Custom Industrial Gear Units Manufacturers

How to avoid deformation and residual stress in the heat treatment process of industrial gearboxes?

1. Mechanism of deformation and residual stress during heat treatment
During the heat treatment process, industrial gearbox parts will undergo complex organizational transformation and temperature field changes, which is the root cause of deformation and residual stress.
From the perspective of organizational transformation, after the steel is heated and austenitized, it will undergo phase transformations such as martensite and bainite during cooling. The specific volumes of different organizations are significantly different (such as the specific volume of martensite is greater than that of austenite). If this volume change is limited by the geometry and cross-sectional size of the parts, organizational stress will be generated. For example, the difference in cooling speed between the gear teeth and the hub may cause the tooth surface to be compressed due to the expansion of the martensitic phase transformation and the core to be tensile, forming internal stress.
Uneven temperature field is the main source of thermal stress. When heating or cooling, the temperature gradient between the surface and the core of the part will cause inconsistent thermal expansion and contraction. Taking the quenching process as an example, rapid cooling causes the surface to shrink rapidly, while the core forms support due to delayed cooling, resulting in tension on the surface and compression on the core. If the part structure is complex (such as with blind holes and bosses), this non-uniformity will be further aggravated.
The existence of residual stress may cause deformation and cracking of parts during subsequent processing or use, and even affect fatigue life. The deformation problem will not only reduce the meshing accuracy of the gear, but also cause vibration and noise, reducing transmission efficiency. Therefore, the optimization of the heat treatment process needs to start from multiple dimensions such as material selection, process parameters, and equipment improvement.
2. Heat treatment process optimization strategy
(I) Material and pretreatment control
The selection of materials with suitable hardenability is the basis. Taking the hardened gear reducer produced by Hangzhou Hengbai Reducer Co., Ltd as an example, industrial gear units's gears are made of high-strength carbon alloy steel (such as 20CrMnTi, 42CrMo). After carburizing and quenching, the tooth surface hardness of this type of material can reach HRC58-62, while the core maintains good toughness. Reasonable alloy composition design (such as adding Cr, Mn, Ni and other elements) can improve hardenability and reduce deformation caused by cross-sectional temperature difference.
The pretreatment process can eliminate the stress of the blank in advance. Hangzhou Hengbai Reducer Co., Ltd has its own foundry, which uses annealing or normalizing treatment for castings: annealing eliminates casting stress and refines grains through slow cooling; normalizing increases hardness and improves machinability through air cooling. The risk of deformation of the pretreated blank in subsequent processing is significantly reduced.
(II) Heating process optimization
Heating speed and temperature uniformity are key. For parts with complex structures, step-by-step heating (such as preheating at 500-600℃ and then heating to austenitizing temperature) can reduce thermal stress. The heat treatment equipment is equipped with an intelligent temperature control system, which controls the temperature deviation in the furnace within ±5℃ through zone temperature control technology to ensure uniform heating of parts.
Protective atmosphere heating can avoid oxidative decarburization and reduce the phase change stress difference caused by changes in surface composition. For example, the use of drip-type controllable atmosphere (such as methanol + acetone) in the carburizing process can not only accurately control the carbon potential, but also make the carbon concentration on the surface of the part uniform and reduce the structural stress during quenching.
(III) Cooling process control
The choice of cooling medium directly affects the phase change process. Water quenching has a fast cooling speed and is prone to large deformation. It is suitable for parts with simple shapes and high hardness requirements; oil quenching has a slow cooling speed and small deformation, and is often used for complex parts. According to the gear modulus and structure, flexibly select PAG water-soluble quenching agent (cooling speed can be adjusted by concentration). For example, for gears with a modulus ≤5, a 3%-5% concentration of PAG solution is used, which can not only ensure the hardness of the tooth surface, but also control the deformation within 0.05mm.
The improvement of cooling method is also crucial. Isothermal quenching (such as bainite isothermal quenching) can significantly reduce stress by maintaining a certain temperature above the Ms point, so that the internal and external temperatures of the parts tend to be consistent, and then slowly cooling. Using isothermal quenching before the gear grinding process, the gear accuracy can reach the international standard level 6, the transmission efficiency ≥96%, and the noise is less than 75dB, which reflects the improvement of precision and performance by optimizing the cooling process.
(IV) Aging treatment and stress relief
Aging treatment relaxes residual stress by heating and heat preservation. For high-precision gearbox parts, low-temperature aging (120-150℃, heat preservation for 10-20 hours) or vibration aging can be used. Vibration aging uses the resonance principle to release microscopic stress, which has the advantages of high efficiency and low energy consumption. Introducing this process in the gear processing process can reduce the deformation of subsequent gear grinding by more than 30%.
For important parts, multiple tempering processes can be used. For example, two temperings after carburizing and quenching (the first 180-200℃ to eliminate quenching stress, the second 200-220℃ to stabilize the structure) can reduce both residual stress and the risk of grinding cracks. After residual stress testing (such as X-ray diffraction method) in the testing laboratory, the surface residual compressive stress of the gear product is controlled between -400MPa and -600MPa, which effectively improves fatigue life.