Introduction: As a crucial component in automobiles, the automotive gears are primarily utilized for power transmission and motion, altering speed ratios between the engine’s crankshaft and the main shaft gear. Given the ever-changing driving conditions encountered on the road, the operational status of automotive gears becomes highly intricate. Consequently, impeccable intrinsic quality is imperative for automotive gears to meet these demands.
Automotive Gear Heat Treatment Process, Characteristics, and Functions
The intrinsic quality of automotive gears mainly refers to fulfilling technical requirements regarding the gear’s microstructure, mechanical properties, and other indicators while ensuring that other defects are controlled within specified technical limits. The quality of intrinsic characteristics significantly influences the overall gear quality and entirely depends on the quality of heat treatment. It is a pivotal factor in achieving low noise, high precision, and extended lifespan of gears.
Automotive gear heat treatment (process) includes two aspects: firstly, conventional heat treatment such as annealing, normalizing, quenching, tempering, and quenching and tempering; secondly, surface heat treatment, which encompasses methods like surface quenching (e.g., induction quenching, laser quenching) and chemical heat treatment (e.g., carburizing, carbonitriding, nitriding, nitrocarburizing).
Quenching and tempering
Quenching and tempering is the operation of tempering at a high temperature (500~650°C) after quenching gears and other parts. Quenching and tempering treatment is often used for gears made of high-quality carbon steel or alloy steel with a carbon content of 0.3%~0.5% (mass fraction). Quenching and tempering can refine the grains and obtain a tempered sorbate structure with uniformity, a certain degree of dispersion, and excellent mechanical properties. Generally, after quenching and tempering treatment, the hardness of the gear can reach 220~285HBW. The performance of quenched and tempered gears is better than that of normalized gears. Quenching and tempering are commonly used in preparatory heat treatment of gears (such as nitriding, quenching, and tempering treatment before induction hardening) and final heat treatment.
Surface quenching
The quenching hardness of the gear tooth surface is generally 45~55HRC. Case-hardened gears have high load-carrying capacity and can withstand shock loads. Usually, the rough surface hardened gear is normalized or quenched and tempered so that the gear core has a certain strength and toughness. Surface hardening mainly includes induction hardening, laser hardening, and flame hardening. Compared with carburizing and quenching, surface quenching has small deformation, low cost, and high efficiency. The surface quenching of automobile gears mainly adopts an induction hardening process. Due to the fast induction heating speed, there is almost no oxidation and decarburization, and the gear deformation is small. It is easy to realize local heating and automatic production, and the heat treatment cost is low. Therefore, it is widely used in the modern automobile industry.
Carburizing and carbonitriding
(1) Carburizing and quenching: Carburizing and quenching are to first put the gear and other parts into the carburizing medium, heat, and keep warm at 880~950°C so that the surface of the gear is carburized, and then quenched. The gas carburizing process is commonly used for automotive gears. After carburizing, quenching, and tempering, the surface hardness of the gear is generally 58~63HRC. Carburizing and quenching has become the dominant heat treatment process for important automotive gears (such as differential gears, driving axle-driven and driven spiral bevel gears, transmission gears, etc.).
(2) Carbonitriding: In recent years, the actual temperature of the tooth surface of the AIT carburizing gear of the automatic transmission for automobiles during work is about 300°C, which is much higher than the normal tempering temperature (150~200°C). The temperature of this surface will cause the hardness to decrease and cause pitting corrosion. Shot peening after carbonitriding improves fatigue strength. During carbonitriding, as the nitrogen content increases, ΔHV (hardness decreases), the tempering resistance performance improves, and the tempering resistance temperature reaches 300°C.
Nitriding and nitrocarburizing
(1) Nitriding: Nitriding is a chemical heat treatment process that infiltrates nitrogen atoms into the surface of gears and other parts to form a nitrided layer. Nitriding can improve gear surface hardness, wear resistance, fatigue strength, and corrosion resistance. The temperature of nitriding treatment is low, so the deformation of the gear is small, and no grinding or fine grinding is required. Japan adopts the nitriding process in the heat treatment of automobile transmission gears, and the German Clocker-ion company applies ion nitriding to automobile gears, improving the gear precision and service life.
(2) Nitrocarburizing: Nitrocarburizing is a chemical heat treatment process in which nitrogen is mainly infiltrated while carbon is infiltrated. Nitrocarburizing can significantly improve the wear resistance, anti-glue and anti-scratch ability, fatigue resistance, and corrosion resistance of gears. At present, gas nitrocarburizing is applied to parts such as transmission gears of cars and light buses.
Development Trends in Automotive Gear Heat Treatment
In the future, automotive gears will develop in the direction of heavy load, high speed, high precision, and high efficiency, and strive to be small in size, light in weight, long in life, and economical and reliable.
High quality
Mainly manifested in the uniformity of the material, which requires the material to have good composition and uniformity of the structure, the temperature field, and the fluid field, that is, to continuously improve the temperature field and various fluid fields, such as carburizing and nitriding, The improvement of the carbonitriding fluid field and the quenching liquid field further improves the internal quality of the gear.
Low energy consumption
Low energy consumption is achieved through the research and development of advanced gear heat treatment equipment. This includes the enhancement of furnace lining heat resistance and the utilization of thermal insulation and energy-saving materials. The objective is to minimize furnace wall temperature rise and reduce heat loss. Additionally, comprehensive waste heat utilization strategies, like applying forging waste heat for processes such as normalizing, are undertaken to lower gear production costs.
Environmental protection
Research and develop new gear technology; this new technology is less (no) pollution; environmental protection, such as the development of technologies such as low-pressure vacuum carburizing, ion nitriding, dual-frequency induction hardening, laser hardening, rare earth, and BH infiltration.
Intelligentization
Intelligentization is an inevitable trend in developing gear heat treatment control technology. Computers, sensors, and intelligent libraries will constitute the core of intelligent heat treatment, which is mainly manifested according to the materials and technical requirements of gears and other parts, the system automatically generates processes; the production process Complete closed-loop automatic control; prediction and prediction of heat treatment quality of gears and other parts; automatic diagnosis and disposal of system faults; online self-adaptation and emergency response capabilities, such as the development of nitrogen potential sensors for ion nitriding and carbonitriding And low-pressure carburizing carbon potential sensor, etc.
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