How Industrial Machinery Gears Are Hardened

Gears used in gearboxes are usually made of hardened steel. Steels are often used in gearbox reducers because they can be heat treated to improve the strength and durability of the gears. The most common are alloy and oil-hardened steels, but plain carbon steel can also be used.

Structural components that are subjected to high operating stresses, such as those in Falk gearboxes, require high-strength hardened structures to reduce the likelihood of gearbox repairs. Machine components such as gears often require hardened structures to resist wear and distortion.

Gear hardening procedures

The basic procedure for hardening a steel part is to heat it until the crystal structure changes at the atomic level. The part is then cooled rapidly by quenching to create a crystal structure that is more resistant to deformation than unhardened steel. This results in greater hardness and strength. Tempering is done to improve the toughness of the machined part, as hardened steel is very brittle and can cause fractures with even minor impacts and require expensive gearbox repairs. Tempering involves reheating the part to a temperature below the hardening temperature after it has hardened and allowing it to cool slowly. This greatly reduces brittleness while slightly reducing hardness and strength. Stress relief is a heat treatment that can also be used to relieve internal stresses in gears within gearbox reducers.

Gears may be integrally hardened, which means that the same hardness is achieved throughout the gear all the time. High toughness can be obtained by hardening only the surface of the gear to a depth that provides sufficient surface hardness while leaving the rest of the gear in a relatively soft condition. This surface hardening also allows the use of cheaper steels and less expensive heat treatment methods. This process is often referred to as surface hardening, which means that a hard casing or shell is required on the finished part.

Gear Hardening Methods

A variety of methods can be used to harden gears used in Falk gearboxes. For fully hardened gears, the part is heated to the proper temperature in a furnace and then dropped into a quenching medium such as oil. For some alloy steels, while the part remains in the oven after the heating cycle, the part can be air cooled with a stream of cold inert gas blown through the part. The gear is then tempered to its final work hardness.

For surface hardening, several options are available, including:

Flame hardening. 

The gear is subjected to a gas flame in a localized cross section and then hardened. The localized area is heated to the hardening temperature and then immediately quenched as the area moves out of the heated area. Typical hardening depths range from 1/8 inch to full depth. Typical steels used are plain carbon and low alloy steels with a carbon content of approximately 0.45%.

Induction Hardening. 

This method is used for smaller parts that cannot be effectively flame hardened. The gear is immersed in a quenching medium such as water or oil. A coil is placed around the part and a high current is passed through the coil. The part is heated to hardening temperature by the induction current from the coil. After about 10 seconds, the current is turned off and the part is rapidly cooled by the surrounding quenching medium. The material used is similar to alloy steel. Typical hardening depths range from 0.01" to full depth.

Carburizing. 

The gear is immersed in a hot carbon-rich medium, such as solid charcoal pellets; a carbon-rich molten salt, such as sodium cyanide; or a carbon-rich fuel gas, such as carbon monoxide - for the desired period of time (typically several hours). They are then removed and transferred to a conventional heat treatment process. The material used is typically low carbon steel with a carbon content of 0.1-0.3%. The depth of hardening is usually limited to 0.050 inch or less because the carbon needs time to diffuse deeply into the gear.

Once hardened, the gears are inspected and shipped to the gear reducer manufacturer for installation.CHAOCHANG's gear hardening equipment provides you with a high quality finished product that will give the gears a long life. Feel free to contact us for a quote.