Through-Hardening Gears

The objective behind through hardening of gears is to harden the gear parts throughout the entire cross section. This process makes the surface resistant to wear, but in the process, the whole part is able to withstand bending or twisting.

The process of Through-hardening is typically used for gears where high surface hardness is not a criteria. Gear tooth hardness ranges from 32 to 48 HRC after undergoing the process of through hardening. Normally steels applied for through-hardened gears have a medium carbon content (0.3-0.6%) as well as a low alloy content (around 3%). The alloy content increases the hardenability. More the hardenability, deeper is the through hardening of gear teeth. High hardenability is a must for through hardening steels.

Types of Through Hardening Process
Primarily there are Four types of method for heat treating of through-hardened gears. Annealing: Annealing primarily refers to the operations of heating and cooling, which is employed for inducing softness. Annealing is also of two types full and process. The purposes of full annealing are to (a) Softening of steel, improve the ductility and machinability (b)Relieving internal stresses as a result of previous treatments (c) improve dimensional stability(d) grain structure refinement. Whereas the purposes of process annealing is partial softening of steel and release of internal stresses.

• Normalizing and annealing: The normalizing and annealing process can be applied singularly or in a typical combination, in the form of a homogenizing grain structure for gears made up alloy steel. The process also helps in minimizing metallurgical non-uniformity.
  
  
• Normalizing and tempering: Normalizing and tempering process means to heat the gears to a temperature of 870- 980 °C (1600 to 1800 °F) and subsequently furnace cooling it in air ( still or circulated). This process can provide more hardness than annealing.
  
  
• Quenching and tempering: In the The quench and temper process there is heating of gears to form austenite at temperatures of 800 to 900 degree centigrade or (1475 to 1650 °F). This is followed by quenching process. Such a quick cooling makes the gears harder and stronger as there is formation of martensite. Hardened gears are subsequently tempered at temperatures below 690 °C (1275 °F). So desired mechanical properties are achieved. Though tempering lowers hardness and strength of the quenched steels but it considerably improves materials properties like ductility, toughness impact etc.