As the core component of mechanical transmission system, gear surface performance directly affects the load capacity, wear resistance and service life of the whole machine.

Heat treatment process is a key technical means to improve the performance of gears. This paper will introduce three common gear heat treatment processes —induction hardening, carburizing and nitriding in detail, analyze their characteristics and advantages and disadvantages, and provide reference for gear design and manufacturing.

01

Gear induction hardening

technical principle

Induction quenching is a surface hardening process that uses the principle of electromagnetic induction to generate eddy currents on the surface of the gear and heat it to the austenitization temperature (usually 850-950℃), and then cool it quickly (water, oil or polymer quenching medium) to obtain the martensitic structure.

Gear material: 42CrMo, 50Mn

Process characteristics

Selective heating: heating only the surface of the gear (usually 1-5mm deep)

Fast and efficient: heating speed can reach 100-300℃/s, short production cycle

Energy saving and environmental protection: energy concentration utilization, high thermal efficiency

Easy to automate: can be integrated into the production line to achieve automatic control

Advantage analysis

Small deformation: local heating significantly reduces the thermal deformation, and the subsequent processing allowance is small

Energy saving and high efficiency: the energy consumption is only 20-30% of the overall quenching, and the production efficiency is high

Strong adaptability: can handle large module gears (module>8mm)

Low cost: moderate equipment input and operating costs

Disadvantage analysis

Hardening layer control is difficult: it is necessary to precisely control the sensor parameters and process parameters

Poor tissue uniformity: the depth and hardness of the hardened layer at the tip and root of the tooth may be uneven

Small gears are not suitable: small module gears (module <2mm) are easy to be hardened as a whole

Insufficient strength of the core: only surface hardening, not suitable for high impact load conditions

02

Gear carburizing and quenching

technical principle

The gear is placed in a carbon-rich medium (gas, liquid or solid), heated and insulated at 900-950℃, so that the carbon atoms infiltrate into the surface (usually 0.5-2mm depth), and then quenched to obtain a composite structure with high hardness on the surface and high toughness in the core.

Gear materials: 20CrMnTi, 20CrMo, 20CrMnMo, 18CrNiMo7-6

Graph of carburizing and quenching process

Process characteristics

Deep sclerosis: The depth of the hardened layer can reach 0.5-2mm

Gradient organization: a gradual transition from surface to core hardness

Composite properties: high hardness on the surface (58-63HRC) and high toughness in the core

Complex process: usually including carburizing, quenching and low temperature tempering

Advantage analysis

Strong load-bearing capacity: deep hardening can withstand heavy load and impact load

Excellent wear resistance: the surface wear resistance of high carbon martensite is 3-5 times that of quenched and tempered treatment

High fatigue intensity: the surface compressive stress state significantly improves the contact fatigue life

Wide range of materials: suitable for low carbon steel, low carbon alloy steel and other materials

Disadvantage analysis

Large deformation: the deformation can reach 0.1-0.3mm due to high temperature and long time heating

Complex process: long production cycle (usually 4-12 hours)

Higher energy consumption: high temperature and long heating energy consumption is large

Higher cost: large equipment investment, strict process control requirements

03

Gear nitriding

technical principle

In the temperature range of 500-580℃, the chemical heat treatment process that makes the active nitrogen atoms penetrate into the gear surface to form a nitrided hardened layer (usually 0.1-0.6mm) is common in the form of gas nitriding, ion nitriding and salt bath nitriding.

Gear material: 42CrMo, 40Cr

Process characteristics

Low temperature treatment: far below the phase transition temperature, deformation is minimal

High hardness: surface hardness up to 1000-1200HV (equivalent to 69-72HRC)

Good corrosion resistance: the formation of dense nitrided layer to improve corrosion resistance

Self-lubrication: low friction coefficient of nitride layer

Advantage analysis

Very small deformation: the thermal deformation is usually <0.02mm, suitable for precision gears

Comprehensive performance: high hardness, wear resistance, corrosion resistance and anti-biting performance

Good thermal stability: the hardness of nitride layer remains stable below 600℃

No quenching: simple process, no risk of quenching crack

Disadvantage analysis

Hardening layer is shallow: usually 0.1-0.6mm, bearing capacity is limited

High brittleness: the white bright layer is highly brittle and often needs to be controlled or removed

Material restrictions: most suitable for steel containing Cr, Mo, Al and other nitride forming elements

Longer cycle: Traditional gas nitriding may take 20-80 hours

04

Comparison of three gear heat treatment processes

• Hardening curve of various gear steels

• Recommended heat treatment method:

• A) Heavy-duty gears (e.g., wind power gearbox, heavy machinery, construction machinery gearbox): carburizing and quenching should be preferred to ensure sufficient hardened layer depth and core toughness.

• b) Small and medium load gears (automobile gearbox, construction machinery): induction quenching (deformation requirements are generally) or carburizing quenching (high precision requirements) can be selected according to the accuracy requirements.

• c) Precision gears (instrumentation, aerospace): Nitriding treatment is preferred, especially ion nitriding, to ensure micron-scale deformation control.

• d) Corrosion and wear resistant gears (food machinery, chemical equipment): select nitriding treatment to have high hardness and corrosion resistance.

• e) Large module gear (mining machinery, large reducer, rotary support): induction hardening can ensure the uniformity of hardened layer.

conclusion

With the development of heat treatment technology, composite processes such as "carburizing + nitriding", "induction quenching + nitriding" are gradually applied in high-end gear manufacturing, which can combine the advantages of various processes to meet the needs of special working conditions.

When selecting gear heat treatment process, many factors such as gear material, service condition, precision requirement and production cost should be considered comprehensively.