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Atmospheric carburizing and low pressure vacuum carburizing of gears

How to reduce the carburizing cost of gears is one of the most concerned issues in today’s gear manufacturing industry, especially the use of traditional atmosphere carburizing for deep carburizing is time-consuming and energy-consuming; secondly, traditional atmosphere carburizing often causes gears The quality of the carburized layer is unstable and the layer is uneven, and it takes multiple tests to obtain repeatable carburizing results; third, due to the accuracy and final requirements of the parts, the parts need to reserve a large dimensional tolerance to meet the requirements after carburizing. However, the best carburized layer is often cut off after carburizing; in addition, traditional atmosphere carburizing also needs to consider environmental pollution, safety and other issues.

The above-mentioned problems in traditional atmosphere carburizing are mainly determined by the method and equipment of atmosphere carburizing. Atmospheric carburizing equipment operation, maintenance and its process conditions are often changing. Due to the limitations of atmospheric carburizing temperature and control methods, it is also impossible to greatly shorten the process time. In addition, atmospheric carburizing can also cause air and water pollution.

Atmospheric carburizing of gears

During atmosphere carburization, the parts are heated to austenite temperature in an endothermic atmosphere or an equivalent atmosphere. The atmosphere contains about 40% hydrogen, 40% nitrogen and 20% carbon monoxide, and there is also a small amount of carbon dioxide (0.5% to 1%), Water vapor (1%), methane (0.5%) and oxygen. This neutral atmosphere (or called carrier atmosphere) generally neither carburizes nor decarburizes the steel surface. For carburizing, enrichment gas needs to be added to the carrier atmosphere. The enriched gas is usually a neutral gas containing 90% to 95% methane (CH4) or propane (C3H4).

In atmospheric carburizing, the enrichment gas is usually introduced after the furnace reaches the carburizing temperature. However, this often results in an uneven layer of infiltration, because the temperature of different parts on the truck and different parts of the parts have not reached uniformity, and the carburizing speed is different.

In the control of the atmosphere carburizing process, the water-gas reaction (CO+H20=CO+H2) is very important. By controlling a certain component in the furnace gas, such as CO2 or H2O, the equilibrium point when the reaction reaches equilibrium can be controlled, thereby Control the carbon potential of the atmosphere.

The dew point method usually used in production is to control the content of H2O, while the infrared method is to control the content of CO2.

The results of atmosphere carburizing + oil quenching of actual gear samples are shown in the figure. The hardness value in the figure is the diagonal length of the indentation measured after the indenter is pressed into the carburized layer of the sample in the Gibbs hardness test, which has not been converted into the Gibbs hardness value. The results show that after carburizing, the effective infiltration depth of the tooth root (the depth of the infiltration layer whose hardness is higher than 50HRC) is 0.76mm, and the effective infiltration depth of the gear pitch diameter is 1.33mm. More importantly, in the gear pitch line and tooth root, the depth of the high hardness (not less than 58HRC) infiltration layer is 0.35mm. From this depth the difference in hardness values between the pitch line and the tooth root of the gear increases rapidly. This result is a representative result after carburizing in the gear atmosphere that is widely used at present.

Low pressure vacuum carburizing of gears

Low-pressure vacuum carburizing is an improved gas carburizing process. The pressure during carburizing is much lower than atmospheric pressure, and the typical pressure range is 300-2500Pa. The advantage is that the steel surface remains very clean, because there is no gas interaction and the carbon reaches the steel surface faster in a vacuum environment.

Hydroxy gas is introduced into the carburizing chamber during carburization, and the cracking produces activated carbon atoms, which freely infiltrate the surface of the steel, while the by-products of hydrogen and remaining hydrocarbons are pumped out of the system by vacuum. Hydroxyl gases currently used for vacuum carburizing are acetylene (C2H2) and propane (C3H3).

In low pressure vacuum carburizing, the cracking of hydroxyl gas is a non-equilibrium reaction, which means that the steel surface can quickly reach the saturation level of carbon in the austenite. By repeating multiple intensive infiltration and diffusion steps, the desired carbon distribution and infiltration depth can be achieved.

The control of the low-pressure vacuum carburizing process is based on the diffusion theory “austenite carbon content saturation value control method”, that is, the entire carburizing process is composed of several sub-carburizing program sets, each sub-carburizing program includes a strong infiltration period and a diffusion period. two stages. Determining the number of sub-carburizing programs and the time of the strong infiltration period and diffusion period in each sub-carburizing program should be based on the composition of the material, the requirements of the depth of the infiltrating layer and the requirements of the surface carbon concentration. After establishing an accurate mathematical model, use The computer calculates it.

The results of low pressure vacuum carburizing + oil quenching of actual gear samples are shown in the figure. The results show that after carburizing, the effective infiltration depth of the tooth root (the depth of the infiltration layer with a hardness higher than 50HRC) is 1.00 mm, and the effective infiltration depth of the gear pitch diameter is 1.33 mm. More importantly, in the gear pitch line and tooth root, the depth of the high hardness (not less than 58HRC) infiltration layer is 0.80mm.

Obviously, the depth of the high hardness layer of vacuum carburized parts is obviously greater than that of atmosphere carburized parts. The hardness of the tooth root and the pitch line of the gear is very consistent in the depth range of the high hardness penetration layer. It can be concluded that the pitch line of the gear and the depth of the root penetration layer are more consistent.

The gears have a larger depth of high-hardness layer after low-pressure vacuum carburization than those after atmosphere carburization, and have a larger effective layer of depth at the root of the tooth.

Features of low pressure vacuum carburizing

From the above research results, it can be considered that low-pressure vacuum carburizing can increase the depth of the carburized layer and make the carburized layer more uniform throughout the gear. In addition, low pressure vacuum carburizing also has the following characteristics:

(1) Low-pressure vacuum carburizing can greatly reduce production costs and greatly improve equipment utilization. This is because the atmosphere of low-pressure vacuum carburizing is very simple, only acetylene or propane is required in the strong infiltration period, and only nitrogen is required in the diffusion period and the pressure is very low, so the cost of using the atmosphere is significantly reduced. In addition, due to the characteristics of low-pressure vacuum carburizing equipment and process, high-temperature carburizing can be performed, so appropriately increasing the carburizing temperature can greatly shorten the carburizing time.

(2) Since the low-pressure vacuum carburizing control system can accurately control the carburizing process, the processed workpiece has good repeatability, and the workpiece has small deformation and bright surface, and does not need to be machined after carburizing.

(3) Low pressure vacuum carburizing technology has greater flexibility. Its equipment can be used as an independent heat treatment equipment, or it can be connected with metal cutting to form a production line. The equipment itself can be either periodic, suitable for small batches and multi-variety production situations, or continuous, consisting of multiple heating carburizing chambers, gas quenching chambers, oil quenching chambers, feeding and discharging chambers, vacuum systems, workpieces Automatic transportation system and other components, suitable for mass production. In addition, it is very simple to start and stop the furnace. It only takes half an hour after starting the furnace to enter the working state, and the furnace can be stopped at any time on weekends and holidays.

(4) Low-pressure vacuum carburizing adopts cold-wall vacuum technology, no ignition device, no fire danger, no pollution, clean and safe, simple equipment operation and easy maintenance.

Selection of vacuum carburizing equipment: The RVC series vacuum carburizing furnace produced by SIMUWU is a high-quality product for the vacuum carburizing process of tools and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum carburizing process.