Qingdao Vanhon Machinery Technology Co.,Ltd.

Industry News
Gear material and heat treatment
Dec 14, 2021

Gears will be impacted by forces such as meshing, collision, rolling, sliding, etc. during the working process, so the selection of gear materials and heat treatment methods is very important. Mainly consider the following aspects:

1. The hardness of the material and the hardness of the tooth surface can improve the anti-fatigue ability of the tooth surface and prevent the unevenness of the tooth surface and cause wear;

2. Core hardness to ensure impact resistance;

3. The material of large and small gears should be different to maintain the difference in hardness and improve the anti-gluing ability;

4. Consider economy and material processability.

Gear material selection

Gear manufacturing materials are mainly steel, cast iron, and non-metallic materials such as plastics.

1. Steel gear

It is mainly divided into forged steel and cast steel. Among them, forged steel has high strength and is the most widely used.

1). Forged steel and rolled steel: A high-quality carbon steel, such as No. 15 and No. 20 steel, can get high tooth surface hardness after proper heat treatment, and the tooth surface has strong bearing capacity, but the core mechanical properties are low and the bending strength is insufficient , Can be used to manufacture impactful medium and small load gears; B high-quality medium carbon steel with 0.3-0.5% carbon content, such as 35, 40, 45, 50, 55 steel, high strength, good toughness and workability, widely used For gear manufacturing, 40 and 45 are the most widely used. C high-quality carbon steel with high manganese content, such as 50Mn, has high elastic strength and hardness, good wear resistance, suitable for manufacturing heavy load gears; D chromium steel, such as 15Cr, 20Cr, 40Cr, chromium can improve strength and hardness , Good wear resistance; E nickel-chromium steel, such as 12CrNi3, 20CrNi, 40CrNi, etc., with high strength, hardness, wear resistance, and toughness. It is used to manufacture gears with high speed, long cycle, and high safety and reliability; F chromium Manganese-titanium steel, chromium-molybdenum steel, chromium-manganese-molybdenum steel, such as 20CrMnTi, 20CrMo, 20CrMnMo, etc., are used for ordinary reducer pinions in mining, metallurgy, chemical lifting and transportation equipment, etc.; G silicon-manganese steel, 35SiMn, 42SiMn, inexpensive, It can be used as a substitute material for 40Cr and 40CrNi for machine tool gears; H other alloy steels such as 20SiMnVB, 35CrMnSi, 40CrNiMoV, 38CrMoAlA, etc.

From economic considerations, generally high-quality carbon steels that are cheap and easily available are generally used, and rare alloy steels, such as chromium-containing alloy steels, are used as little as possible.

2). Cast steel: commonly used carbon steel or alloy steel with a carbon content of about 0.4%, such as ZG310-570, ZG340-640, ZG35SiMn, and ZG42SiMn, etc. The mechanical properties of cast steel are inferior to that of forged and rolled steel, but it has good fluidity. It is often used to manufacture gears with low strength requirements, complex shapes and larger diameters.

2. Cast iron.

The mechanical properties of cast iron are not as good as steel, but it has good workability, good resistance to adhesion and pitting corrosion, high wear resistance, low noise and low cost. It is often used to manufacture gears in open gear transmissions.

Commonly used materials such as: HT200, HT300, HT400, QT50-5, QT60-2, etc.

3. Non-steel gears.

Copper alloys: HA160-1-1, QA15, etc., have good wear and corrosion resistance, and are often used in worm manufacturing.

Non-metal: mainly plastic gears, which have low vibration and noise, but low load-bearing capacity. Commonly used materials such as: A general light load, medium load, working at 90 degrees Celsius, POM; B heavy load, high impact, PA46+50%GF; C general heavy load, the temperature does not exceed 120 degrees, PA66+30%GF; D High-speed sliding, such as worm gear, PA46 or TW371.


Heat treatment of steel gears

    1. Normalizing

    Heat the steel to 30-50 degrees above Ac3 (the starting line of austenite precipitation in the iron-carbon alloy balance diagram) or Acm (the starting line of cementite precipitation from the austenite in the iron-carbon alloy balance diagram), After keeping it for a period of time, it is cooled in the air. This kind of heat treatment is called normalizing. Normalizing treatment can eliminate excessive stress inside the gear, increase toughness, and improve the cutting performance of the material. Normalizing is often used for gears made of high-quality carbon steel or alloy steel with a carbon content of 0.3-0.5%.

    The strength and hardness of normalized gears are lower than those of quenched and quenched and tempered gears. The hardness is generally HB163-217. Therefore, for large-diameter gears that do not require high mechanical properties or cannot be quenched and tempered, normalizing is used for long ones.

    2. Tempering treatment

    The steel is quenched and tempered at a high temperature (500-650 degrees Celsius), which is called quenching and tempering treatment. It is often used for gears made of high-quality carbon steel or alloy steel with a carbon content of 0.3-0.5%.

The hardness of the gear teeth after quenching and tempering can reach HB220-285, and the overall performance is higher than that of the normalizing gear, especially the yield limit and impact toughness, which account for a large proportion in the heavy gear transmission.

In addition, considering the improvement of the anti-gluing ability of the gears, that is, the hardness difference of the large and small gears, the quenched and tempered small gears are commonly used in conjunction with the normalizing large gears.

    3. Surface hardening.

    Commonly used in medium carbon steel or medium carbon alloy steel gears, it is to obtain martensite with high hardness by changing the surface layer structure of the parts, while retaining the toughness and plasticity of the core. After surface hardening, the gear must be tempered at low temperature to reduce internal stress and brittleness. The tooth surface hardness is generally HRC45-55. The surface-hardened gear has a high load-bearing capacity and can withstand high impact loads. Generally, the roughened gear can be normalized or quenched and tempered first to give the core a certain strength and toughness.

    4. Carburizing and quenching

    Carburizing and quenching is to first put the gear in a carburizing medium, heat and keep it at 900-950 degrees Celsius to increase the carbon on the surface of the gear, and then quench it to obtain martensite on the surface. Commonly used for alloy steel gears with a carbon content of 0.10-0.25%. After carburizing and quenching, the tooth surface hardness is HRC58-62. Generally, tooth grinding or honing is required to eliminate the deformation caused by heat treatment. This type of gear has high contact strength and bending strength, and can withstand greater impact loads. The gears of heavy-duty vehicles need this treatment.

    5. Nitriding

    Nitriding is to infiltrate nitrogen atoms into the surface of gear teeth to form a nitrided layer, which can improve surface hardness, wear resistance, fatigue strength and corrosion resistance. The nitriding treatment temperature is low, so the deformation is very small, no grinding is required, or only fine grinding is required. The nitriding materials mainly include 38CrMoAlA, 30CrMoSiA, 20CrMnTi, etc.

    Nitrided gears have a thin nitriding layer and the hardened layer is in danger of peeling off, so the bearing capacity is not as high as that of carburized gears, and it is not suitable for use in places with impact loads or strong wear.

    6. Other heat treatment of steel gears

    Such as overall quenching, cyanidation (carbonitriding). The overall hardened gear has high hardness, but large deformation, poor toughness, impact resistance, and less application. Cyanide gears have high hardness, good wear resistance, small deformation, and high productivity. They are suitable for carbon steel and alloy steel, but their hardened layer is brittle, not resistant to impact, and cyanide is highly toxic, so safety measures must be taken.

    7. Heat treatment of cast steel gears.

    Annealing and normalizing are commonly used to refine grains, eliminate internal stress, and improve the mechanical properties of cast steel. Annealing is to heat the gear to the critical point of steel austenite, and then slowly cool it in a furnace or bury it in a medium with poor thermal conductivity after keeping it warm.

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