In order to improve the material structure, improve material properties, development of aircraft engine technology to meet the requirements of the bearing, ship loader bearing three different existing materials were studied by experiments illustrate the relationship between microstructure and properties of materials. These three materials are A casting hot rolled steel (M2), one kind of powder metallurgy material (ASP23) and a carbonaceous 0.65 (weight percent), low content of alloying elements of high speed steel derived material.
1. Fracture Toughness
In aircraft engines, since the high speed of the bearing, and the resulting strong centrifugal force, and therefore the fracture toughness of the bearing material is particularly important.
In the hardened high speed steel, carbide fracture process including initial gap caused by the emergence of crack or cracks at the end of the elastic region due to the martensitic matrix microstructure caused decohesion produce. Studies have shown that these three materials, the material with the improvement of toughness, hardness decreased, and the quenching temperature decreases.
This is mainly due to the lower half of the martensitic matrix content and improve tissue elasticity actions. Most carbide steel hardened by a low temperature, they choose adversely affect fracture toughness. In the case of low hardness, since the material yield strength decreased, the difference between the low hardness of the steel depends on Fracture explants taken to the distribution of carbides at high hardness, the decrease in the volume occupied by carbides, carbide and the size of the gap between the elastic material is about the same.
In this case, the fracture toughness is no longer primarily depends on the distribution of carbides and matrix tissue elasticity is also the smallest. Therefore, the fracture toughness values of the three material concentrated in 750HV and higher hardness. Under low hardness, a large area of the elastic case, by increasing the diameter of the carbide or carbides in the reduced hardness of the same conditions the ratio of the fracture toughness can be improved. Further, when the hardness, fracture toughness value of the rate of change is lowered. When the elastic region carbide gap between small and large, the matrix effect
on the fracture toughness of the largest organizations.In short, the fracture toughness of high speed steel mainly depends on the elastic matrix structure, and matrix organizational flexibility also affected by the carbon content of martensite and its largely. The researchers also pointed out that of course can not be ruled out starting carbide, residual stress, and other factors affecting the residual austenite and martensite tempered from precipitation of carbides, such as fracture toughness.
2. The development of fatigue cracks
Researchers crack development process into the following phases:
The first stage, cracks began to expand. Although the critical stress intensity high-speed steel can be changed by heat treatment, but the range is not large absolute value, between 3 ~ 5MPa / m2.
The second stage, crack propagation. Crack growth rate was also similar to the high-speed steel, as in the case 10Mpa / m2, the approximately 5 × 10-6mm / cycle. But with a large steel toughness can slow down the speed of crack propagation.
The third stage, the rapid expansion of the crack. When the crack reaches this stage, mobile crane bearing soon fail.
3. Surface Treatment
According to the study of the relationship between the fracture toughness and other materials crack development process and analysis, we can see that in the presence of high and dn value hoop stress of the situation, the low carbon content of the surface treatment and high-speed steel HSS hardened surface depth change is significant. For this reason, development of a surface treatment process, can enhance the surface hardness and compressive stresses. Surface-treated material has the following characteristics: has good resistance to fatigue crack resistance, the second stage, fracture toughness values have
lower crack growth rate and insufficient hardening or due to the low carbon content of the core material of higher and so on.Such surface treatment method developed comprising:
(1) with a laser, electron beam or induction hardening methods such as inadequate hardened local heat treatment, in order to obtain a surface hardened layer. It should be noted that hardening is insufficient to increase the fracture toughness.
(2) obtained by chemical treatment hardened surface layer. Such methods include carbon or nitrogen to penetrate into the ordinary high-speed steel microstructure, or a small change in the carbon content of the degree.
In short, the company believes in the development of new or improved analysis method of steel heat treatment process for the next generation of aircraft engines, considerable benefit can be obtained from the resulting effect of surface treatment. It can be said that if this microstructure and heat treatment related to the development of the means of mechanical properties once people fully grasp, hardenability steel and high speed steel is bound to be fully utilized in the aero-engine bearings.
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