Precision carburizing heat treatment of gears

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The key to obtaining low-noise and long-life gears lies in the control of heat treatment quality. Precision heat treatment of gears includes the mass dispersion of technical indicators such as carburized layer microstructure, core and surface hardness, effective hardened layer depth, residual stress and thermal distortion. The smaller the dispersion, the higher the heat treatment quality. The basic conditions required for gear precision heat treatment include: ① the uniformity of gear raw materials. ②The heat treatment equipment has a well-distributed temperature field and fluid field. To achieve precise carburization of gears, the key lies in the precise control of furnace temperature and carbon potential (temperature control accuracy≤±1.5℃, temperature difference of effective heating zone≤±5℃, carbon potential≤±0.05%, deep fluctuation of carburized layer ≤±0.1mm).

1. Temperature control
The main measure to ensure the low dispersion of the quality of the same batch of gear products and the reproducibility of the quality of different batches is to ensure the accuracy and stability of the process parameters during the heat treatment process. For heating equipment, it is mainly the fluctuation degree of furnace temperature over time (that is, the accuracy of temperature control), and the temperature difference of each edge position in the effective heating area (that is, furnace temperature uniformity).

2. Carbon Potential Control
Carbon potential control accuracy
The gas carburizing (carbonitriding) furnace should not only meet the furnace temperature uniformity performance requirements in the effective heating zone, but also accurately control the furnace gas carbon potential to precisely control the carbon content on the gear surface, the depth of the carburizing layer, and even the surface carbon concentration gradient , to meet the requirements of the uniformity of the infiltrated layer (metallographic structure, layer depth) and the uniformity of the surface hardness. Using oxygen probes as sensors, various types of carbon control instruments, as well as databases and process software suitable for different carburizing processes of various steels, online control of the heat treatment process can be realized. The placement position of the sample can refer to the position of the thermocouple in the furnace temperature uniformity test. The precise carbon potential control requires ≤±0.05%, and the depth deviation of the seepage layer should be ≤±0.1mm.

Carbon potential measurement method
(1) Direct measurement method Put the low carbon steel foil in a carbon-containing atmosphere for a sufficient time, and then measure the carbon content of the steel foil, which is the carbon potential of the atmosphere at the test temperature.
(2) Indirect measurement method. At present, the measurement of carbon potential is mostly through the primary instrument (sensor) to continuously measure the relevant composition parameters and temperature of the carburizing atmosphere online, and then the secondary instrument (carbon potential controller) is used to measure the parameters according to the primary instrument. And the established atmospheric carbon potential mathematical model can automatically calculate the real-time carbon potential of the atmosphere.
3. Carbon potential control system
Under the premise of ensuring the uniformity of furnace temperature and accurate temperature control, carbon potential sensor (such as oxygen probe) and carbon controller are used for carbon potential control. However, under the conditions of dripping atmosphere and direct generation atmosphere, when the content of CH4 in the carburizing atmosphere is high, it is easy to control the carbon potential by using the oxygen probe alone. The carbon potential value can only truly reflect the carbon potential of the atmosphere. From the single “oxygen potential” or “CO2” parameter control to the “oxygen potential-CO2-temperature” three-parameter control, the carbon potential control accuracy is ±(0.025%~0.05%).
Selection of carburizing equipment: GH-GCF series gas carburizing furnaces produced by SIMUWU are high-quality products for vacuum heat treatment of tools and molds. Good temperature control accuracy and uniformity ensure effective vacuum heat treatment.

Model	TemperatureUniformity (Dia x H )(mm)	Rated power（KW）	Power supply		Max . Temperature (℃)	Loading Capacity
			Phase	Voltage(V)
GH-GCF-0304	Ø300 x 450	25	3	380	950	100Kg
GH-GCF-0306	Ø300 x 600	35	3	380	950	150Kg
GH-GCF-0406	Ø450 x 600	60	3	380	950	200Kg
GH-GCF-0409	Ø450 x 900	75	3	380	950	300Kg
GH-GCF-0609	Ø600 x 900	90	3	380	950	400Kg
GH-GCF-0612	Ø600 x 1200	105	3	380	950	650Kg
Remark: The working zone of equipment could be customized base on customer’s production.

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