In order to ensure the reliability of gear operation and improve its service life, gear material and its heat treatment should be selected according to working conditions and material characteristics.
The basic requirements for gear materials are: the tooth surface should have enough hardness and wear resistance, and the tooth center should have enough toughness to prevent all kinds of failure of the tooth surface. At the same time, it should have good cold and hot processing technology to meet all kinds of technical requirements of the gear.
The commonly used gear materials are all kinds of high-quality carbon structural steel, alloy structural steel, cast steel, cast iron and non-metallic materials. Generally, forgings or rolled steel are used. When the gear structure size is large and the wheel blank is not easy to forge, cast steel can be used. Open low-speed transmission, can use gray iron or ductile iron. The gear with low speed and heavy load is easy to produce plastic deformation of tooth surface, and the gear teeth are easy to break, so the steel with better comprehensive performance should be selected. High speed gears are prone to pitting, so it is better to select materials with high hardness. The gear under impact load should be made of materials with good toughness. For high-speed, light load and low-noise gear drive, non-metallic materials, such as laminated bakelite and nylon, can also be used.
The heat treatment methods of steel gears are as follows:
- Surface quenching is commonly used for medium carbon steel and medium carbon alloy steel, such as 45, 40Cr steel, etc. After surface quenching, the hardness of tooth surface is generally 40-55hrc. It is characterized by high resistance to fatigue pitting, scuffing and abrasion. Due to the final hardening of the tooth center, the gear still has enough toughness and can bear little impact load.
- Carburizing and quenching are often used for low carbon steel and low carbon alloy steel, such as 20, 20Cr steel, etc. After carburizing and quenching, the hardness of the tooth surface can reach 56-62hrc, but the tooth center still maintains high toughness, the bending strength and contact strength of the tooth surface are high, and the wear resistance is good, which is commonly used in the important gear transmission under impact load. After carburizing and quenching, the gear tooth is deformed greatly, so it should be grinded.
- Nitriding is a kind of surface chemical heat treatment. After nitriding, no other heat treatment is needed, and the hardness of tooth surface can reach 700-900hv. Due to the high hardness, low process temperature and small deformation of the gear after nitriding treatment, it is suitable for internal gear and hard to grind gear. It is commonly used for nitriding steel containing chromium, copper, lead and other alloy elements, such as 38CrMoAlA.
- Quenching and tempering are generally used for medium carbon steel and medium carbon alloy steel, such as 45, 40Cr, 35SiMn steel, etc. The hardness of tooth surface after tempering treatment is generally 220-280hbs. Because the hardness is not high, gear finishing can be carried out after heat treatment.
- Normalizing can eliminate internal stress, refine grains, improve mechanical properties and cutting performance. The gears with low mechanical strength requirements can be normalized with medium carbon steel, and the gears with large diameter can be normalized with cast steel.
Generally, the gear drive with soft tooth surface can be used. In order to reduce the possibility of gluing and make the life of the matched big and small gears equal, the hardness of the pinion tooth surface is usually 30-50hbs higher than that of the big gear tooth surface. For high-speed, heavy load or important gear transmission, the combination of hard tooth surface gear can be adopted, and the hardness of tooth surface can be approximately the same.