Vacuum carburizing process of gear steel

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Carburized steel usually refers to the steel used after vacuum carburizing, vacuum quenching and low temperature tempering. 20CrMnTi carburizing steel is a low-carbon alloy steel with excellent process performance. It is widely used in important carburizing parts with a cross-section less than 30mm that are subjected to high-speed, medium or heavy loads, impact loads and friction, such as transmission gears in automobiles and tractors. Cams, heavy-duty gears used in mining machinery, etc., but often due to poor heat treatment quality of gears, it will cause processing difficulties, cracks in gear grinding, unqualified structure and mechanical properties, etc. In this paper, through the analysis of several aspects of gear vacuum carburizing and vacuum quenching and tempering, the process is optimized to improve the performance of the material and reduce the production cost.

20CrMnTi gear steel must be subjected to vacuum heat treatment in order to achieve the required properties for processing and use. The purpose is to improve the surface hardness, wear resistance and fatigue strength, and the core has sufficient strength and toughness. The general gear processing process route is as follows:

Forging→normalizing→tooth profile processing→vacuum carburizing→vacuum quenching, low temperature tempering→shot peening→correcting spline hole→grinding.

1 Vacuum carburizing heat treatment process

1.1 Chemical composition of steel

The carbon content of 20CrMnTi is 0.20%, which belongs to low carbon steel, and the normal infiltration of carbon element is guaranteed during vacuum carburizing. After vacuum quenching heat treatment, the core obtains low-carbon martensite to ensure that the core has sufficient plasticity and toughness to resist impact loads. The alloying elements in the steel are Cr<1.5%, Mn<1.5%, Ti<1.5%. Cr and Mn alloying elements can increase the strength of the steel cable body, and at the same time improve the hardenability of the steel. Ti element can prevent the growth of austenite grains in steel and improve the tempering stability of steel. The surface of 20CrMnTi gears is required to be wear-resistant according to the service performance, and the core is required to have good strength and toughness. Therefore, the surface of 20CrMnTi steel should be carburized. After carburizing and quenching, the surface will obtain high-carbon martensite, which has high hardness and durability Abrasive.

1.2 Vacuum carburizing process

In the vacuum carburizing and quenching process, to prevent gear deformation, the surface carbon concentration and carburizing layer depth of carburizing gears must be strictly controlled. Because they will affect the expansion coefficient of the carburized layer structure, if the carbide distribution is formed on the surface after carburizing, the deformation of the tooth shape, tooth direction and spline hole will be increased. Therefore, the carbon potential during vacuum carburizing must be controlled to ensure Prevent excessive carbon concentration and uneven carbon content on the surface. The thicker the carburized layer, the greater the distortion. The carbon content on the surface affects the hardenability of the vacuum carburized and quenched gear, and the hardenability of the material has a direct impact on the structure, performance and distortion. Therefore, the depth of the carburized layer and its surface carbon content should be controlled within a reasonable and appropriate range.

2 vacuum quenching low temperature tempering

2.1 Vacuum quenching

The heating temperature of steel can generally be selected according to the Fe-Fe3C phase diagram. The vacuum quenching heating temperature of hypoeutectoid steel is 30°C ~ 50°C above Ac3, and the vacuum quenching heating temperature of hypereutectoid steel is 30°C ~ 50°C above Ac1. According to the distribution of carbon content in the surface layer of the gear after vacuum carburizing and practical experience, oil cooling from 900°C to about 820°C can get good results. If the heating temperature is too high or the holding time is too long, the austenite grains will be coarse, causing overheating or grain boundary oxidation and partial melting. Therefore, vacuum carburizing at a temperature of 900°C can be selected, and oil cooling and quenching at about 820°C can be pre-cooled. The cooling rate of vacuum quenching is too fast, and the transformation from austenite to martensite is severe. Volume shrinkage will cause a large internal stress, which will easily cause deformation and cracking of the gear. Since 20CrMnTi is an alloy steel with good hardenability, oil cooling is selected to reduce the cooling rate. The steel structure after vacuum quenching is martensite and a small amount of retained austenite. They are all unstable structures and tend to transform into stable structures. At the same time, internal stress is generated during vacuum quenching. In order to reduce or eliminate the internal stress of vacuum quenching, stabilize the structure and size, and obtain the required mechanical properties, it has been proved that the low-temperature tempering process at 200°C for 4 hours is better for heavy-duty gears. During low-temperature tempering, the supersaturated carbon atoms in the martensite gradually precipitated in the form of carbides, the degree of martensite lattice distortion weakened, and the internal stress decreased. At this time, the tempered structure is composed of martensite and carbide, which is called tempered martensite. Although the decomposition of martensite reduces the degree of supersaturation of carbon in α-Fe, and the hardness of the steel decreases accordingly, the precipitated carbides strengthen the matrix, and part of the retained austenite decomposes into tempered martensite. Therefore, the steel still maintains high hardness, wear resistance and certain toughness.

Vacuum carburizing equipment selection: A process technology must be matched with excellent equipment to play a role, among which the vacuum carburizing furnace selected in vacuum carburizing is very important. The RVC vacuum carburizing furnace produced by SIMUWU is an excellent product for this type of process. With its excellent temperature control accuracy and temperature uniformity, it can realize the brazing of workpieces evenly, so as to achieve high-level brazing standards, reduce scrap rate, increase production efficiency and reduce costs.

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