Effect of microstructure of bainite martensite composite wear resistant cast steel

As shown in Figure 1, the types of bainite structure include upper bainite, lower bainite, carbide free bainite, granular bainite, quasi bainite, etc.

(a) Upper bainite; (b) Lower bainite; (C) Carbide free bainite; (d) Granular bainite; (e) Quasi bainite

The upper bainite is feathery and has obvious crystal trend. The carbides in the upper bainite are rod-shaped and distributed in parallel between the ferrite laths. Due to the high formation temperature, the ferrite and carbides are relatively coarse. The lower bainite is acicular, the ferrite needle is fine and evenly distributed, there are a large number of carbides in it, and the dislocation density is very high. Therefore, the lower bainite has high strength and good toughness. In carbide free bainite, there are only bainite ferrite and retained austenite without carbide, or carbon free bainite, which usually appears in low carbon and low alloy steel. The ferrite strips are directly arranged in parallel, the size of ferrite is large, the spacing between ferrite strips is wide, and there is carbon rich austenite between ferrite strips. Granular bainite is granular or island shaped. The island is formed by a large number of decomposition products of retained austenite, martensite and austenite. The island is distributed in the ferrite matrix and plays a role in strengthening the second phase of the structure. Quasi bainite is similar to carbide free bainite, there is no carbide, and there is thin film carbon rich austenite between ferrite laths.

According to the different carbon content, bainitic martensitic steel is mainly divided into high-carbon bainitic martensitic steel, medium carbon bainitic martensitic steel and low-carbon bainitic martensitic steel. Low alloy bainite martensite steel has attracted extensive attention as a wear-resistant material because of its high hardness and sufficient toughness. By adjusting the composition and heat treatment process, the reasonable combination of hardness and toughness of low alloy wear-resistant steel can be controlled in a large range. In recent years, China has successively developed zg42cr2mnsi2mo, zg40crmnsimore, zg70crmnmobre and other low-alloy martensitic wear-resistant steels, which are applied to the lining plate of ball mill, and their service life is more than 1.6 times that of high manganese steel lining plate.

Lath martensite is obtained after quenching of low-carbon bainite martensite steel, and low-temperature tempering is carried out after quenching to ensure wear resistance, eliminate quenching stress and obtain certain toughness. On the premise of ensuring sufficient bremsstrahlung, it also has high hardness, which is about HRC50.

The internal structure of medium carbon bainite martensite steel is mainly residual austenite, twin martensite and carbide. The residual austenite film is discontinuous distributed between the pins and plates of martensite, which improves the toughness and wear resistance of the steel.

High carbon bainite martensite steel adopts high carbon content to obtain high hardness matrix and a certain amount of carbide hard phase to resist the wear of hard materials. The microstructure of twin martensite and retained austenite can be obtained by reasonably adjusting the chemical composition, and the retained austenite is distributed between the martensite needles in the form of discontinuous film; At the same time, the carbides are distributed on the martensitic matrix in the form of fine particles. The carbides of this shape can hinder the local deformation and uniformly strengthen the matrix, so as to improve the wear resistance.

Bainite / martensite multiphase structure, as an incomplete bainite transformation structure, has gradually entered people’s vision and attracted the attention of scientific and technological workers because of its good combination of strength and toughness. Studies by Tomita et al. Show that the multiphase structure containing an appropriate amount of lower bainite or carbide free bainite can increase the strength and initial impact properties of steel.

(a, b) deformed structure; (C) Bainite axial deformation; (D, e) interlaced martensite laths; (f) twin martensite

The research of Huang Weigang et al. And song Yujiu et al. Shows that the strength of medium low carbon and medium high carbon bainite / martensite multiphase steel containing an appropriate amount of lower bainite is slightly lower than that of single martensite steel, but the initial impact resistance is significantly increased. The research of Jiang Yehua and others shows that the medium carbon low alloy Bainite / martensite multiphase steel has good strong bremsstrahlung fit, excellent abrasive wear resistance and good comprehensive mechanical properties. Cai Minghui et al. Showed that replacing martensite with appropriate amount of bainite can reduce the hardness ratio between hard phase and soft phase in the structure, help to improve the initial properties of hard phase and improve the comprehensive properties of multiphase steel. Wang et al. Pointed out that in Bainite / martensite multiphase steel, flake retained austenite is sandwiched between bainite or martensite laths. As a softer phase, retained austenite is more sensitive to deformation or crack. When deformation or crack occurs, this form of retained austenite generates martensitic transformation to resist deformation or hinder crack propagation, Thus, the strength and toughness of Bainite / martensite multiphase steel are improved. Figure 2 shows some microstructure strengthening mechanisms in bainite martensite multiphase steel.

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