Heat treatment and quenching of gears is a complex process. Previous processing variables, as well as the heat treatment parameters of time, temperature, agitation, and quenchant must be carefully scrutinized for proper control of microstructure, distortion, and control of microstructure. D. Scott MacKenzie** Houghton International Inc. Valley Forge, Pa. **Member of ASM International and member, ASM Heat Treating Society he heat treatment of gears is a delicate balancing act. It is necessary to achieve specific mechanical properties at the case and core while achieving low part distortion. Excessive distortion can result in the need for excessive machining to achieve the desired di- mensional properties and can cause too much noise in a gear train. Exces- sive residual stresses can also shorten the life of a gear because of mismatch or shortened fatigue life. Quenching is defined as “the con- trolled extraction of heat.” The most important word in this definition is controlled. A quenchant is any medium that extracts heat from the part, and can be a liquid, solid, or gas. Three stages of quenching (Fig. 1) when a hot part comes into contact with a liquid quenchant are: • Vapor stage (Stage Aor Vapor blanket) • Boiling stage (Stage B or nucleate boiling) • Convection stage (Stage C or convection cooling) The vapor stage is encountered when the hot part surface initially comes into contact with the liquid quenchant, and the part becomes sur- rounded with a blanket of vapor. In this stage, heat transfer is very slow, and occurs primarily by radiation through the vapor blanket. Some conduction also occurs through the vapor phase. The blanket is very stable and its removal can only be en- hanced by agitation or speed-im- proving additives. This stage is re- sponsible for many of the surface soft spots encountered in quenching. High-pressure sprays and strong ag- itation can eliminate this stage, but undesirable microconstituents can form if they are allowed to persist. The second stage encountered in quenching is nucleate boiling, where the vapor stage starts to collapse and all liquid in contact with the compo- nent surface erupts into boiling bub- bles. This is the fastest stage of quenching; high heat extraction rates are due to carrying heat away from the hot surface and transferring it far- ther into the liquid quenchant, which allows cooled liquid to replace it at the surface. Many quenchants contain ad- ditives to enhance maximum cooling rates obtained by a given fluid. The boiling stage stops when the temper- ature of the component’s surface reaches a temperature below the boiling point of the liquid. For many distortion-prone components, high boiling temperature oils and liquid salts are used if the media is fast enough to harden the steel, but both of these quenchants see relatively little use in induction hardening. The final stage of quenching is the convection cooling, which occurs when the component reaches a tem- HEAT TREATMENT OF GEARS : CONTROL OF RESIDUAL STRESS AND DISTORTION HEAT TREATING PROGRESS • JULY 2007 47 T Fig. 1 — Schematic of the three phases of quenching: from left to right, vapor blanket, nu- cleate boiling, and convection.