Knowledge

What kinds of steel are used for gear nitriding? And some microstructure characteristics of commonly used steels. Understanding the microstructure characteristics can help to bring out the good performance of gears in use, and at the same time optimize the gear manufacturing process, which is conducive to improving the service life of gears.

 

01

The core purpose of nitriding

 

First, we need to understand why nitriding treatment is carried out? The core purpose of nitriding treatment is to achieve a performance match of "hard outside and tough inside" by forming a nitride layer on the surface. Therefore, the steel used for nitriding needs to meet the following key requirements: 1) Nitriding sensitivity: The steel should contain sufficient nitride-forming elements (such as Al, Cr, Mo, V, etc.), which can combine with nitrogen to form stable alloy nitrides (such as AlN, CrN, MoN, etc.), significantly improving the hardness and wear resistance of the nitride layer. 2) Core mechanical properties: After quenching and tempering treatment, the core should have appropriate strength and toughness to withstand loads and prevent fractures. Generally, the tensile strength is required to be around 1000 MPa, the hardness around HRC 35, and the impact toughness ≥ 40 J/cm². 3) Microstructure uniformity: There should be no severe segregation, non-metallic inclusions or porosity defects in the steel, otherwise it will lead to uneven thickness of the nitride layer, hardness fluctuations, and even cracks. 4) Machinability: It should have good machinability and heat treatment processability to meet the processing requirements of gears with complex shapes.

 

02

Steel for gear nitriding

 

From the previous section, we know the core purpose of nitriding. To form these nitrides on the surface of the steel, the steel must contain these chemical elements. Moreover, to maintain the toughness of the core, we usually need to use medium carbon alloy steel for quenching and tempering (quenching + high-temperature tempering) before nitriding. The nitriding temperature should be lower than the tempering temperature after quenching to reduce deformation.

 

 

03

Commonly used nitrided steels and their microstructure characteristics

 

1) 38CrMoAlA is the most widely used nitrided steel.

The main chemical composition elements (mass fraction) are:

0.35% - 0.42% C, 0.20% - 0.45% Si, 0.30% - 0.60% Mn, 1.35% - 1.65% Cr, 0.15% - 0.25% Mo, 0.70% - 1.10% Al.

Microstructure after quenching and tempering: After oil quenching at 850 - 880°C and high-temperature tempering at 600 - 650°C, the microstructure is uniform tempered sorbite, and the grain size is usually 6 - 8 grades. The Al element exists in a solid solution form, providing a material basis for subsequent nitriding reactions.

Microstructure of the nitrided layer: After nitriding treatment (500 - 560°C, ammonia atmosphere), a three-layer structure is formed on the surface. After ion nitriding, the compound layer is mainly γ' phase, containing a small amount of ε phase and alloy nitrides, with simple and sharp diffraction lines and fewer phases.

The hardness of the ε phase can reach 900-1100HV; the γ' phase, in needle-like or block form, has a hardness of 800-900HV; the transition layer is a diffusion layer composed of alloy nitrides (such as AlN) and pearlite, with a gentle hardness gradient. The core still retains tempered sorbite structure, ensuring good toughness.

 

2) 42CrMo steel is often used for nitriding treatment of heavy-duty gears due to its high hardenability and strength. Its chemical composition (mass fraction) is: 0.38% - 0.45% C, 0.20% - 0.40% Si, 0.50% - 0.80% Mn, 0.90% - 1.20% Cr, and 0.15% - 0.25% Mo.

Tempered microstructure: After quenching at 840 - 860℃ and tempering at 580 - 620℃, the microstructure is fine tempered sorbite with finer grains (7 - 9 grades), and it has a higher dislocation density than 38CrMoAlA. The hardness of the core is controlled at 33 - 36 HRC.

Nitrided layer microstructure: After nitriding, the surface is mainly composed of γ' phase, with a relatively thin ε phase. Due to the presence of Cr and Mo elements, a large amount of alloy nitrides such as CrN and MoN are formed in the diffusion layer, which are dispersedly distributed, making the hardness of the nitrided layer reach 850 - 1000 HV. Since it does not contain Al element, the depth of ion nitriding is slightly shallower, usually about 0.1 mm.

 

 

3) Properties of Nitrided Layer

Hardness and Wear Resistance: The type and distribution of alloy nitrides are crucial. The high dispersion of AlN in 38CrMoAlA results in the highest hardness of the nitrided layer; CrN and MoN in 42CrMo can enhance wear resistance.

Fatigue Performance: The uniformity of the quenched and tempered microstructure is of vital importance. Coarse grains or carbide segregation can cause stress concentration in the nitrided layer, reducing the bending fatigue strength. Fine-grained tempered sorbite can increase the fatigue life of gears by 30% to 50%.

Anti-seizing Performance: Although the ε phase has high hardness, it is brittle. Excessive ε phase can lead to the peeling of the nitrided layer. The γ' phase-dominated microstructure has better toughness and anti-seizing performance. Therefore, for heavy-duty gears, the thickness of the ε phase is usually controlled to be ≤ 5 μm.

 

04

Summary

In practical applications, the selection of steel for gear nitriding should be guided by the operational requirements. Microstructure analysis is a key method for determining the material's suitability and the rationality of the process - a uniform tempered sorbite matrix is a prerequisite for ensuring the core performance, while the type, distribution and phase composition of the nitrides in the nitrided layer directly determine the surface strengthening effect. That's all for today. Welcome friends to exchange in the comment section and give a thumbs up to support!

 

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