The Steel Hardening Process

When heated to its critical temperature and then cooled rapidly steel containing sufficient carbon transforms from austenite into martensite. Martensite is a hard brittle material.

This phase transformation involves an increase in volume and the volume change in turn generates stresses in the component. It is these stresses that can create distortion and in some cases, cracking.

The amount of stress depends on the hardening method, material composition, product geometry, hardness level and depth of hardness. These factors are critical in achieving the right outcome.

There are 4 basic methods employed with flame hardening.
Flame hardening uses a burner as shown below, induction hardening uses a coil.

The choice of method depends largely on the material, the geometry of the part and the desired outcome. The same part, say a plain wheel made from 4140, may be spin hardened or progressive-spin hardened. If spin hardened it will have a heavier case and a lower hardness compared to a progressive-spin part.

Material Selection

Carbon is the most important hardening element in steel or cast iron.

There should be at least 0.3% carbon and preferably more than 0.35% to obtain a response to flame of induction hardening. As the carbon content increases, so does the hardness obtained.

There are other elements which affect the hardening process such as manganese, chromium, molybdenum, nickel and silicon, but carbon is by far the most influential.

Below we list some commonly available steels and typical results obtained by flame hardening. The final surface hardness achieved depends on the chemical composition of the steel, the quenchant used (air, water, oil or polymer) and the section thickness of the component. The values quoted below are representative only.

1045 carbon steel (0.45%carbon). Most common steel used for gears, pins, bushes, washers and wear plates. Typical hardness 45-55HRc

4140/709M alloy steel (0.40%carbon). Higher core strength than 1045. Used for the same items but more highly stressed applications. Typical hardness 50-60HRc

4340 alloy steel (0.40%carbon). Nickel content provides a good combination of hardness and toughness. 4340 is used in highly stressed applications. More difficult to heat treat than 1045 or4140 since it is prone to quench cracking. Typical hardness 50-60HRc

EN25 alloy steel (0.30%carbon). High strength alloy steel with high hardenability, toughness and fatigue resistance. Used for gears, shafts and axles of large section. Typical hardness 45-55HRc

EN26 alloy steel (0.40%carbon). EN26 is similar to EN25 but higher levels of hardness and wear resistance. Typical hardness 55-60HRc

XK1340 carbon steel (0.40%carbon). Used for similar applications as 1045. Manganese content provides improved toughness over equivalent plain carbon steel. Typical hardness 45-55HRc

K245 tool steel (0.65% carbon). Shock resisting tool steel with extreme toughness, edge retention and wear resistance. Typical hardness 55-65HRc

Martensitic stainless steel (400 series) and many other carbon steels, alloy steels, tool steels and cast irons may be successfully flame or induction hardened.

Remember to centre pins both ends and leave machining of any details in the heat affected zone until after hardening. These simple actions will save you time and money.

Nitriding Steels are steels that contain the strong nitride-forming elements aluminium, chromium and molybdenum. Nitriding consists of heating the part in an atmosphere containing ammonia at about 500 degrees Celsius. The parts are not quenched which minimises distortion. A thin, very hard case results from the formation of nitrides. 4140 is commonly available steel suitable for nitriding. It is ideal for small intricate parts that require hardening. 

Cast Irons Malleable iron, gray iron and ductile iron (also called SG or nodular iron) may be successfully flame hardened to hardness levels of 45-60 Rockwell C depending on the composition and microstructure of the iron.

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