Gear Materials and Heat Treatment: Material Selection and Carburizing Practice
Covers gear material grades like 20CrMnTi and 42CrMo, hardness ranges, and the carburizing process route from forging to final machining.
The design decision behind it
Before a single tooth profile is cut, someone has to decide what the part is made of and how it will be heat treated. On a custom-machined gear, that decision is not a catalogue pick — it is a trade-off the drawing has to record, because the shop will build exactly what the drawing says.
The first fork is through-hardened versus case-hardened. Through-hardening (quench and temper) suits low-to-medium speed, moderate load gearing where distortion control and cost matter more than surface durability. Case carburizing suits high-speed, heavy-duty gearing where the surface must resist contact fatigue while the core stays tough enough to carry bending load. Nitriding is a third route for wear-resistant precision parts where minimal distortion after the final cut is the priority. Each route changes what the shop must leave as grinding stock, how much distortion to plan around, and how the part is inspected.
The second fork is material grade within a route. For carburizing, the choice is driven by hardenability and grain behaviour during the long hot cycle, not by price alone. A grade that holds fine grain through carburizing lets you quench directly from the furnace; a grade that does not forces extra process steps. For through-hardened parts, the question is whether the section size can actually achieve the core hardness the design assumes.
The third fork is case depth and core hardness as a pair. They are a system: a hard case on a soft core spalls or fractures; a shallow case on a hard core wears through. Neither number means anything alone, and both should be derived from the tooth root and flank stress the design expects — not copied from a neighbouring project.
Numbers that matter on the drawing
A gear drawing that will be quoted and machined without back-and-forth carries a specific set of numbers. These are the ones that change the process route.
| Drawing item | Why the shop needs it | How to state it |
|---|---|---|
| Module, tooth count, pressure angle, helix angle | Defines the cutter and the inspection setup | Full basic geometry, not just OD and ratio |
| Material grade and condition | Sets the process route and stock allowance | Grade plus heat-treatment callout, e.g. carburize and quench |
| Effective case depth | Drives furnace time and grinding stock | A range, with the measurement definition (see below) |
| Core hardness | Determines hardenability requirement and temper | HRC range, measured at a defined location |
| Surface hardness | Inspection acceptance | HRC or HV, with scale and load stated |
| Distortion allowance | Determines pre-heat-treatment dimensions | State whether grind-after-heat is required |
| Datum and inspection scheme | Makes the acceptance criteria checkable | Datum faces, gear inspection standard referenced as the marking basis |
| Root fillet and tip chamfer | Affects tooling and stress concentration | Explicit radii, not “standard” |
Two measurement conventions are worth spelling out, because they are the most common source of dispute. Effective case depth is conventionally defined as the depth to a specified hardness limit (often around 550 HV) measured on a cross-section — the drawing should name the limit and the location. Core hardness must be tied to a position, typically mid-tooth or a defined distance below the case, otherwise two labs will report two different numbers on the same part.
Gear accuracy should be called out by referencing a standard as the basis of marking — ISO 1328, DIN 3962, AGMA 2001 and similar — together with the specific grade you want marked. The standard tells the inspector which evaluation system applies; it does not by itself fix a tolerance.
Where custom manufacturing changes the answer
On a made-to-drawing part, the parameters that move cost and feasibility are rarely the ones buyers expect.
Section thickness and transitions. A gear with abrupt shoulders, thin webs, or heavy-to-light transitions distorts more in quench. That distortion is not a defect to be argued away — it is a dimensional consequence that has to be absorbed either by extra grinding stock or by a stress-relief step. A drawing that shows generous fillets and gradual transitions is cheaper to hold than one that does not.
Grinding stock and the heat-treat sequence. If the drawing requires the final tooth flank to be ground after hardening, the pre-heat-treatment profile must be cut oversize by an amount the shop and the heat treater agree on. If the drawing does not say whether grinding follows heat treatment, the quote is ambiguous and the part may come back undersize or with an unground flank.
Case depth versus available furnace time. Deeper case means longer at temperature, and the relationship is steep. A case depth specified “to be safe” can multiply cycle time without improving the part. The right number is the minimum that satisfies the stress case, with a small allowance.
Material availability in the specified grade and size. A grade that is common in bar stock may be awkward in the required diameter or forging size. When the drawing allows an equivalent grade with stated hardenability, the shop has room to work; when it names one grade only, lead time and cost follow that grade.
Quantity and repeatability. A single prototype and a repeat order are different problems. The first part proves the process; the repeat order depends on the drawing being complete enough that the same decisions get made again.
Common mistakes in drawings and enquiries
Case depth given as a single number with no measurement definition. Consequence: the shop and the customer measure at different hardness limits and both are “right.” Fix: state the range, the hardness limit, and the location on the cross-section.
Surface hardness specified without core hardness. Consequence: the part passes surface inspection and fails in service at the root. Fix: specify both, with the core measurement position.
No statement of whether grinding follows heat treatment. Consequence: stock allowance is guessed, and the flank may end up under case or out of tolerance. Fix: add an explicit note on the process sequence and the finishing operation.
“Standard” fillets, chamfers, and tolerances. Consequence: the shop applies its own default, which may not match the design intent, and the first article is rejected on a detail nobody specified. Fix: put numbers on every feature that carries stress or fits another part.
Enquiry with a photo or a sample but no key dimensions. Consequence: the shop cannot assess machinability or heat-treat route, so the quote is either padded or wrong. Fix: send the drawing, or for a sample, the critical dimensions, material, and intended duty.
A short checklist
Print this and run it against the drawing before it goes out for quotation.
- Full basic gear geometry: module, tooth count, pressure angle, helix angle, hand of helix
- Material grade and heat-treatment route named explicitly
- Effective case depth as a range, with hardness limit and measurement location
- Core hardness range, with measurement position
- Surface hardness with scale and load
- Note on whether tooth flanks are ground after heat treatment
- Distortion allowance or pre-heat-treatment dimensions stated
- Datum faces and gear inspection standard identified as the marking basis
- All fillets, chamfers, and radii given as numbers
- Quantity, and whether the enquiry is a prototype or a repeat
FAQ
Does the drawing need to name a specific material grade, or can it name a property?
Either works, but they are quoted differently. A named grade fixes the process route and makes the quote comparable; a property specification (hardenability, core hardness, case depth) lets the shop propose a grade it can actually source in your size. If you name a grade, add the properties you actually need so an equivalent can be discussed.
How do I decide case depth if I don’t have stress analysis?
Start from the tooth root bending stress case and the contact stress case, then take the shallower of the two results as a floor and add a small grinding allowance. If neither is available, module-based estimation is a reasonable first pass, but treat it as a placeholder to be confirmed, not a final number.
Why does the shop ask about grinding after heat treatment on a part that is already finish-dimensioned?
Because carburizing and quenching move the part. If the flank is ground afterward, the pre-treatment profile must be cut oversize; if it is not, the heat-treat distortion has to sit inside the final tolerance. The answer changes the stock allowance, the process sequence, and the price.
Send your drawing to sales@dgpinyi.com or call +86 18002857601 and we will review the geometry, material, and heat-treatment callouts for machinability before quoting.