5 Types of Steel Materials for Gears: Selection, Heat Treatment Characteristics and Applicable Conditions

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.

Various materials are used for gears, such as various cast iron, steel, powder metallurgy materials, non-ferrous alloys (such as copper alloys) and non-metallic materials.

Among them, steel is the most widely used material, including various low carbon steel, medium carbon steel, high carbon steel and alloy steel.

The proper heat treatment (such as normalizing and annealing, overall quenching and tempering, quenching and tempering, carburizing, nitriding, surface quenching, etc.) for steel gears is intended to improve the service performance of steel, give full play to the ability of materials, and also improve the cutting performance of steel, improve the processing quality of gears and prolong the service life of gears.

The characteristics and applicable conditions of various steel materials and heat treatment for gears are as follows:

1. Quenched and tempered steel

Steel grades: 45, 35SiMn, 42SiMn, 37SiMn2MoV, 40MnB, 45MnB, 40Cr, 45Cr, 35CrMo, 42CrMo, etc.

Process 1. Tempering or normalizing

1) After quenching and tempering, it has good strength and toughness, and is often used in the range of 220 ~ 300hbw.

2) When the hardness of the tempered pinion cannot be improved due to the limitation of the tool, the normalized large gear can be used to maintain the hardness difference between the large and small gears, but the strength is worse than that of the tempered one.

3) The fine cutting of the tooth surface can be carried out after the heat treatment to eliminate the heat treatment distortion and maintain the gear accuracy.

4) No special heat treatment equipment and tooth surface finishing equipment are required, and the manufacturing cost is low.

5) The hardness of the tooth surface is low and it is easy to run in, but the bearing capacity of the material cannot be fully exerted.

Applicable conditions: it is widely used for general medium and low speed gears with low requirements for strength and accuracy, and large gears with difficult heat treatment and tooth surface finishing.

Process 2. Surface quenching (induction quenching, flame quenching)

1) The tooth surface has high hardness and strong pitting resistance and wear resistance;

The core has good toughness, and the surface is hardened to produce residual stress, which greatly improves the strength of the tooth root;

The general tooth surface hardness range is: alloy steel 45 ~ 55HRC, carbon steel 40 ~ 50HRC.

2) In order to further improve the strength of the core, quenching and tempering treatment is often carried out before surface quenching.

3) Induction hardening time is short.

4) The depth and hardness of the case hardening layer are different along the tooth surface.

5) It is easy to crack due to rapid heating and cooling.

Applicable conditions: widely used for gears with high load capacity and small volume.

2. Carburizing steel

Steel grades: 20Cr, 20CrMnTi, 20CrMnMo, 20CrMo, 22CrMo, 20CrNiMo, 18Cr2Ni4W, 20Cr2Ni4A, etc.

Process: carburizing and quenching

1) The tooth surface has high hardness and strong pitting resistance and wear resistance;

The core has good toughness, and the residual stress generated after the surface is hardened greatly improves the strength of the tooth root;

The general tooth surface hardness range is 56 ~ 63HRC.

2) Good cutting performance.

3) The distortion of heat treatment is large, and the teeth should be ground after heat treatment, which increases the processing time and cost, but high accuracy can be obtained.

Applicable conditions: it is widely used for medium and small gears with high bearing capacity, good impact resistance, high accuracy and small volume.

3. Nitriding steel

Steel grades: 38CrMoAlA, 30CrMoSiA, 25Cr2MoV, etc.

Process: nitriding treatment

1) It can obtain very high tooth surface hardness and has strong pitting corrosion resistance and wear resistance;

The core has good toughness.

In order to improve the strength of the core, the medium carbon steel is often quenched and tempered first.

2) Due to the low heating temperature, the distortion of heat treatment is very small, and there is no need to grind the teeth after nitriding treatment.

3) The hardened layer is very thin, so the bearing capacity is not as good as that of carburized quenched gear, and it is not suitable for the occasion with impact load.

4) Nitriding treatment cycle is long and processing cost is high.

Applicable conditions: it is suitable for gears working under large and stable loads, and for occasions where there is no tooth surface finishing equipment and hard tooth surface is required.

4. Cast steel

Steel grades: ZG310-570, ZG340-640, ZG42SiMn, ZG50SiMn, ZG40Cr1, ZG35CrMnSi, etc.

Process: normalizing or quenching and tempering, and surface quenching

1) Large gears with complex shapes can be manufactured

2) Its strength is lower than that of quenched and tempered steel of the same grade and heat treatment

3) Easy to produce casting defects

Applicable conditions: for large gears that cannot be forged.

5. Cast iron

Steel grade: various gray cast iron, ductile iron, malleable cast iron, etc.

1) The material is low.

2) Good wear resistance.

3) Large gears with complex shapes can be manufactured.

4) It has good casting and cutting technology.

5) Low bearing capacity.

Applicable conditions: gray cast iron and malleable cast iron are used for low-speed, light load and impact free gears;

Ductile iron can be used for gears with large load and impact.

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