When selecting the gear material and its heat treatment, it is mainly based on the working conditions such as the transmission mode, load property and size, transmission speed and accuracy requirements of the gear.
At the same time, it is also necessary to consider the steel hardenability and tooth surface hardening requirements, the matching of the material and hardness value of the gear pair according to the gear module and section size.
Gears can be made from various materials, including cast iron, steel, powder metallurgy materials, non-ferrous alloys (such as copper alloys), and non-metallic materials.
Steel is the most commonly used material for gears and can be low carbon steel, medium carbon steel, high carbon steel, or alloy steel. Proper heat treatment, such as normalizing, annealing, quenching and tempering, carburizing, nitriding, or surface quenching, can improve the material’s performance and cutting ability, as well as the processing quality and service life of the gears.
Below are the characteristics and applicable conditions of various steel materials and heat treatment methods for gears.
1. Quenched and tempered steel
Steel grades: 45, 35SiMn, 42SiMn, 37SiMn2MoV, 40MnB, 45MnB, 40Cr, 45Cr, 35CrMo, 42CrMo, etc.
Process 1: Tempering or Normalizing
(1) Quenched and tempered steel gears exhibit good strength and toughness, typically with a hardness range of 220-300 HBW.
(2) If the hardness of a tempered pinion cannot be improved due to tool limitations, a normalized large gear may be used to maintain the hardness difference between the large and small gears. However, normalized gears have lower strength than tempered gears.
(3) Fine cutting can be used to eliminate distortion caused by heat treatment and maintain gear accuracy.
(4) Normalized gears do not require special heat treatment or tooth surface finishing equipment, making them relatively inexpensive to manufacture.
(5) Normalized gears have lower tooth surface hardness, which may limit their bearing capacity.
Applicable conditions: Normalized gears are widely used for general medium and low-speed applications with low strength and accuracy requirements, as well as for large gears that are difficult to heat treat and finish.
Process 2: Surface Quenching (Induction Quenching, Flame Quenching)
(1) Surface quenched gears have high tooth surface hardness, pitting resistance, and wear resistance. The hardened surface produces residual stress, greatly improving tooth root strength. The general tooth surface hardness range is 45-55 HRC for alloy steel and 40-50 HRC for carbon steel.
(2) Quenching and tempering treatment may be carried out before surface quenching to further improve core strength.
(3) Induction hardening time is short.
(4) The case hardening layer’s depth and hardness may vary along the tooth surface.
(5) Rapid heating and cooling can cause cracking.
Applicable conditions: Surface quenched gears are widely used for high-load, small-volume applications.
2. Carburizing steel
Steel grades: 20Cr, 20CrMnTi, 20CrMnMo, 20CrMo, 22CrMo, 20CrNiMo, 18Cr2Ni4W, 20Cr2Ni4A, etc.
Process: Carburizing and Quenching
(1) Carburized and quenched gears have high tooth surface hardness, pitting resistance, and wear resistance. The hardened surface produces residual stress, greatly improving tooth root strength. The general tooth surface hardness range is 56-63 HRC.
(2) Carburized gears exhibit good cutting performance.
(3) Carburizing and quenching cause significant heat treatment distortion, requiring post-heat treatment grinding to achieve high accuracy. This increases processing time and cost.
Applicable conditions: Carburized and quenched gears are widely used for medium and small gears with high bearing capacity, impact resistance, accuracy, and small volume.
3. Nitriding steel
Steel grades: 38CrMoAlA, 30CrMoSiA, 25Cr2MoV, etc.
Process: nitriding treatment
(1) Nitrided gears have very high tooth surface hardness, pitting corrosion resistance, and wear resistance. The core has good toughness. Medium carbon steel is often quenched and tempered first to improve core strength.
(2) Due to low heating temperatures, heat treatment distortion is minimal, and teeth do not require grinding after nitriding treatment.
(3) The hardened layer is thin, making the gear less suitable for impact loads and with lower bearing capacity than carburized and quenched gears.
(4) Nitriding treatment takes longer and is more expensive than other heat treatments.
Applicable conditions: Nitrided gears are suitable for large, stable load applications and for situations where tooth surface finishing equipment is not available, but hard tooth surfaces are required.
4. Cast steel
Steel grades: ZG310-570, ZG340-640, ZG42SiMn, ZG50SiMn, ZG40Cr1, ZG35CrMnSi, etc.
(1) This process is suitable for manufacturing large gears with complex shapes.
(2) The strength of gears manufactured with this process is lower than quenched and tempered steel of the same grade and heat treatment.
(3) This process can result in casting defects.
Applicable conditions: This process is suitable for large gears that cannot be forged.
5. Cast iron
Steel grade: various gray cast iron, ductile iron, malleable cast iron, etc.
(1) Cast iron has low material cost.
(2) Cast iron gears exhibit good wear resistance.
(3) This process is suitable for manufacturing large gears with complex shapes.
(4) Cast iron has good casting and cutting technologies.
(5) Cast iron gears have lower bearing capacity than other materials.
Applicable conditions: Gray cast iron and malleable cast iron are suitable for low-speed, light-load, and impact-free gears. Ductile iron can be used for gears with large loads and impact.
