Heat treatment turns a soft, easily machined blank into a gear that can carry load for years, but it also moves the metal. Every quenched gear changes shape a little, and on a badly designed or poorly loaded part the change can be large enough to scrap it. Distortion is not a random defect that a supplier either fixes or does not; it is a predictable mechanical response to heat, phase change and restraint, and it must be designed for, allowed for and finally corrected by finishing work. Buyers who treat heat treatment as a black box often receive bores out of round, teeth with lead error, end faces that are not flat and pitch errors that only show up when the gearbox whines. The good news is that most distortion is well understood, and once the mechanism is clear the remaining work is a matter of allowance, fixturing and process control.
Two different stresses act on a gear during quenching, and each one bends the part in its own way. Thermal stress comes from uneven cooling: the surface chills and tries to contract first while the core is still hot and plastic, so the surface is pulled in tension and later, as the core cools and contracts, the surface ends up compressed. Transformation stress comes from the austenite to martensite change itself, because martensite occupies more volume than the austenite it grows from. The surface transforms first, expanding while the core is still soft austenite; when the core finally transforms and expands, it pushes the now-hard case outward. Which stress dominates depends on steel hardenability, section size, quench speed and the way the part is fixtured, and the result is a mixture of growth, contraction, bowing and ovality that is hard to guess without trial data. Carburizing adds a further twist because the high-carbon case transforms differently from the low-carbon core: the case usually grows while the core shrinks, and if carburizing is uneven across the tooth, the root or one side of the face width receives more carbon and more expansion than the other, bending the tooth lead. Tempering then relaxes the quenching residual stresses, but the relaxation itself can shift the dimensions slightly, which is why stable parts are tempered twice or held at temperature long enough to finish the movement.
Large and lightly sectioned gears are where distortion bites hardest. A thin-wall internal ring gear with a large diameter and a small radial wall thickness tends to go oval in quenching, because the thin rim cools faster than the heavier web and bolted ring that support it. Section changes do the same thing: a hub that is much heavier than the web, or a rim much heavier than the web, cools at its own pace and drags the lighter section out of true. Bolt holes, keyways and lightening holes placed on one side of the bore break rotational symmetry and act as local coolers and stress raisers, so the gear grows unevenly around its circumference. Long slender pinion shafts and gear shafts behave like a bow: one side of the shaft cools a little faster than the other, the transformation expansion is lopsided, and the part exits the quench with a permanent lead error that can only be removed by straightening or grinding. In practice the worst offenders are thin ring gears, one-sided flanges, splined shafts and webs with asymmetric cutouts.
The standard answer is to leave enough material for the heat treatment to move into, then machine the true surfaces back after hardening. The sequence starts with rough machining: turn, bore and hob the blank near final size but leave a generous layer on every surface that must later be accurate. Typical allowances are 0.3 to 1.0 mm on the bore diameter, 0.2 to 0.5 mm on the outer diameter, a few tenths on each end face and 0.15 to 0.35 mm per tooth flank, depending on module, case depth and the supplier's measured distortion data. The allowance must be symmetric: leaving more stock on one side of a web or one side of a tooth flank only hides the distortion, it does not remove it. A stress relief anneal or normalizing step between roughing and final heat treatment removes machining residual stress that would otherwise add to the quenching stress. After quenching and tempering the part comes back to the machine shop: the bore is ground or honed to size, the end faces are ground flat and parallel, and only then are the teeth finish-ground, because grinding the teeth before the bore is trued would lock the runout into the pitch.
Grinding the hardened teeth is the finishing step that actually corrects post-heat-treatment distortion, but it introduces its own risk. A grinding wheel that is dull, a feed rate that is too high, or coolant that does not reach the contact zone can heat the tooth flank enough to temper the case back, producing a soft, untempered zone or even a network of fine grinding cracks that propagate under load. The usual countermeasures are a sharp and frequently dressed wheel, small infeed, plenty of high-pressure coolant aimed at the contact arc, and a spark-out pass. It is worth understanding the limits of the other finishing processes: shaving is always done on a soft gear before heat treatment, so it cannot correct quench distortion at all; honing removes only a few micrometres and is used after grinding to improve surface finish and introduce a compressive residual stress that lowers noise; grinding is the only process that can actually correct profile, lead and pitch errors after hardening. For medium-precision gears shaving plus heat treatment is enough cost-wise, but for high-precision or high-speed gears there is no substitute for grind-after-quench, often followed by a light honing pass.
A practical control list is worth keeping on every project. Before heat treatment, rough machine the part and stress relieve it, because green-machined blanks carry their own locked-in stress. In design, favour symmetry: uniform wall sections, symmetric bolt-hole patterns, added dummy holes where mass must be balanced, and avoid thin flanges that cool first. For loading, hang shafts vertically on mandrels, support ring gears on flat racks or with internal fixtures, and pack the furnace so that no part touches another at a critical surface. Choose the quench medium carefully: oil or polymer quenches move the part much less than water, and controlled agitation beats still quench. Keep furnace temperature uniform across the load, with calibrated zone control and a slow ramp through the transformation range. Measure before and after: run quench test bars or a representative sample, record bore ovality, runout and lead, and keep a distortion map for each gear family so that the allowance can be tightened over time instead of guessed on every order.
Distortion is best treated as a number that must be predicted, not as an accident to be avoided. When you ask a supplier for gears, make the conversation concrete: state the accuracy grade you need, accept that post-heat-treatment grinding will be used to reach it, specify the surfaces that must be finished after hardening, and ask for sample parts first so that the allowance can be tuned. A supplier who refuses to measure distortion, or who promises zero movement from quench, is promising something no heat treatment can deliver. With realistic allowance, sensible fixturing and a documented process, the same quenching that hardens your gears can also be made to leave them close enough to true that a finishing pass brings them in.