In industrial equipment, aerospace, precision machine tools, construction machinery and other high-end manufacturing fields, heavy-duty precision transmission system requires gears not only to withstand huge loads, but also need to have very high dimensional stability, transmission accuracy and long-term reliability. In order to meet these stringent requirements, the heat treatment process of gears has become a key part of enhancing their comprehensive performance. This article focuses on three typical heat treatment methods - carburising quenching, nitriding and medium frequency quenching - and analyses their advantages and differences in the application of heavy-duty precision gears.

1. Carburising quenching: the ‘king process’ for heavy-duty precision transmission

Principle and advantages:

Carburising quenching will gears in the carbon potential environment at high temperatures for carburising, so that the surface layer of the formation of carbon-rich layer, followed by rapid quenching to form a high hardness martensitic organisation. After tempering, the ideal organisation of hard surface layer and tough heart is achieved.

Suitable for:

High speed and heavy load, high impact load working conditions

Transmission systems with high requirements for wear resistance and pitting resistance.

Engineering machinery, high-end reducer, aviation power gears

Technical advantages:

Deep penetration layer (up to 1.2~2.0mm) to support long-term service under high loads.

Extremely high surface hardness (HRC58~62) to enhance contact fatigue life.

Good core toughness to improve fracture resistance

Suitable for high-precision grinding to ensure the accuracy of tooth shape.

Caution:

Carburising process has high thermal stress, fine control of deformation and subsequent grinding or fine honing are required to ensure accuracy.

2.Nitriding treatment: ‘Golden armour’ for precision transmission.

Principle and advantages:

Nitriding is in the low temperature so that nitrogen atoms penetrate into the surface layer of gear steel, and alloying elements to form a hard nitride layer. Its advantage is that the size of the deformation is very small, and the treatment is gentle, does not affect the matrix organisation.

Suitable for:

Gear wheels with extremely high requirements for dimensional accuracy

Products that do not require subsequent grinding, or where grinding is not advisable

Precision equipment for long-term stable operation without shock loads

Technical advantages:

High surface hardness (HV800~1100), with strong wear resistance

Low-temperature treatment, minimal thermal deformation, suitable for direct use in precision gears

Good corrosion resistance and fatigue resistance

No or less grinding after treatment, shorten the manufacturing cycle

Limitations:

Thin carburised layer (about 0.3~0.6mm), not as good as carburised.

Not suitable for strong impact or variable load applications

3. Medium-frequency quenching: an efficient solution for localised strengthening

Principle and advantages:

Medium-frequency induction heating quickly on the tooth surface or tooth root, and spray quenching treatment, to achieve local hardening, retaining the overall toughness of the gear.

Suitable for:

Heavy-duty gears that are cost-sensitive but still require some surface hardening

Short cycle times with fast processing times

Large gears with high modulus and difficult to carburise as a whole.

Technical Advantage:

Fast heating and high process efficiency

Selective hardening of tooth flanks, tooth roots and other high-stress areas

Small deformation after treatment, easy to maintain the accuracy of tooth shape

Automated production, suitable for batch

Limitations:

Depth and hardness of the penetration layer are limited, usually less than carburising.

Overall performance improvement is not as systematic as carburising quenching.

4. Summary Recommendations

In heavy-duty precision transmission applications:

Carburising quenching + grinding is the first choice if comprehensive gear performance improvement is required;

If the pursuit of high dimensional stability and anti-wear performance, suitable for the selection of nitriding treatment;

If efficiency and economy are of concern, and the gear structure allows, medium frequency quenching can be considered as a complementary programme.

Selection of the appropriate heat treatment process, often need to be combined with the load characteristics of the gear, the working environment, precision requirements and cost control for a comprehensive trade-off.