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Nitriding process for heavy duty gears

Commonly used surface treatments and mechanical properties of gears In order to ensure the reliability of gear operation and increase its service life, gear materials and their vacuum heat treatment should be selected based on the working conditions and material characteristics. The basic requirements for gear materials are: the tooth surface should have sufficient hardness and wear resistance, the tooth center should have sufficient toughness to prevent various failures of the tooth surface, and it should have good cold and hot processing technology. To meet various technical requirements of gears. Commonly used gear materials are various grades 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; when open low-speed transmission, gray cast iron or ductile iron can be used. Low-speed and heavy-loaded gears are prone to plastic deformation of the tooth surface, and the gear teeth are also easy to break. Steel with better comprehensive properties should be selected; high-speed gears are prone to tooth surface pitting, so materials with high tooth surface hardness should be selected; gears subject to impact loads should be selected with good toughness. For gear transmission that requires high speed, light load and low noise, non-metallic materials, such as bakelite, nylon, etc., can also be used.

List of commonly used gear materials and their mechanical properties. Vacuum heat treatment methods for steel gears mainly include the following:

Vacuum hardening

Vacuum hardening is commonly used for medium carbon steel and medium carbon alloy steel, such as 45, 40Cr steel, etc. After vacuum 0 quenching, the tooth surface hardness is generally 40~55HRC. It is characterized by high resistance to fatigue pitting corrosion and gluing resistance. Good wear resistance; since the tooth center part is not hardened, the gear still has enough toughness and can withstand moderate impact loads.

Vacuum carburizing and hardening

Vacuum carburizing and hardening is often used for low carbon steel and low carbon gold-containing steel, such as 20, 20Cr steel, etc. After vacuum carburizing and hardening, the tooth surface hardness can reach 56~62HRC, while the core of the gear still maintains high toughness. The gear teeth have high bending strength and tooth surface contact strength, and have good wear resistance. They are often used in places subject to impact loads. Important gear transmission. After the gear is carburized and quenched, the gear teeth will deform greatly and should be ground.

Vacuum nitriding

No other heat treatment is required after vacuum nitriding, and the tooth surface hardness can reach 700~900HV. Since the gears after nitriding treatment have high hardness, low process temperature and small deformation, they are suitable for internal gears and gears that are difficult to grind. It is often used for nitriding steel containing lead, molybdenum, aluminum and other alloying elements, such as 38CrMoAl.

Conditioning

Quenching and tempering is generally used for medium carbon steel and medium carbon alloy steel, such as 45, 40Cr, 35SiMn steel, etc. The tooth surface hardness after quenching and tempering treatment is generally 220~280HBS. Because the hardness is not high, gear tooth finishing can be performed after vacuum heat treatment.

Normalizing

Normalizing can eliminate internal stress, refine grains, and improve mechanical properties and cutting performance. Gears with low mechanical strength requirements can be normalized by medium carbon steel, and gears with large diameters can be normalized by cast steel. According to the difference in tooth surface hardness after vacuum heat treatment, gears can be divided into soft tooth surface gears (≤350HBS) and hard tooth surface gears (>350HBS). Generally required gear transmission can use soft tooth surface gears. In order to reduce the possibility of gluing and make the matched large and small gears have the same service life, the tooth surface hardness of the small gear is usually 30 to 50 HBS higher than the tooth surface hardness of the large gear. For high speed, heavy load or important gear transmission, hard tooth surface gear combinations can be used, and the tooth surface hardness can be approximately the same.