Carburizing and Quenching of Gears: Controlling Case Depth Uniformity and Distortion

The design decision behind it

Case depth is not a number you pick once and forget. It is a consequence of how you expect the tooth to fail. A part that pits at the flank under high contact stress wants a deeper, well-supported case; a thin-ribbed or long-shafted part that must stay straight through quench wants the shallowest case that still meets the contact duty, because every extra tenth of a millimetre of case depth buys fatigue resistance at the price of quench response and dimensional movement.

The practical trade-off runs along three axes. First, case depth against core strength: a case that is deep relative to the tooth root section leaves little ductile core to absorb bending, so the drawing should state both the case requirement and the core hardness expectation, not just one. Second, hardness against toughness: harder cases resist pitting but raise the risk of edge chipping on teeth with sharp corners or thin tips. Third, distortion allowance against grinding stock: the more you intend to grind after heat treatment, the more room you must leave in the pre-heat-treatment tooth thickness, and the more you must decide in advance which surfaces are ground and which are left as-quenched.

For a designer, this means the heat-treatment decision belongs on the drawing at the same time as the geometry. If you specify a case depth without saying how it will be verified, or a hardness without saying where on the tooth it applies, the shop has to guess, and the guess will be conservative in a way that costs you money. Decide the verification method before you fix the number.

Medium choice follows the same logic. Atmosphere carburizing suits general transmission work and is forgiving on fixturing; vacuum carburizing avoids intergranular oxidation and gives tighter control of distortion, which matters when the finished part has no grinding allowance on the flanks. Salt bath is a small-module option. None of these is a default — each one changes what you must put on the drawing.

Numbers that matter on the drawing

The single most important callout is the effective case depth, defined as the perpendicular distance from the surface to the point where hardness falls to a stated limit, commonly 550 HV. Because the measurement is hardness-based, the drawing must name the load and the standard used for verification, for example ISO 2639 for determination of effective case depth. Without that, two inspectors can legitimately report different values on the same part.

Surface and core hardness should be stated as separate requirements with their measurement locations. Tooth flank hardness is not the same as root hardness, and neither is the same as a test coupon. Say which one governs acceptance.

The table below gives a starting relationship between module and effective case depth that is widely used in practice. Treat it as a reference point to be confirmed against your contact stress calculation, not as a rule.

Distortion is the second number that must appear. It is normally expressed as an allowed deviation in lead, profile, or runout after quench, and it only has meaning alongside the grinding allowance. If the drawing says “distortion ≤ 0.10 mm” but leaves no stock to correct it, the requirement is unachievable by any process. State the post-quench allowance and the surfaces to be ground.

Finally, record the pre-heat-treatment condition. Normalizing plus high-temperature tempering before finish machining is a drawing-level instruction as much as a process one, because it determines how much residual stress the part carries into the furnace.

Where custom manufacturing changes the answer

On a made-to-drawing part, the parameters that move cost and feasibility are rarely the ones printed largest.

Geometry that quenches unevenly is the first driver. Asymmetric webs, unequal rib thickness, blind holes near the tooth root, and keyways that break the section all create thermal gradients, and thermal gradients become distortion. A design that is fine in service can be expensive to harden because it needs fixtures, slower quench sequences, or straightening afterwards. When you review a drawing for quote, look for section changes within one tooth width.

Grinding allowance is the second. Every surface you intend to grind after heat treatment adds operations, and it also constrains how much distortion the process is allowed to produce. A part that is finish-machined before hardening and ground only on the bore is a different cost profile from one ground on the flanks.

Lot structure matters more than people expect. Quenching behaviour depends on how parts sit in the load, so a drawing that specifies vertical hanging or dedicated fixtures is telling the shop something about cost as well as quality. Small, thin, or long parts that cannot be stacked will consume fixture space.

Material specification is the fourth. The hardenability of the grade you name determines whether the core hardness you asked for is reachable at the section size you drew. If the section is heavy and the grade is mild, the answer is a different grade or a different core requirement — better settled at enquiry than after the first furnace run.

Common mistakes in drawings and enquiries

1. Case depth stated without a verification method. The consequence is argument at incoming inspection. Fix it by naming the hardness limit, the load, and the standard, for example “effective case depth per ISO 2639, limit 550 HV, HV1.”

2. Surface hardness given as a single number with no location. A flank reading and a coupon reading are different measurements. State the location and whether the coupon is acceptable as the acceptance basis.

3. No grinding allowance defined after heat treatment. Distortion then has nowhere to go, and parts are scrapped or accepted with compromised lead. Mark which surfaces are ground and how much stock is left.

4. Sharp corners and thin tooth tips left on the drawing. These are the first places to chip during handling and service. Add edge break callouts and reconsider tip thickness at the design stage rather than asking the shop to work around it.

5. Material and core hardness specified independently. The two must be consistent at the drawn section size. Ask for a hardenability review at enquiry if the section is heavy, and be prepared to change one of the two.

A sixth, quieter mistake: sending a drawing without the pre-heat-treatment condition. The shop will assume a normalizing route, and if your part needs stress relief before finish machining, that assumption surfaces only when the first batch distorts.

A short checklist

• Effective case depth stated as a range, with the hardness limit and verification standard named.
• Surface hardness given with measurement location; core hardness given separately.
• Material grade and section size checked for consistency with the core requirement.
• Pre-heat-treatment condition (normalizing, tempering, stress relief) noted on the drawing.
• Grinding allowance defined per surface, with ground surfaces marked.
• Distortion limit expressed in a measurable way (lead, profile, runout) and tied to the allowance.
• Fixturing or orientation requirement noted for thin, long, or asymmetric parts.
• Edge break and corner radii called out on tooth tips and flank ends.
• Coupon and sampling basis agreed at enquiry, including how many parts per lot are tested.
• Any special cleanliness, decarburization, or intergranular oxidation limit stated explicitly.

FAQ

Does a deeper case always mean a longer-lasting gear?

No. Beyond the depth needed for the contact stress, additional case reduces the ductile core that absorbs bending and makes distortion harder to control. The useful depth is the one that satisfies your contact duty while leaving enough core to carry the root bending load at the section you drew.

Can I specify case depth on the finished part after grinding?

You can, but you must then tell the shop how much stock will be removed, because the as-quenched case must be deeper than the final requirement. If the drawing gives only the finished value, the process route cannot be planned correctly at quotation.

What do you need to quote a carburized and quenched gear?

The drawing or sample, the material grade, the case depth and hardness requirements with their verification basis, which surfaces are ground and with how much allowance, and the distortion limit you can accept. Send these to sales@dgpinyi.com or call +86 18002857601 and we will review manufacturability against the drawing before quoting.

Send your drawing to sales@dgpinyi.com or call +86 18002857601 for a manufacturability and process review.