How to Choose Gear Material: Where 20CrMnTi, 40Cr, 42CrMo and 17CrNiMo6 Each Fit

Bottom line: Pick the heat treatment route first, then the steel grade. For low- and medium-speed gears with moderate loads, quench-and-tempered 40Cr (HB 260–300 after Q&T) is usually enough; 42CrMo (HB 280–320) covers larger, heavier sections. When the flank must be hard for heavy loads, go carburized: 20CrMnTi reaches HRC 58–62 on the surface with a HRC 30–42 core. For small-module precision gears where distortion matters, 38CrMoAl nitriding gives a surface hardness around HV 950–1100. For the same blank, carburizing steel costs roughly 1.3–1.5× a Q&T steel — material selection is basically trading cost for fatigue life.

1. Choose the heat treatment route first; the steel grade follows#

The steel grade alone does not decide gear performance; steel + heat treatment does. The same 40Cr bar can be quenched and tempered or surface-hardened, and the two routes differ in performance by more than a factor of two.

Three mainstream routes:

• Quenching and tempering (Q&T): quench, then temper at high temperature, to obtain tempered sorbite. Hardness typically HB 240–320, strength and toughness come together, good enough for low- and medium-speed gears with modest flank hardness requirements.
• Carburizing and quenching: low-carbon steel is carburized at 920–950°C and quenched. High-carbon martensite on the flank, tough low-carbon core — hard outside, tough inside, suited to heavy-load gears with high contact stress.
• Nitriding: nitrogen diffuses into the surface in an ammonia-bearing atmosphere at roughly 500–570°C, forming a nitride-hardened layer. Low process temperature, small distortion, but the case is thin (normally 0.3–0.6 mm), so it fits small-module precision gears that skip final grinding.

Selection order: calculate flank contact stress and root bending stress first, decide whether surface hardening is needed; if yes, choose between carburizing steel and nitriding steel.

2. Where the common gear steels fit#

Costs are for the same blank size, excluding heat treatment. The extra money for carburizing steel buys contact fatigue life — under the same load, a carburized flank generally resists pitting one step better than a Q&T flank.

Quench-and-temper steels: 40Cr and 42CrMo#

40Cr is the most widely used Q&T gear steel in China — cheap and easy to source. 42CrMo has better hardenability, so thicker sections harden through, which is why large-module and large-diameter shaft gears use it. For moderate loads with module below 8 and modest face width, Q&T 40Cr is usually enough.

Carburizing steels: 20CrMnTi and 17CrNiMo6#

20CrMnTi has middling hardenability but reaches the required hardness with an oil quench, so it dominates small- and medium-module heavy-duty gears in China; it shows up constantly in automotive transmissions and custom gear projects. 17CrNiMo6 offers better hardenability and a tougher core, suited to large modules and obvious impact loads — at a higher price.

Nitriding steel: 38CrMoAl#

Nitrided 38CrMoAl gives high surface hardness with small distortion, and usually skips final grinding. The catch is the thin case: limited contact fatigue capacity, so it cannot carry high flank loads. Use it only when the contact stress calculation comes in under the limit.

3. Wrong material fails first#

Three cases from practice:

1. 20CrMnTi swapped to 40Cr. The reducer output gear pitted, and pits appeared after roughly 2,000 running hours. A Q&T 40Cr flank at HB 285 has lower contact fatigue strength than a carburized layer, so the failure was expected — the drawing just never checked contact stress.
2. Large-module gear carburized in 20CrMnTi. A module-12 gear with 120 mm face width reached only HRC 25–28 in the core, below the required HRC 30, so root bending strength fell short and tooth fracture risk rose. 20CrMnTi cannot harden through thick sections; this job belongs to 17CrNiMo6.
3. 40Cr Q&T only, no surface hardening, flank wear. On fast flanks with high sliding ratios, a Q&T hardness cannot resist wear. After induction surface hardening to HRC 45–52, the wear stopped.

Wrong material cannot be fixed by better downstream processes.

4. The drawing must state the material completely#

A complete gear drawing lists three things in the material block: steel grade, heat treatment, hardness range. Writing only “40Cr” without heat treatment leaves the supplier guessing, and acceptance turns into a debate.

Ask the steel supplier for a mill certificate — heat number, chemical composition, macrostructure — normally to GB/T 3077. Hardenability bands differ between mills for the same grade, so for batch production, fix the mill.

Practice notes: during drawing approval at our precision gear machining stage, we check three things: whether flank contact stress stays under the material’s allowable value, whether the section size from module and face width is within hardenability, and whether hardness requirements and measurement positions are written down. Most rework is not a machining problem; it is a material or annotation problem. For carburized parts we keep a companion test bar with every batch, measure CHD and surface hardness, and file the data so any later failure traces back to the heat. For selection support browse the gear product range ; for process details see our capabilities page.

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