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High temperature vacuum carburizing gear steel

When the gear carburizing temperature is increased from 930 ℃ to 1 000 ℃, the carburizing time can be shortened by more than 50%, and the production efficiency can be significantly improved. However, the austenite grains tend to grow during high temperature carburization of commonly used gear steels, which greatly reduces the fatigue properties. Therefore, in recent years, people have devoted themselves to the research and development of high temperature carburizing gear steel. In order to overcome the problem of grain growth during high temperature carburization, microalloying elements such as Nb, Ti, and B are added to the gear steel.

1 High temperature carburizing grain growth control

The prior austenite grain size of gear steel is an important microstructure parameter, and secondary quenching and microalloying are important ways to control the grain growth. In the secondary quenching process, a large number of dispersed carbides are formed, which hinder the migration of grain boundaries during the heating homogenization process, that is, can control the grain size. However, the secondary quenching process complicates the industrial production process of gears and increases the cost. Therefore, it is necessary to study and find a grain control method with simpler process and lower production cost.

Now, it has been found that microalloying is an effective means to inhibit the growth of prior austenite grains in gear steels. In the process of vacuum carburizing heat treatment, the undissolved precipitates can make the migration of austenite grain boundaries difficult. After carburizing, the furnace temperature was lowered to 830-840 °C for 30 min, so that the carbon distribution and structure were uniform, and the material strength was improved. Therefore, the microalloying plus homogenization process, followed by a quenching simplifies the production process, while also ensuring the performance of the gear steel.

Most of the commonly used gear steels add Al to refine the grains with AlN. Since AIN is unstable at high temperatures above 1000 °C, the austenite grains of commonly used gear steels are easily coarsened during high temperature carburization. The high temperature carburizing gear steel mainly prevents austenite by adding Nb, Ti, B and other microalloying elements to form more stable Nb(C,N) and Ti(C,N) precipitation phases at 1 000 ~ 1 100 ℃ Grains grow.

A key feature of high temperature carburized gear steels is the use of Nb microalloying, which is considered to be the best element for grain refinement. Because, the Nb-containing precipitates not only have high temperature stability, but also have small particle size, which is spherical or ellipsoid, which has a good effect of grain refinement.

In Nb microalloyed gear steel, a small amount of Ti or B is often added to fix N, so that the NbC content in the precipitation phase increases. Because both TiN and BN are precipitated before NbC, the N content in the steel decreases, and the NbC content of the smaller size increases.

2 Microstructure control of high temperature carburizing carburizing layer

Gear steel is carburized at high temperature above 1 000 ℃, and the degree of surface oxidation is very serious, so a vacuum carburizing furnace is required. After the gear steel is carburized by vacuum high temperature, the content of retained austenite and carbides in the carWarm extrusion die vacuum heat treatmentburized layer will change greatly. Therefore, it is necessary to control the structure of the carburized layer so as not to have an excessive influence on the material properties.

The existence of retained austenite in gear steel is inevitable and necessary, and an appropriate amount of retained austenite is beneficial to the properties of the material, especially the rotational bending fatigue properties. When the carburizing temperature increases, the diffusion coefficient of carbon increases exponentially, and the carbon content in the carburized layer increases, resulting in a decrease in the start and termination lines of martensitic transformation. Carbon will rise. It is generally believed that the content of retained austenite in gear steel should be maintained at 15% to 25%, and the content of retained austenite in this range is beneficial to material properties. However, if it is higher than 50%, it will seriously reduce the surface hardness of the gear and reduce the rotational bending fatigue performance.

In addition, high-temperature carburizing tends to increase the carbon content of the carburized layer, and the tendency to form a large amount of carbides increases, which tends to gather at the prior austenite grain boundaries to form reticulated or massive carbides, making the grain boundaries brittle and reducing the material fatigue life. To sum up the above, the carbon potential during high temperature carburizing should not be too high, and some scholars have proposed that the carbon content on the surface of gear steel should be controlled at about 0.8%.

The micro-alloyed gear steel is carburized at high temperature within an appropriate time range, the austenite grains can still be kept small and uniform, and it has good fatigue properties. For example, the hot-rolled Nb-Ti-containing SAE8620 gear steel containing Nb-Ti in the hot-rolled state of SAE8620 gear steel containing Nb-Ti increases with the increase of Nb content, and the prior austenite is The grain size decreases, and the cantilever bending fatigue limit shows an upward trend. The corresponding fatigue limits of the three Nb contents are 758, 827, and 965 MPa, respectively. The SCr420Nb steel was added with “blast” cooling in the high-temperature vacuum carburizing process at 1050 °C, and then Recrystallization occurred during heating. Compared with vacuum carburizing at 930°C, the grains were refined (1 to 2 grades), and the fatigue strength was increased by 10%. It can be seen that the fatigue strength of the microalloyed gear steel containing Nb and Nb-Ti after high temperature carburizing treatment, adding a specific vacuum heat treatment process, compared with conventional temperature carburizing, the fatigue strength will not be reduced, but a quantitative improvement .