Types and Effects of Gear Heat Treatment

Gears are made of different materials, sizes, working environments, and required properties, all of which affect the choice of gear heat treatment process. So,what kind of heat treatment is required for gear processing?

One. Types of gear heat treatment

1. Gear normalizing heat treatment

Normalizing treatment can eliminate excessive stress inside the gear, increase the toughness of the gear, and improve the cutting performance of the material. Normalizing treatment is often used for gears made of high-quality carbon steel or alloy steel with a carbon content of 0.3% to 0.5%. The strength and hardness of normalized gears are lower than those of quenched or tempered gears, with a hardness of HB163~217. Therefore, normalizing treatment is often used for large-diameter gears that do not require very high mechanical properties or are not suitable for quenching or tempering.

2. Gear quenching and tempering heat treatment

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% to 0.5%. Quenching and tempering treatment can refine the grains and obtain a uniform, fine spherical pearlite-like structure-tempered sorbite with a certain degree of dispersion and excellent comprehensive mechanical properties. Generally, after quenching and tempering treatment, the hardness of gear teeth can reach HB220~285. For smaller gears, the hardness can be higher. The comprehensive performance of quenched and tempered gears is higher than that of normalized gears. Its yield limit and impact toughness are about 40% higher than that of normalized gears, and its strength limit and section shrinkage are also 5% to 6% higher (for carbon steel) . Tempered gears are easy to run in during operation, have large tooth root strength margin, and have strong impact resistance. They occupy a considerable proportion in heavy-duty gear transmissions. In order to improve the anti-gluing ability of soft tooth surface gears and considering that the work of small gears is heavier than that of large gears, quenched and tempered small gears are often matched with normalized or quenched and tempered large gears. The hardness difference is often HB20~50.

3. Gear surface quenching

After surface quenching, the gear needs to be tempered at low temperature to reduce internal stress and brittleness. The tooth surface hardness is generally HRC45~55. Case-hardened gears have high load-carrying capacity and can withstand impact loads. Usually, the blank of the quenched gear can be normalized or quenched and tempered first, so that the center of the gear teeth has a certain strength and toughness.

4. Gear carburizing and quenching

Carburizing and quenching gears are commonly made of alloy steel or high alloy steel with a carbon content of 0.10% to 0.25%. After carburizing and quenching, the tooth surface hardness is HRC58~62. Generally, gear grinding or honing is required to eliminate deformation caused by heat treatment. This type of gear has high contact strength and bending strength, and can withstand large impact loads. Important gears in various load-carrying vehicles are often carburized and quenched.

5. Gear nitriding

Nitriding can improve the surface hardness, wear resistance, fatigue strength and corrosion resistance of gear teeth. The temperature of nitriding treatment is low, so the gear deformation is very small, and no grinding is required or only fine grinding is required. The main materials for nitriding gears include 38CrMoAlA, 30CrMoSiA, 20CrMnTi, etc. Because the nitrided layer of nitrided gears is thin (0.15~0.75mm), the hardened layer is in danger of peeling off. Its load-bearing capacity is generally not as high as that of carburized gears, and it is not suitable for use in situations where it can withstand impact loads or has strong wear.

6. Other heat treatment methods for steel gears

Steel gears can also be treated by methods such as overall quenching or cyanidation (carbonitriding). Overall quenched gears have higher hardness, but have large deformation, poor toughness, and are not resistant to impact, so they are rarely used. Cyanide gears have the advantages of high hardness, good wear resistance, small deformation, and high productivity, and are suitable for carbon steel and alloy steel. However, its hardened layer is brittle and not impact-resistant, and cyanide is highly toxic, so safety facilities must be provided.

Two. Effect of gear heat treatment

1. Influence of metallurgical factors of gear materials on deformation

Tests show that the higher the hardenability of steel, the greater the deformation. When the core hardness is higher than 40HRC, the deformation will increase significantly. Therefore, there are certain requirements for the hardenability zone of steel. The narrower the hardenability zone, the more stable the deformation will be. Steel plants are required to provide "low and stable deformation" steel. Controlling the A1/N content ratio within the range of 1 to 2.5 can narrow the hardenability zone and reduce deformation. In addition, the box segregation and band-like structure of the material affect the uneven deformation and uneven carburization of the gear spline holes.

2. Effect of preliminary heat treatment on gear deformation

Excessive normalizing hardness, mixed crystals, a large amount of sorbite or Widmanstatten structure will increase the deformation of the inner hole, so temperature-controlled normalizing or isothermal annealing should be used to process the forgings.

3. Effect of carburizing process on deformation

The uniformity of temperature, the uniformity of the carbon layer, and the uniformity of the cooling medium temperature all affect gear deformation. At the same time, the higher the carburizing temperature, the thicker the carburizing layer. The lower the oil temperature, the greater the gear deformation. Therefore, it is necessary to improve equipment, optimize processes, and improve the quality of gear heat treatment.

4. Effect of quenching on deformation

Quenching cooling behavior is the most important factor affecting gear deformation. Hot oil quenching has smaller deformation than cold oil quenching, and is generally controlled at 100℃±120℃. The cooling ability of oil is also crucial to deformation. The mixing method and intensity both affect the deformation. For the disc gears quenched on the quenching press, according to the deformation conditions of various gears, adjust the parameters of the punching press to reduce the deformation. Adjust the pressures of the inner and outer dies and expansion blocks and each The deformation can be controlled by the size of the fuel injection volume and the working table.