The types of gear material are so many. There are also many factors should to consider when choosing.

1.Gear material must meet the need of working conditions. For example, The gear used in aircraft should meet the requirements of light weight, high transmission power and high reliability. So we must choose high mechanical properties of gold and silver. The gear drive in mining machinery has high power, low working speed and high dust content in the surrounding environment. Therefore, cast steel or cast iron are often chosen. The power of household and office machinery is very small, but it requires smooth transmission, low noise or no noise, and can work normally in the condition of less lubrication or no lubrication, so engineering plastics are often used as gear materials. In short, the first factor to be considered when choosing gear materials is the requirement of working conditions.

2.The size of the gear, the method of forming the blank, the heat treatment and the manufacturing process should be taken into account. Cast billet is generally used for large size gears, and cast steel or cast iron can be selected as gear material. Forged blank is often used for gear with medium or lower size requirements, which can be made of forged steel. When the size is small and the level of requirements are not high, round steel can be used as blank.

The methods of surface hardening of gears are: carburizing, ammoniating and surface hardening. When using carburizing, low carbon steel or low carbon steel should be used as gear material. Ammoniated steel and quenched and tempered steel can adopt nitriding process. When surface hardening is used, there are no special requirements for materials.

3. Normalized carbon steel, no matter how the blank is made, can only be used to make gears that work under stable load or mild impact, and can not withstand large impact load. 

The tempered carbon steel can be used to make gears that work under moderate impact loads.

4. Alloy steel is often used to make high speed, heavy load and work under impact load gear.

5. The gear transmission in the aircraft, the size of the gear is required to be as small as possible,  surface hardening treatment of high strength alloy steel should be chosen.

6. Soft face gear of metal, the hardness difference between the two tooth surface should be maintained at 30 ~ 50HBS or more.

When the tooth surface of the pinion and the big gear has a large hardness difference (such as the pinion tooth surface is quenched and grinding, the big gear tooth surface is normalized or tempered). 

When the speed is higher, the hard pinion teeth will have a significant cold-hardening effect on the soft tooth surface of the big gear, and the fatigue limit of the big gear tooth surface will be increased.

Therefore, the allowable contact fatigue stress of the big gear can be increased by about 20% when the hardness difference between the two pairs of gear tooth surfaces is large. However, attention should be paid to the tooth surfaces with high hardness, and the roughness value should be decreased accordingly.