bainitic steel has attracted great attention in academic and engineering circles, and bainitic steel has also been widely used in industrial production.

Mo-B bainitic steel: in the 1950s, British P.B. Pickering and others invented Mo-B air-cooled bainitic steel. The combination of Mo and B can make the steel obtain bainite structure in a wide range of continuous cooling rate. The emergence of Mo-B or Mo bainitic steel has attracted worldwide attention. However, due to the high price of Mo raw materials, high initial transformation temperature of Mo-B steel and poor strength and toughness of products, in order to reduce this temperature, Mo-B steel must be metallized, which increases the production cost, so its development is limited to a certain extent.

Mn-B bainitic steel: Fang Hongsheng of Tsinghua University found that when Mn content is certain, there is obvious separation of upper and lower C curves on the isothermal transformation curve of undercooled austenite; The combination of Mn and B makes the incubation period of high temperature transformation significantly longer than that of medium temperature transformation. Therefore, Mn-B air-cooled bainitic steel is successfully alloyed with common elements. Because an appropriate amount of Mn can lead to its enrichment at the phase boundary at medium temperature and drag the phase boundary migration, it is easy to obtain bainite together with B; At the same time, Mn significantly reduces the driving force of bainite transformation, reduces the bainite transformation temperature, refines the bainite size, and improves the initial properties and strength. It breaks through the traditional design idea that Mo and W must be added to air-cooled bainitic steel, and develops medium high carbon, medium carbon, medium low carbon and low carbon Mn-B series bainitic steel. On the basis of Mn-B bainitic steel, some scholars obtain the CCT curve shown in Figure 1 through reasonable alloy design and the favorable effect of Mn on bainite formation, so that the bainite structure can be obtained in a large cooling rate range.

Bainitic martensitic wear-resistant cast steel is a microstructure dominated by bainite, which contains a certain amount of austenite. It has high advantages in strength and hardness. Low alloy wear-resistant cast steel whose main microstructure is bainite has become a common research direction. Generally, the low stress impact wear is dominated by cutting mechanism, and the hardness of the material has a major impact on its wear resistance; The high stress impact wear is mainly white layer spalling, and the strong initial property of the material has a major impact on its wear resistance. It has excellent strength and toughness, so it has high wear resistance and can take into account cutting wear and peeling failure. As shown in Figure 2, under the same hardness, the wear resistance of bainite structure is significantly better than that of martensite structure. In addition, the addition content of alloying elements in bainitic Martian multiphase wear-resistant cast steel is small, and there is no or only a small amount of precious elements such as Ni and Mo. the cost is low. The cost is the lowest among several kinds of materials such as high chromium cast iron, high manganese steel and low alloy wear-resistant cast steel. The hardenability is also very high. Water pan can penetrate a cylinder with a diameter of about 600mm. The tensile strength of Mn-B air-cooled bainite Martian multiphase wear-resistant cast steel developed by domestic and foreign scholars can exceed 2000mPa, the impact toughness can reach 40-300j / cm2, and the wear resistance exceeds more than 30% of high manganese steel.

China has made some achievements in the research of Bei Ma multiphase wear-resistant cast steel technology, and has been well applied in the fields of mining, building materials and so on. In the future, the research work on bainitic Martian multiphase wear-resistant cast steel will continue, and the main research direction is the research on its stability and uniformity; At the same time, the research focus can be on improving its initial properties and hardness at the same time, so as to comprehensively improve its wear resistance; It can also improve the casting process and improve the quality of steel wear-resistant materials; Carry out large-scale production of large castings and expand the application scope of bainite Martian multiphase wear-resistant cast steel. The research and application situation of bainitic Martian multiphase wear-resistant cast steel is very good at home and abroad, and has a very broad development prospect. On this basis, the research and application of bainitic Martian multiphase wear-resistant cast steel can be strengthened.

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