Heat Treatment

Normalizing or Annealing are preparatory or softening treatments, often done on the raw materials or after forgings. The gear is heated to its austenitizing temperature (above the critical point) and then cooled in still air (Normalizing) or furnace-cooled slowly (Annealing). The process is in order to homogenize the microstructure, relieve internal stresses, improve machinability, and prepare the steel for subsequent hardening, to achieve a softer, more uniform, and ductile structure (like pearlite or ferrite-pearlite). A standard step for gear blanks before tooth cutting. Annealing produces the softest state for easiest machining.

QT is a bulk hardening process affecting the entire gear cross-section which is in order to create a uniform, high hardness throughout the gear for strength and wear resistance in smaller or highly loaded gears. The quenching process is heating to austenitize, then rapidly cool (in oil, polymer, or water).Tempering is reheating to a specific lower temperature and hold, then cool. This process could perform a hard, strong, and tough martensitic structure. Higher tempering temperatures trade some hardness for increased toughness and stress relief. QT are used for gears where the entire tooth needs high strength (e.g., pinions, heavily loaded industrial gears). Distortion can be high, requiring subsequent grinding.

1. Specific areas of the gear tooth, typically the flank and root. Induction Hardening is in order to rapidly harden the tooth flanks and root using electromagnetic induction heating followed by quenching. The process is an induction coil (often tooth-shaped) is placed near the gear tooth, a high-frequency current induces heat in a thin surface layer, which is then immediately quenched by a spray.This is highly productive for medium-to-large gears.

2. Laser Hardening is in order to create precise, localized hardened tracks with minimal heat input and distortion. The process is a focused laser beam scans the tooth flank, rapidly heating a thin layer. The surrounding cold metal acts as a “heat sink,” causing self-quenching (no external spray usually needed).This process is Ideal for complex or thin-walled gears, selective hardening of specific wear-prone areas, and repairs which offers the highest flexibility in hardening pattern design.

Carburizing is a surface hardening process which is used for low carbon steel.The process is to add carbon to the surface of low-carbon steel, enabling a deep, hard case after quenching. The process is to make the gear be heated in a carbon-rich atmosphere like endothermic gas, carbon diffuses into the surface, creating a high-carbon “case”, then to be quenched. There will be a deep hardened case (0.5 – 2.0 mm) with very high surface hardness (58-62HRC) and excellent bending fatigue strength.

Nitriding is a surface hardening process which is used for middle carbon steel. The process is in order to diffuse nitrogen into the surface, creating extreme hardness without quenching. The process is making the gear be heated in a nitrogen-rich environment. Then Nitrogen forms hard nitrides, which leads to a very hard surface (65-72 HRC) with a shallow case (0.1 – 0.6 mm). Since there is no phase change that means the distortion is very minimal. Nitriding is very regular to use for finished or near-net-shape gears requiring high wear resistance and minimal distortion (large gears, precision gears, those with complex shapes), however because of less hardening depth, which is not effective for high bending loads gears.