Gear Materials and Heat Treatments

6. Gear Materials and Heat Treatments / It is essential to select proper materials and heat treatments in accordance with the intended application of the gear.

Since gears are applied for various usages, such as industrial machinery, electric/ electronic devices, household goods and toys, and composed of many kinds of materials, we like to introduce typical materials and their heat treatment methods.

6-1 Types of Gear Materials

S45C (Carbon Steel for Structural Machine Usage)

S45C is one of the most commonly used steel, containing moderate amounts of carbon (0.45% ). S45C is easily obtainable and is used in the production of spur gears, helical gears, gear racks, bevel gears and worm gears.

Heat Treatment and Hardness

Heat Treatment	Hardness
None	less than 194HB
Thermal Refining	225 – 260HB
Induction Hardening	45 – 55HRC

SCM440 (Chrome-molybdenum Alloy Steel)

An alloy steel containing moderate amounts of carbon (0.40% ). It also contains chrome / molybdenum. SCM440 has more strength than S45C and is used with thermal-refining or induction-hardening treatment for producing gears.

Heat Treatment and Hardness

Heat Treatment	Hardness
Thermal Refining	225 – 260HB
Induction Hardening	45 – 60HRC

SCM415 (Chrome-molybdenum Alloy Steel)

SCM415 is one of the most commonly used low-carbon alloy steel (C = 0.15%). Generally, it is carburized for use. It has more strength than S45C or SCM440. Surface hardness should be between 55 and 60HRC for use.

SUS303 (Stainless Steel：18Cr-8Ni Stainless Steel)

Since it is called “stainless steel”, it is a rust-resistant steel. This authentic stainless steel is basically non-magnetic. Most commonly used for gears in applications where rust contamination is undesirable, such as in food-processing machinery. There is a similar stainless steel called SUS304 which has more corrosion resistance than SUS303.

Copper Alloy Casting

Frequently used as a material for worm wheels. Phosphor bronze casting (CAC502) or aluminum-bronze casting(CAC702) are commonly used. For mating worms, iron metals such as S45C/SCM44/SCM415 are used. To prevent galling / seizure by slippage, different materials are used for each of the paired worm and worm wheel.

6-2 Heat Treatments

What is Quenching ?

Quenching is a treatment performed on steel, applying rapid cooling after heating at high temperature (Approximate 800C). Quenching is applied to adjust the hardness of steel. There are several types of quenching in accordance with cooling conditions; oil quenching, water quenching, and spray quenching. After quenching, tempering must be applied to give toughness back to the steel, that might become brittle. Quenching cannot harden genuine steel, however, quenching can work for steel containing more than 0.35% carbon.

What is Thermal Refining ?

Thermal Refining is a heat treatment applied to adjust hardness / strength / toughness of steel. This treatment involves quenching and tempering. Since machining is applied to products after thermal refining, the hardness should not be raised too high in quenching.

What is Induction Hardening ?

Induction Hardening is a heat treatment performed to harden the surface of the steel containing carbon more than 0.35%, such as S45C or SCM440. For gear products, induction hardening is effective to harden tooth areas including tooth surface and the tip, however, the root may not be hardened in some cases. The precision of gears declines by induction hardening. To encourage the gear accuracy, grinding must be applied.

Fig. 6.1 Quenching Machine

Fig. 6.2 Heating Coil
(Gear and High frequency inductor (coil))

What is Carburizing ?

Carburizing is a heat treatment performed to harden only the surface of low-carbon steel. The surface, in which carbon is present and penetrated the surface, gets especially hardened. Inner material structure (with low-carbon C=0.15%) is also hardened by some level of carburizing, however, it is not as hard as the surface. The precision of carburized gears declines by 1 grade or so, due to deformation (dimensional change) or distortion. To encourage the gear accuracy, grinding is essential.

Fig. 6.3 Carburizing Furnace

Fig. 6.4 Carbon layer by carburizing

What is Nitriding ?

Nitriding is a heat treatment performed to harden the surface by introducing nitrogen into the surface of steel. If the steel alloy includes aluminum, chrome, and molybdenum, it enhances nitriding and hardness can be obtained. A representative nitride steel is SACM645 (Aluminum chromium molybdenum steel).

Related article – Heat treatment of gears (Carburizing, Induction hardening, Nitriding)

When mating gears transmit power, both tooth flanks are in sliding contact and rolling contact. In other words, the pressure which the tooth flank get is same as the pressure when a cylinder slides while rolling on a flat surface. The tooth flank needs to have durability against such pressure.
However, some rigid materials such as ceramic are vulnerable to impacts and break easily. Bending force is frequently applied to the dedendum and impact may be generated when gears start moving. The material of gear needs to be tenacious in order to achieve high durability against external forces and impact.
Therefore, gears need to be hard on the outside and tenacious inside just like a Japanese sword. Heat treating is the method to give such characteristics.

Types of heat treatment for gears are :

Carburizing
Induction hardening
Nitriding

Let’s see the details and characteristics of each type.

1. Carburizing

A typical treatment for gears made of low-carbon steel like S15CK.
After being penetrated by “carbon” just like the name, the metal surface is quenched and tempered to harden. The thickness of the hardened layer is 0.2mm to 2mm depending on gear size.
In the past, people used to cover metal with crushed carbon before sealing and heating. In Europe, products were dipped in molten sodium cyanide-based inorganic salts at high temperature, but the toxicity of sodium cyanide was a problem. Currently, gas carburization in which products are heated in carbon dioxide gas, methane, propane and steam are mainly used. Although the gas carburization is expensive because large special equipment is needed, a large volume of products can be processed effectively.
Carburizing is usually performed after gear is cut and gear teeth are formed. After that, tooth surface is lapped and ground to finish.

2. Induction hardening

A typical treatment for gears made of medium-carbon steel like S45C.
The eddy current flows by electromagnetic induction when the metal gear, wound with wire, is electrified. The induction hardening takes advantage of the property of the eddy current which concentrates at the metal surface and heats metal.
As the common method for induction hardening, the electric current is applied to the wired coil overlaid on the gear tooth formed by cutting.
Induction hardening varies in the precision widely and fewer products can be processed at once compared to other methods like carburizing. On the other hand, this method is suited to process products with gear and axis integrated or large gears because the gear tooth can be hardened partially.

3．Nitriding

Frequently used for gears made of steel which contains chrome or molybdenum. Among the various kinds of steel which are suitable for nitriding, aluminum-containing steel produces particularly a powerful effect and are called “nitride steel”.
In nitration, gears are heated in nitrogen gas to form a layer of iron nitride of 0.1mm to 1mm thickness on the gear surface.
The nitriding can give hardness higher than carburizing and induction hardening, but the layer hardened by nitriding is thinner than other heat treatments. Meanwhile, the temperature necessary for nitriding is relatively low, 500 to 600 degrees Celsius. In contrast, products need to be heated up to 800 degrees Celsius in carburizing and induction hardening. Therefore, nitriding causes no quenching cracks or distortion unlike other heat treatments.
Generally, the nitriding is carried out in the final step of processing gears as it doesn’t cause deformation and gives high hardness.