Carburized Gear Steels: Selection Guide for Industrial Gearboxes
Carburized gear steels combine a hard, wear-resistant case with a tougher core. That combination is widely used for heavily loaded gearbox gears because the tooth surface must resist pitting and wear while the core carries bending and impact loads.
This guide compares commonly used grades such as 20CrMnTi, 17CrNiMo6, 18CrNiMo7-6 and 8620H, and explains how material choice, case depth and heat treatment affect gear performance.
What Is Carburizing?
Carburizing enriches the surface of a low-carbon alloy steel with carbon at elevated temperature. The part is then quenched and tempered to create a hard case over a tougher core. For industrial gears, surface hardness is commonly specified around the high-50s to low-60s HRC, but the correct hardness and effective case depth depend on the steel grade, gear size, tooth geometry, required life and heat-treatment specification.
Common Carburized Gear Steel Grades
| Steel Grade | Common Standard / Region | Typical Characteristics | Typical Gear Applications |
|---|---|---|---|
| 20CrMnTi | GB / China | Widely used carburizing steel with good machinability and availability | General industrial gears, planetary stages, reducers |
| 17CrNiMo6 | DIN / Europe | Nickel-chromium-molybdenum steel with good core toughness and hardenability | Mining, marine and larger industrial gears |
| 18CrNiMo7-6 | EN / Europe | High hardenability and fatigue capability for demanding carburized gears | Heavy-duty gearboxes, mills and high-load drives |
| 8620 / 8620H | AISI / North America | Common Ni-Cr-Mo carburizing steel with broad industrial use | OEM gearboxes, shafts and custom industrial gears |
| 16MnCr5 / 20MnCr5 | EN / Europe | Economical carburizing steels for smaller and medium-size components | Gearmotors, reducers and general machinery |
Typical Hardness and Case-Depth Considerations
Values below are representative engineering ranges rather than universal specifications. Actual requirements should be confirmed from the applicable drawing, standard and load calculation.
| Material | Typical Surface Hardness | Case-Depth Consideration | Engineering Comment |
|---|---|---|---|
| 20CrMnTi | Often around 58–62 HRC | Selected to suit tooth size and load | Good balance of cost, availability and gear performance |
| 17CrNiMo6 | Often around 58–63 HRC | Can support deeper hardened cases on larger gears | Useful where core toughness and fatigue strength are important |
| 18CrNiMo7-6 | Often around 58–63 HRC | Frequently selected for demanding carburized gear sections | Common choice for high-load industrial gearing |
| 8620H | Often around 58–62 HRC | Specified according to section size and duty | Widely used in North American gear applications |
How to Select a Carburized Gear Steel
- Tooth bending load: core strength and toughness matter under cyclic bending.
- Contact stress: case hardness and effective case depth influence pitting resistance.
- Gear size: larger sections require sufficient hardenability and controlled heat treatment.
- Shock loading: mining, marine and lifting applications may require greater core toughness.
- Availability and standard: equivalent grades may be selected to match the customer drawing or regional material standard.
- Post-heat-treatment machining: grinding allowance and distortion control should be considered before carburizing.
Carburized Gear Manufacturing Process
A typical route may include blank preparation, rough machining, gear cutting, carburizing and quenching, stress-relief or tempering as specified, hard finishing or gear grinding, and final inspection. The exact route depends on gear size, accuracy class and drawing requirements.
For precision gear cutting and grinding capability, see KNGEAR Gear Works and our guide to CNC gear machining.
Where These Steels Are Used
Carburized gears are widely used in planetary gearboxes, helical gear stages, pinions, industrial gearboxes and other high-load drive systems. The correct steel grade should be selected together with tooth geometry, service factor, lubrication and bearing design rather than as an isolated material decision.
Material Review for Custom Gears
For a custom or replacement gear project, KNGEAR can review the drawing, operating load, existing material specification and required heat treatment before manufacturing. If the original grade is unavailable, an equivalent material should only be selected after comparing chemistry, hardenability, core properties and the required case specification.
Contact KNGEAR for a custom gear or gearbox material review.
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