Forged Gear Blanks

Before a gear has teeth, it is a blank: a disc, hub or ring with the right outline, bore and faces to be cut into a gear. The blank is turned, then its teeth are hobbed, shaped or milled, then the gear is usually carburised and hardened, and finally its teeth may be ground.

It is easy to treat the blank as a simple piece of steel. It is not. The blank's internal structure determines how the gear cuts, how much it distorts in heat treatment and how long its teeth survive. Many gear quality problems that appear at hardening or in service trace back to the blank.

Common blank forms

• Plain discs for spur and helical gears
• Hub-and-web blanks with a thinner web between hub and rim, reducing weight and machining
• Shaft-integral blanks for pinion shafts, where the gear is part of the shaft — see forged pinions
• Bevel gear blanks with a conical face
• Rings for ring gears and large internal gears, often produced by ring rolling

Gear teeth fail in two principal ways: bending fatigue at the tooth root, which snaps teeth off, and contact fatigue on the flank, which pits the working surface. Both are sensitive to the quality of the metal where the teeth are cut.

Upsetting a billet into a disc drives grain radially outward, so in the rim the fibre is arranged around the circumference rather than being cut across by tooth spaces in the way it would be in a bar-cut gear. Together with the absence of porosity just below the flank surface, this is why forged blanks are standard for loaded transmission gears. See forged versus cast gears.

Forging also saves material and machining. A hub-and-web forged blank avoids turning away large volumes of steel from a solid bar or disc, and a near-net profile shortens the turning cycle before hobbing.

Why case hardening steels dominate

A gear tooth needs a very hard surface to resist pitting and wear, and a tough, relatively ductile core to resist bending fatigue and shock. Case hardening steels have low carbon, typically about 0.14–0.22%. Carburising diffuses carbon into the surface, which then hardens to around 58–62 HRC, while the low-carbon core hardens only moderately and stays tough. See 20MnCr5 versus 16MnCr5.

Restricted hardenability (+H) steels

Standard case hardening grades allow a fairly wide range of hardenability from one heat to the next. Because hardenability affects both core hardness and distortion, gear makers frequently specify restricted hardenability bands — designated +H in EN ISO 683-3, or H-steels such as SAE 8620H in American practice — so that successive heats respond to heat treatment consistently.

Fine grain size

Carburising holds steel at high temperature for hours, which can coarsen grain. Gear steels are therefore normally fine-grain, aluminium-treated steels, commonly specified with an austenitic grain size of ASTM 5 or finer, so that the case and core remain tough and distortion stays controlled.

1. 01Material verification. The heat is spectro tested and, for gear steels, its hardenability and grain size requirements are checked against the certificate.
2. 02Billet cutting. Billets are cut to calculated weight.
3. 03Heating. Induction heating to forging temperature with pyrometer verification.
4. 04Upsetting. The billet is upset into a pancake, driving grain radially outward.
5. 05Closed die forging. The pancake is forged to the hub, web and rim profile; the centre may be pierced to form the bore.
6. 06Trimming. Flash is removed hot.
7. 07Isothermal annealing or normalising. Controls microstructure and hardness for machining and heat treatment.
8. 08Shot blasting and proof machining. Scale is removed, and blanks may be rough turned to the gear maker's allowance.
9. 09Inspection. Hardness, dimensions and, where specified, microstructure are checked.

Large ring gears are usually produced by piercing an upset preform and ring rolling it to size, which aligns grain continuously around the circumference.

The heat treatment a blank receives after forging has a large, and often underestimated, influence on gear quality.

A forged blank cools unevenly. Left alone, or merely air cooled, it can develop a mixed microstructure — patches of bainite among ferrite and pearlite — and variable hardness. That causes two problems: inconsistent machinability during turning and hobbing, and, more seriously, unpredictable distortion during carburising, because different structures change volume differently when the gear is hardened.

Isothermal annealing, often called FP (ferrite–pearlite) annealing, addresses both. The blank is heated to austenitising temperature, then cooled rapidly to a holding temperature — typically in the region of 600–650°C — and held there so the steel transforms fully to a uniform ferrite–pearlite structure before cooling. The result is a consistent structure and hardness from blank to blank.

Consistent distortion does not mean zero distortion. It means the gear maker can compensate for it reliably in pre-hardening dimensions, reducing grinding stock and scrap. See forging distortion problems.

Gear accuracy grades are defined in standards such as ISO 1328-1, and recommendations for blank tolerances, including runout of reference faces and bores, are given in ISO/TR 10064-3. Load capacity calculations under ISO 6336 or AGMA 2001 assume material quality consistent with the grade selected — which begins with the blank.

Although carburising is carried out after the teeth are cut, it is worth understanding because it is where the blank's quality shows.

• Carburising — typically at about 900–950°C in a carbon-rich atmosphere, diffusing carbon into the surface for a time set by the required case depth.
• Hardening — quenching, usually in oil, to transform the carbon-rich case to martensite.
• Tempering — at a low temperature, typically about 150–200°C, to reduce brittleness while keeping case hardness around 58–62 HRC.
• Grinding — where tooth accuracy requires it, removing distortion from the flanks.

Effective case depth for transmission gears is commonly in the range of about 0.6–1.5 mm, scaled to the gear module and load. Too shallow and the case wears or fatigues through; too deep and the tooth tips can become brittle. Our carburising versus induction hardening comparison explains when each method suits.

See why gears fail for failure diagnosis.

Two heats of the same gear steel grade can harden differently, and in gears that difference shows up as varying core hardness and varying distortion. Hardenability is therefore measured, not assumed.

The standard method is the Jominy end-quench test, described in ASTM A255 and ISO 642. A bar of the steel is austenitised, then water is sprayed on one end only. The quenched end cools fastest and the far end slowest. Hardness is then measured at intervals along the bar, producing a curve that shows how hardness falls with distance from the quenched end — effectively, with slower cooling.

• A steel with high hardenability keeps its hardness further along the bar.
• A steel with low hardenability loses hardness quickly away from the quenched end.
• Restricted hardenability (+H) grades guarantee that the curve falls within a defined band, so gear core hardness and distortion are consistent from heat to heat.

Gear manufacturers who have tuned their heat treatment and grinding allowances to one hardenability level can suffer rising scrap if the next heat of steel sits at the other end of an unrestricted range. Specifying a restricted band avoids that.

Most gear blank problems are discovered by the gear maker — during hobbing or after carburising — rather than at the forge. Knowing their origins makes them easier to prevent.

Several of these can only be seen with metallurgical examination, which is why blank microstructure and hardness should be part of a gear blank specification rather than left to chance. See why gears fail.

We supply blanks to gear manufacturers who cut and harden the gears themselves. Carburising is arranged with qualified processors where a customer requires hardened parts. Related components include forged pinions, sprockets and worm shafts. Request a quote.