The service life of a gear is decided long before the first tooth is cut. It is decided the day the material grade is chosen and the heat-treatment route is set. Two gears with identical geometry, accuracy and surface finish can last ten times apart in service purely because one was made from the right steel and case-hardened correctly, while the other was not. For design and procurement engineers, the material and treatment call is the highest-leverage decision in a gear program: it sets the contact fatigue limit, the tooth-root bending strength, the wear rate, the noise behaviour and, ultimately, whether the gear reaches its rated life or fails in the first year. This guide explains the working gear materials, the heat-treatment routes, and how to balance a hard, wear-resistant tooth surface against a tough, shock-resistant core.
The single most common mistake is treating "hardness" as the only target. A gear that is too hard through the section becomes brittle and breaks under shock load; a gear that stays soft wears quickly and pits at the pitch line. The real objective is a controlled gradient: a hard, smooth, compressive surface that resists pitting and wear, supported by a ductile, tough core that absorbs bending and impact without cracking. Nearly every gear heat treatment exists to create exactly this gradient, and the material grade decides how steep and how reliable that gradient can be.
Carbon and low-alloy steels are the workhorses of custom gearing. Plain carbon steel 45# (roughly equivalent to AISI 1045) is the economical default for light and medium loads, low-speed reducers and general machinery; it is usually normalized or quenched and tempered to 220–280 HB, and can be induction hardened on the tooth flank for extra wear resistance where contact stress is moderate. For higher loads, 40Cr (similar to AISI 5140) and especially 42CrMo (similar to AISI 4140) offer markedly better hardenability and core strength, which matters once the gear section grows beyond roughly 50 mm; these medium-alloy steels are typically induction hardened or quenched and tempered to 280–340 HB for robust bending strength without the cost of through-carburizing.
When the load is heavy, the speed is high, or shock and reversing loads are expected, designers move to carburizing steels. 20CrMnTi is the standard carburizing gear steel in China and the most common grade we machine for gearboxes, reducers and automotive drives; 20CrMo and 20CrMnMo are close alternatives with higher hardenability for larger cross-sections. These steels start with low carbon, so the core stays naturally tough. After machining, the teeth are carburized to raise carbon only in a thin surface layer, then quenched and tempered, producing a tooth surface of 58–62 HRC over a ductile core of roughly 30–45 HRC. That combination — hard skin, tough core — is what gives a carburized gear its high contact-fatigue and bending-fatigue capacity, and it is the default choice for any gear expected to run for tens of thousands of hours.
Beyond alloy steels, other materials enter where a specific condition demands them. Stainless steels such as 304 and 316 are chosen for corrosive, food, medical or wash-down environments, but they cannot be through-hardened to gear-grade hardness and are limited to light duties; where stainless strength is needed, precipitation-hardening 17-4PH (630) reaches about 300–380 HB and is a practical compromise. Grey and ductile cast iron (for example GG25 and QT600) are valued for excellent machinability, vibration damping and low noise, which is why they appear in pumps, light gearboxes and noise-sensitive drives, but their lower strength confines them to moderate contact and bending stresses. Sintered powder-metallurgy gears are near-net-shape, consistent in mass production, and can even be made self-lubricating through controlled porosity, but their lower effective strength puts them in low-torque, high-volume applications such as office equipment and small motors.
The heat-treatment route is chosen together with the grade. Carburizing and quench is the premium route for 20CrMnTi-type steels: it gives the deepest, most fatigue-resistant case and the best surface compressive residual stress, at the price of some distortion that must be removed by finish grinding. Induction hardening suits medium-carbon 40Cr and 45# gears: the tooth surface is selectively heated and quenched, the cycle is fast, distortion is lower than carburizing, and it is economical for large, single or small-batch gears where through-carburizing would be costly. Nitriding is the low-distortion finishing treatment: parts are tempered and the surface is enriched with nitrogen at low temperature, reaching 650 HV or more with almost no quenching distortion, which is ideal for precision gears that cannot tolerate grinding stock; the case is thin, however, so it fits high-wear, low-impact rather than heavy shock. Quench and tempering (the Chinese "tiao zhi") gives uniform through-hardness of 220–320 HB for balanced, predictable bending strength, and normalizing ("zheng huo") refines the microstructure as a baseline for low-stress gears that will not be surface hardened.
Effective case depth (CHD) is the next decision and is where many gears are underspecified. The case must be deep enough to support the Hertzian contact stress and the bending stress at the root, yet shallow enough to remain tough and not to require expensive grinding. A shallow case wears through quickly and then pits; an excessively deep, brittle case can spall or crack under shock. As a rule of thumb, medium-duty carburized gears run an effective case depth of roughly 0.6–1.2 mm (measured to 550 HV on the hardness gradient), with larger modules and heavier loads calling for deeper cases. The case must be specified together with the finish: if the gear will be ground after heat treatment, the as-treated case must be deeper than the finished CHD to leave hardened material after grinding.
The link between material, treatment and gear life is direct. Contact fatigue pitting, the most common failure, starts at the surface, so it is governed mainly by surface hardness, material cleanliness (inclusion content) and compressive residual stress — which is why a fine-carburized, grain-refined 20CrMnTi outperforms a cheap through-hardened steel on pitting by a wide margin. Root bending fractures start at the fillet, so they depend on core toughness, root hardness and a smooth fillet surface; this is exactly where a brittle over-hardened gear lets you down. Adhesive wear and scuffing are controlled by surface hardness and lubricant film, while abrasive wear falls sharply once the surface passes about 55 HRC. Choosing the grade and route therefore means mapping the expected failure mode first: if the risk is pitting, optimize surface hardness and case depth; if the risk is tooth breakage under shock, protect the core and the root fillet.
For RFQ and purchasing, a heat-treated gear should never be ordered by material name alone. The enquiry should state the grade, the surface hardness range and the core hardness, the effective case depth, whether grinding stock is left after treatment, and the inspection records the buyer expects: a hardness survey across the tooth profile and root, an etched case-depth section, and mill test or heat-treatment certificates. Gaps here are the usual source of the "two gears look identical but perform differently" problem, because hardness and case depth vary within a part as well as between parts.
What surface hardness should a case-hardened gear reach? For carburized automotive and reducer gears, 58–62 HRC on the tooth flank is the normal working target; induction-hardened medium-carbon gears usually land at 50–55 HRC, and nitrided gears are quoted around 650–900 HV rather than HRC.
How deep should the effective case be? Medium-duty carburized gears commonly use 0.6–1.2 mm to the 550 HV limit, scaled up with module and load. The deeper the case, the more the core is diluted in toughness, so deeper is not always better.
When is carburizing better than induction hardening? Carburizing wins for heavy, high-speed, shock-loaded gears that need a deep, fatigue-resistant case and a tough core; induction hardening wins for medium-load carbon or 40Cr gears where speed, low distortion and cost matter more than maximum contact capacity.
Do harder gears always last longer? No. Surface hardness resists wear and pitting, but if the whole part is too hard and brittle the teeth break. The durable gear is the one with the right surface-hardness-to-core-toughness gradient, not the hardest number on a drawing.
At our factory in Luoyang we machine and heat-treat custom gears from 20CrMnTi, 42CrMo, 40Cr, 45# and stainless grades, and we provide hardness, case-depth and material certificates with every order. Send us your drawing, load, speed and expected life, and we will recommend the material grade, heat-treatment route and case depth that balance wear resistance against shock toughness for your application — before the gear is cut, not after it fails in service.