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17Cr2Ni2Mo Steel Gear Vacuum Carburizing

When the gear is working, it must not only bear a large impact load, but also bear alternating bending stress and contact stress. Therefore, the gear should have high strength, impact toughness and good fatigue resistance. When the gear pair meshes, the pressure and friction coefficient at the contact point are both very large. It causes serious wear at the contact area, therefore, the tooth surface must have good wear resistance. Considering these factors, in the general principles of gear design, it is determined that the safety factor of the gear should be 2.5~3.5. A lower safety factor reduces the service life of the machine. Therefore, in order to ensure the normal operation of the gear and increase the service life of the gear, it is necessary to select materials with suitable properties and determine the best heat treatment process.

1 Test materials and test methods
The test material is 17Cr2Ni2Mo steel.

2 vacuum carburizing

There are three common methods of gas carburizing: one-stage carburizing method, segmented carburizing method and constant temperature variable carburizing potential carburizing method. After analysis and comparison, combined with the existing test equipment (vacuum carburizing furnace), this test decided to use the method of constant temperature and variable carbon potential for carburizing. This process is intensive infiltration and diffusion, that is, a higher carbon potential is used in the intensive infiltration period (the carbon potential is controlled at 1.1%~1.4%), so that the surface of the gear is strongly carbonized, resulting in a higher carbon concentration gradient from the tooth surface to the center. During the diffusion period, the carbon potential is controlled at 0.75 % ~ 0.85 %. At this time, the carbon potential of the medium is lower than the carbon concentration of the gear surface layer. The surface layer enters the gaseous medium. The diffusion period increases the depth of the infiltration layer, decreases the carbon concentration in the surface layer, and decreases the concentration gradient.

2.1 Heat treatment after vacuum carburizing

Vacuum quenching (oil quenching) is carried out after the parts are vacuum carburized to improve the strength, hardness and wear resistance of the surface of the carburized layer, and at the same time improve the strength and toughness of the core and refine the grains. High-temperature tempering is carried out before vacuum quenching to make the structure more uniform and prepare the structure for vacuum quenching.
The principle of determining the quenching heating temperature after vacuum carburizing is to obtain high hardness and good structure of the carburized layer as much as possible under the premise of ensuring the best performance of the core. When the quenching temperature is low, the performance of the core is poor, and when the quenching temperature is high, coarse martensite and a large amount of retained austenite tend to appear in the infiltrated layer.

2.2 Determine the best vacuum carburizing and quenching process

The optimal carburizing and quenching process of 17Cr2Ni2Mo steel can be determined as shown in the figure below. Among them, the high temperature tempering at 700°C is to prepare the structure for vacuum quenching, and the secondary tempering can stabilize the structure to the greatest extent.

3 Productive test and trial production

The picture below shows the 17Cr2Ni2Mo steel gear shaft. According to the performance and technical requirements of the gear, the production process of the gear can be determined as: forging – normalizing – rough machining – quenching and tempering – milling – carburizing and quenching – grinding – assembly. Among them, normalizing after forging and quenching and tempering after rough machining are pre-heat treatment. The purpose of normalizing after forging is to eliminate the stress after forging, and the normalizing temperature is set at 890 ℃. Quenching and tempering treatment can make the steel obtain good mechanical properties before carburizing, so as to reduce gear deformation during carburizing and quenching. The quenching and tempering process selected in the test is: heating to 860 °C, oil cooling after heat preservation, and then vacuum tempering at 660 °C.
In the production test, anti-seepage measures are taken for all parts except the teeth before vacuum carburizing. After the vacuum carburizing and quenching process shown in the figure below, the tooth surface hardness is HRC58~60, the core hardness is HRC33.7~37.5, and the effective hardened layer depth is 1.6~2.0 (mm).

4 Conclusion

(1) When the safety factor of the gear is small, it should be made of 17Cr2Ni2Mo steel with better comprehensive performance.
(2) The best vacuum quenching temperature for 17Cr2Ni2Mo steel after vacuum carburizing is 840 ℃.
(3) After the vacuum carburizing and quenching process shown in the above figure, the tooth surface hardness of the 17Cr2Ni2Mo steel gear is HRC58~60, and the core hardness is HRC33.7~37.5, which meets the design requirements.