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Heat treatment of gear materials - carburizing and quenching

There are usually several methods for surface hardening treatment of gear materials,

Including carburizing quenching, nitriding, induction hardening, and flame hardening, among which carburizing quenching is the most widely used method for gear surface hardening treatment. So what gear materials are commonly used for carburizing and quenching, and what are the design quality control requirements?

(1) Definition and application materials of carburizing and quenching

Carburizing is the process of heating gears in a carburizing medium above the critical temperature, allowing carbon to be absorbed and penetrate into the surface of the gear teeth. After carburizing, gears can be quenched immediately or slowly cooled before quenching. Due to the increased carbon concentration on the surface after carburizing, high surface hardness can be obtained after quenching.

The carbon content of carburized quenched steel is usually in the range of 0.10% to 0.25%, which belongs to low carbon steel. The commonly used materials include 20Cr, 20CrMo, 20CrMnTi, 20CrMnMo, 20Cr2Ni4, 20MnCr5, 17CrNiMo6, 18CrNiMo7-6, 9310, 8620, and so on.

(2) Quality control requirements for carburizing and quenching

Control of surface carbon content

The control of surface carbon concentration is the key to carburizing process. In order to achieve optimal hardness and fatigue strength, the carbon content should be within the range of 0.8~0.9%, and the maximum should not exceed 1%. By adjusting the carbon concentration of the carburizing atmosphere, carbon content control can be achieved.

Core hardness requirements

The core hardness mainly affects the bending fatigue strength of gears. If the core hardness is too high, it will increase the brittleness of the gear teeth and reduce the bending fatigue strength; The hardness of the heart is too low to provide sufficient support for the high load-bearing capacity of the tooth surface, and the surface is prone to peeling off. Studies have shown that the optimal range of heart hardness is between HRC35 and 42. The minimum value of heart hardness should not be less than HRC30. The measurement site for heart hardness is generally taken at the intersection of the tooth centerline and the tooth root circle, or according to ISO standards, in the normal direction of the 30 ° tangent to the tooth root at the mid tooth width, with a depth of 5 times the effective hardening layer depth, but the minimum depth shall not be less than 1 modulus.

Surface hardness requirements

Surface hardness mainly affects contact fatigue strength, and the range of surface hardness is generally between HRC58 and 64. Different applications may have different requirements, which can be designed according to the following table.

Requirements for effective hardening layer depth

The effective hardening layer depth refers to the distance from the surface layer to a hardness of HRC52 or HV550. The depth of the infiltration layer required for carburized gears is mainly related to the modulus. The larger the teeth, the thicker the infiltration layer is needed to withstand the load applied to the teeth. However, it should be noted that excessive infiltration layer can make the teeth brittle, and there is a tendency for brittle cracking at the tooth tip, which is called tooth center separation; Thin infiltration layer can reduce the strength and pitting resistance of teeth, which may lead to tooth surface crushing. The following table provides the effective hardening layer depth corresponding to different moduli, which can be used as a design reference. For more detailed calculation, please refer to the article Determination and Design Considerations of Gear Carburizing Layer Depth in this official account.

Surface organization requirements

The microstructure of the carburized layer that affects the strength performance of gears mainly includes carbides, residual austenite, and martensite.

1) Carbides are hard and brittle phases, and their morphology, quantity, and distribution have a significant impact on the strength performance of gears. Especially coarse block shaped, pointed, and network like carbides have a significant adverse effect on the fatigue strength of gears. The automotive industry generally requires carbides to be controlled below level 4 (the smaller the number, the better). According to ISO standards, the MQ level of heat treatment quality requires that the carbide form be discontinuous carbide, and the length of all carbides should not exceed 0.02mm.

2) Residual austenite is an important constituent phase in carburizing and quenching structures, generally ranging from 5% to 30%. Excessive residual austenite content can cause a decrease in surface residual compressive stress, thereby reducing the fatigue strength and wear resistance of gears. The automotive industry generally requires a residual austenite content of no more than level 5. ISO standards stipulate that the MQ level of heat treatment quality requires the residual austenite content to be controlled below 30%.

3) Martensitic structure is the basic structure of the carburized layer. The control of martensitic structure mainly depends on the needle like size of martensite, and generally requires fine needle like martensite to be optimal. The automotive industry generally requires the martensite level to be within level 5, and the deepest non martensite structure on the surface should not exceed 0.02mm. The ISO standard stipulates that the MQ level of heat treatment quality requires the non martensitic structure of the surface layer to be within 5%, rather than the depth of the non martensitic structure, and there are different requirements for different infiltration depths.