Selecting Steel Materials for Gears: 5 Types Compared

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.

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(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.

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(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.

Frequently Asked Questions

What are the common steel grades used for manufacturing gears?

There are several steel grades commonly used for manufacturing gears, which include carbon steels, alloy steels, and stainless steels. Popular carbon steel grades include SAE 1045, SAE 1050, and SAE 1060. Alloy steel grades such as SAE 4140, SAE 4340, and SAE 8620 are also widely used. For applications requiring corrosion resistance, stainless steel grades like AISI 304 and AISI 316 are preferred.

How do material properties affect the performance of gears?

Material properties play a crucial role in determining the performance, efficiency, and reliability of gears. Key properties that impact gear performance include strength, surface hardness, toughness, fatigue resistance, and wear resistance. The selection of the appropriate steel grade ensures that gears can withstand the various stresses and loads they may experience during operation, while also providing adequate longevity and minimal maintenance requirements.

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Which type of steel is best suited for high load applications?

For high load applications, alloy steels, such as SAE 4340 and SAE 8620, are often recommended due to their high strength and fatigue resistance. These steels are subjected to heat treatments like carburizing, quenching, and tempering to achieve the desired combination of high core strength and surface hardness. These properties enable the gears to withstand high stresses and loads without premature wear or failure.

What factors should be considered when selecting steel for gear manufacturing?

When selecting steel for gear manufacturing, several factors need to be considered, including strength, toughness, wear resistance, machinability, hardenability, and cost. Gear application and operating conditions, such as load capacity, torque transmission, speed, environment, and temperature, are also important considerations. It is vital to strike a balance between material properties, performance requirements, and overall cost-effectiveness in the selection process.

How is the material selection different for helical, bevel, and spur gears?

Material selection for helical, bevel, and spur gears may vary depending on the gear type’s specific requirements and load capacities. For instance, helical gears often experience higher contact stresses compared to spur gears and may require steel with higher strength and wear resistance. On the other hand, bevel gears are subjected to complex load conditions and may demand materials with high toughness and fatigue resistance. While some materials may be suitable for multiple gear types, it is essential to consider the unique requirements and operating conditions for each gear type when selecting materials.

What benefits do advanced steel materials offer in gear manufacturing?

Advanced steel materials, such as ultra-high strength steels, offer several benefits in gear manufacturing. They provide higher strength-to-weight ratios, improved wear resistance, and enhanced fatigue resistance. This can translate into lighter and more compact gear designs capable of handling higher loads and operate at higher efficiency. Additionally, advanced steel materials often require less frequent maintenance due to their improved durability. These benefits can contribute to increased performance, reliability, and overall cost savings in gear manufacturing applications.

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