Comparing Gear Steels: 20CrMnTi, 40Cr, 42CrMo and 17CrNiMo6

The heat treatment route comes first; the grade name comes second

No steel grade on its own determines how a gear performs. What determines it is the pairing of a grade with a heat treatment. Take one 40Cr bar: quench-and-temper it, or surface-harden it, and the two outcomes differ by more than double.

Three routes account for most work:

• Quenching and tempering (Q&T) — the part is quenched, then held at elevated temperature to produce tempered sorbite. HB 240–320 is the usual hardness window. Ductility and strength are obtained at the same time, enough for slow- and medium-speed gears whose flanks need not be especially hard.
• Carburizing and quenching — a low-carbon grade is held at 920–950°C in a carbon-rich atmosphere, then quenched. Carbon raises the flank to martensite while the interior remains low-carbon and ductile: a hard skin over a soft core. Gears under severe contact stress are the natural application.
• Nitriding — at roughly 500–570°C, ammonia-bearing gas drives nitrogen into the surface, creating a nitride-hardened zone. Distortion stays low because the temperature is low, yet the layer is shallow (0.3–0.6 mm in normal practice), which limits it to small-module precision gears that need no final grinding.

Decision sequence: work out root bending stress and flank contact stress first. That tells you whether the surface must be hardened. If it must, the next question is carburizing steel or nitriding steel.

In short: settle the heat treatment route before the grade. Slow- and medium-speed gears under moderate load are usually served by quench-and-tempered 40Cr (HB 260–300 after Q&T); 42CrMo (HB 280–320) takes over where sections are larger and heavier. Where a hard flank is mandatory under heavy load, carburize: 20CrMnTi gives HRC 58–62 at the surface over a HRC 30–42 core. Small-module precision gears, where distortion is the concern, suit 38CrMoAl nitriding, with surface hardness near HV 950–1100. On an identical blank, a carburizing steel runs about 1.3–1.5× the cost of a Q&T steel — choosing a material is essentially paying more to gain fatigue life.

Fitting the common gear steels to the job

These costs assume the same blank size and leave heat treatment out of the figure. What the premium on a carburizing steel purchases is contact fatigue life: at equal load, a carburized flank typically holds off pitting one grade better than a Q&T flank does.

40Cr and 42CrMo: the quench-and-temper pair

In China, 40Cr is the Q&T gear steel seen most often — cheap and readily available. 42CrMo hardens to greater depth, so heavy sections through-harden; that property is why large-module and large-diameter shaft gears are cut from it. When the load is moderate, the module sits below 8 and face width is limited, Q&T 40Cr generally does the job.

20CrMnTi and 17CrNiMo6: the carburizing pair

Hardenability in 20CrMnTi is only moderate, yet an oil quench still brings it to the required hardness — the reason it holds sway over small- and medium-module heavy-duty gears in China and shows up repeatedly in automotive transmissions and custom gear work. Deeper hardening and a tougher core are what 17CrNiMo6 offers, which suits large modules and strong impact loading, at a higher price.

38CrMoAl: the nitriding choice

A nitrided 38CrMoAl part combines high surface hardness with minimal distortion, and final grinding is usually unnecessary. The shallow case is the catch: contact fatigue capacity is limited, so heavy flank loads are out of reach. Use it only where the contact stress calculation stays below the limit.

A complete drawing states the material in full

The material block of a finished gear drawing carries three entries: steel grade, heat treatment, hardness range. Leave heat treatment off and write only “40Cr”, and the supplier is left to guess while acceptance turns into a dispute.

Obtain a mill certificate from the steel supplier — heat number, chemical composition, macrostructure — normally to GB/T 3077. Because hardenability bands differ from mill to mill even for one grade, lock in a single mill for series production.

Practice notes: at drawing approval during our precision gear machining stage, three checks are made: does flank contact stress remain below the allowable value for the material, does the section size implied by module and face width fall within hardenability, and are hardness requirements and measurement positions spelled out. Rework usually traces to material or annotation, not to machining. Every carburized batch we run carries a companion test bar; CHD and surface hardness are measured and the records filed, so a later failure can be traced to its heat. To review selection options, browse the gear product range; process information sits on our capabilities page.

Wrong material shows up as failure

Our records hold three instructive jobs:

1. 40Cr used where 20CrMnTi was specified. Pitting broke out on a reducer output gear after about 2,000 running hours. Contact fatigue strength of a Q&T 40Cr flank at HB 285 sits below that of a carburized layer, so the outcome was predictable — nobody had run a contact stress check on the drawing.
2. A large-module gear carburized in 20CrMnTi. Core hardness on a module-12 gear with 120 mm face width came out at only HRC 25–28, under the HRC 30 requirement; root bending strength was therefore short and the tooth fracture risk climbed. Thick sections will not harden through in 20CrMnTi; the job called for 17CrNiMo6.
3. 40Cr Q&T with no surface hardening, flank worn away. Where flanks move fast and sliding ratios are high, Q&T hardness offers no defence against wear. Induction surface hardening to HRC 45–52 put an end to it.

Nothing done downstream can make up for a material that was wrong from the start.

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