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Gear material heat treatment - Nitriding

Gear nitriding is one of the common methods for gear surface hardening treatment. Gear nitriding is a chemical heat treatment process that allows active nitrogen atoms to penetrate the surface of gears at a certain temperature. Through nitriding, the surface hardness, wear resistance, anti bonding performance, fatigue strength, and corrosion resistance of gears can be improved. So what gear materials are commonly used for nitriding and what are the quality control requirements?

(1) Definition and Application of Nitriding Material 

Nitriding is a surface hardening process that absorbs nitrogen from a nitrogen-containing medium (usually decomposed ammonia gas) to form a nitride hardening component on the surface of steel. The heat treatment temperature for nitriding is usually between 500~560 ℃, and the nitriding temperature is much lower than the critical temperature of steel, so the gear blank will not undergo phase transformation. And before nitriding, the blank is usually tempered at a temperature higher than the nitriding temperature (540~620 ℃), so that the gear will not undergo significant deformation. Therefore, nitriding is usually carried out after the precision machining of the gear is completed.

Gear steel using nitriding process usually belongs to medium carbon steel, and the steel usually contains a certain amount of aluminum Al, chromium Cr, molybdenum Mo, which can obtain a useful hardening layer during the nitriding process. Elements such as nickel, nickel, and vanadium V are also needed due to their favorable properties for nitride steel. Common nitriding materials include 40Cr, 42CrMo, 31CrMoV9, 35CrMoV, 38CrMoAl, 40CrNiMo, 42CrMo4, 4340, 4140, and so on.

(2) Quality control requirements for nitriding

Requirements for raw materials and pre-processing

Before nitriding, the blank needs to undergo quenching and tempering treatment (quenching+high-temperature tempering) to avoid decarburization. At the same time, the tempering temperature should be higher than the subsequent nitriding temperature to avoid a decrease in the hardness of the central part during the nitriding process.

Surface hardness requirements

The higher the surface hardness of nitriding gears, the more favorable it is for the wear resistance and contact fatigue strength of the gears. According to ISO standards, surface hardness of nitride steel (aluminum containing or high chromium nitride steel) is required to be within the range of 650HV~900HV; For quenched and tempered steel, the surface hardness is required to be no less than 450HV, while actual design usually requires a hardness of 500HV or higher.

Heart hardness requirements

Gears need to undergo quenching and tempering treatment before nitriding to achieve moderate core hardness. The higher the core hardness, the higher the surface hardness obtained by nitriding. Due to the relatively thin nitride layer, the hardness of the core directly affects the support capacity of the nitride layer. Research has shown that when the gear core is increased from 240~260HB to 310~330HB, the contact fatigue strength can be increased by more than 30%. So in actual design, try to ensure that the core hardness is above 300HB as much as possible. In addition, according to ISO standards, the core hardness requires a tensile strength greater than 900MPa, corresponding to a core hardness of approximately 270HB or higher.

Requirements for effective hardening layer depth

The effective hardening layer depth of nitriding gears refers to the distance from the surface to a hardness of 400HV or 40.8HRC. If the core hardness exceeds 380HV, an additional 50HV can be added to the core hardness as the critical hardness value. The required depth of the nitriding layer for gears is usually lower than that for carburized gears, and the depth of the hardening layer mainly affects the contact fatigue strength of the gears. According to relevant literature recommendations, there are the following recommended values for nitride layer depth for different gear module sizes.

Surface organization requirements

For gears that rely mainly on transmission power, the teeth bear high contact stress. The surface structure of nitride gears should be dominated by the γ 'phase and the ε phase should be minimized as much as possible. If there is a white bright layer dominated by the ε phase on the tooth surface, it may become a fatigue crack source due to brittle fracture. For gears that mainly require wear resistance for transmitting motion, it is allowed to use the ε phase as the main component. In addition, according to ISO standards, nitride gears generally require a white bright layer thickness of ≤ 25um, and the ratio of γ '/ε phase nitride should be greater than 8.

Finished product state after nitriding

In some cases where high-precision gears are required, nitride gears can be ground, but it should be avoided to reduce the load-bearing capacity of the tooth surface (ensuring sufficient hardening layer depth), and magnetic particle inspection should be carried out according to ASTM E1444 or EN10228-1 after grinding.