What Is Gear Steel?
Gear steel is the family of steels used to manufacture gears, splines, shafts and similar power-transmission parts. A gear tooth has to survive repeated contact stress, sliding wear and bending fatigue at the root, so the material is selected for a combination of surface hardness and core toughness that plain structural steel cannot deliver. Most gear steels are low-carbon alloy grades that are carburized and hardened: the surface reaches a high hardness for wear and pitting resistance, while the core stays tough enough to absorb shock loading.
Carbon, chromium and nickel are the elements that define the behaviour of these steels. Carbon, typically between 0.15% and 0.25% in a carburizing grade, controls the hardness that can be achieved after carburizing and quenching; chromium raises hardenability and wear resistance; nickel improves core toughness and case ductility. Molybdenum, where present, raises hardenability further and allows heavy sections to harden through.
Carburizing Gear Steel Grades and Their Standards
Gear steels are specified through several national and international systems, and the grades below cover the great majority of automotive, industrial and off-highway gear applications. The list also gives the standard that governs each designation.
| Standard | Grade | Region | Notes |
|---|---|---|---|
| SAE / AISI | 8620, 8620H | North America | Nickel-chromium-molybdenum carburizing grade for gears and shafts |
| SAE / AISI | 4320H | North America | Higher hardenability variant for heavier sections |
| ASTM A304 | 8620H, 4320H | North America | End-quench hardenability requirements for alloy steel bars |
| EN 10084 | 20MnCr5 | Europe | Case-hardening steel with high core strength |
| DIN | 1.6523 | Germany | Material number for 20MnCr5 |
| EN 10084 | C10E, C15E, C16E | Europe | Unalloyed case-hardening steels for light-duty gears |
| JIS G4053 | SCM415, SCM420, SCM421 | Japan | Chromium-molybdenum carburizing grades |
| JIS G4053 | SNCM220 | Japan | Nickel-chromium-molybdenum grade comparable to 8620 |
| ISO 683-18 | C10E to C60M2 series | International | Bright bar products of unalloyed and low alloy steels |
Grades are often described as equivalents across these systems, but the equivalence is never exact. SCM420, 20MnCr5 and 8620 all carburize and harden to a comparable surface hardness, yet their hardenability curves differ, which changes the core hardness obtained in a given section size. That is why a grade substitution on a gear drawing should always be checked against the required core hardness at the critical section, not just against the surface hardness.
Chemical Composition of Common Gear Steels
| Element, % | SAE 8620 | EN 20MnCr5 | JIS SNCM220 |
|---|---|---|---|
| C | 0.18-0.23 | 0.17-0.22 | 0.17-0.23 |
| Si | 0.15-0.35 | max 0.40 | 0.15-0.35 |
| Mn | 0.70-0.90 | 1.10-1.40 | 0.60-0.90 |
| Cr | 0.40-0.60 | 1.00-1.30 | 0.40-0.60 |
| Ni | 0.40-0.70 | - | 0.40-0.70 |
| Mo | 0.15-0.25 | - | 0.15-0.30 |
The carburizing grades are supplied in the as-rolled, normalized or soft-annealed condition. Soft annealing is common where the part will be cut and hobbed before heat treatment, because it lowers hardness and improves machinability and dimensional stability during gear cutting. Phosphorus and sulphur are usually held to low maximum values in these grades, since inclusions reduce fatigue life at the tooth root.
Heat Treatment of Gear Steel
Carburizing: the part is held in a carbon-rich atmosphere at roughly 900-950 degrees C so that carbon diffuses into the surface, producing a case typically 0.5-2.0 mm deep.
Direct quenching or reheat quenching: the carburized part is quenched to form a hard martensitic case; reheat quenching gives finer grain and better core toughness.
Tempering: a low-temperature temper around 150-200 degrees C relieves quenching stress while preserving case hardness, commonly 58-62 HRC at the surface with a core in the region of 30-42 HRC depending on grade and section.
Controlled atmosphere and case depth verification: case depth and surface carbon are checked on test coupons or samples from the batch, since both govern pitting resistance and tooth bending fatigue.
Not every gear is carburized. Through-hardened and nitrided routes are also used: medium-carbon grades such as C45 quenched and tempered, or nitriding steels, suit larger, slower-running gears and applications where distortion after heat treatment must be minimal.
Applications and Product Forms
Automotive transmissions: gears, synchronizer parts and shafts that need high surface hardness with a tough core.
Commercial vehicle and off-highway drivelines: heavy-duty gearboxes and final drives operating under shock loads.
Industrial machinery: conveyor drives, reducers, winches and crusher gearing.
Aerospace and precision gearboxes: components where fatigue resistance and consistent hardenability are critical.
Gear steel is normally produced as round bar or wire rod. Wire rod is used for forged or machined small gears, while round bar covers medium and large parts; bar diameters from roughly 12 mm up to 400 mm and wire rod from about 5.5 mm to 23 mm cover most gear and shaft sizes. Bars are supplied in as-rolled, normalized, soft-annealed or peeled and ground condition depending on the machining route, and can be certified to a specified hardenability band when the application requires it.
Frequently Asked Questions
Q: What is gear steel made of?
Most gear steels are low-carbon alloy steels containing roughly 0.15-0.25% carbon with chromium, nickel and often molybdenum to control hardenability, wear resistance and core toughness.
Q: Is 8620 the same as 20MnCr5?
No. Both are carburizing grades and are often described as equivalents, but 8620 uses nickel and molybdenum while 20MnCr5 relies on manganese and chromium, so their hardenability and core hardness differ.
Q: Why is gear steel carburized?
Carburizing creates a hard, wear-resistant case for contact fatigue resistance while the low-carbon core stays tough enough to resist tooth root bending and impact loads.
Q: What hardness does a carburized gear reach?
Surface hardness is typically 58-62 HRC after carburizing and low-temperature tempering, with a core hardness in the region of 30-42 HRC depending on grade and section size.
Q: Which standards cover gear steel?
The main ones are SAE and AISI for 8620 and 4320H, ASTM A304 for hardenability, EN 10084 and DIN material numbers for 20MnCr5, JIS G4053 for SCM and SNCM grades, and ISO 683-18 for bright bar products.
Q: Can gear steel be welded?
Weldability is limited by the alloy content, and these grades are rarely welded in service parts. Where welding is unavoidable, procedures must be qualified and post-weld heat treatment considered.
