Analysis of Tooth Fracture of Carburized and Quenched Gears

The gear is the most important part in the gearbox.

During operation, the tooth surface bears the contact stress and the tooth root bears the bending stress.

Therefore, the failure forms of gears are mainly divided into pitting corrosion and peeling caused by tooth surface fatigue and tooth fracture caused by tooth root fatigue.

As the most commonly used and mature process for high-speed and heavy-duty gears, carburizing and quenching can simultaneously improve the tooth surface contact fatigue strength and tooth root bending fatigue strength.

However, due to the complex carburizing and quenching process, various types of defects will be produced during the heat treatment process, which will lead to the early failure of gears.

During the fatigue life test of a transmission assembly newly developed by our company, a gear broke.

The design requires that the gear material is 8620H.

After carburizing, it is quenched and tempered at low temperature.

The effective hardened layer depth is 0.8~1.3mm, the surface hardness is 58~64HRC, and the core hardness is 30~45HRC.

In order to determine the cause of tooth fracture, the fracture morphology, material and heat treatment quality were tested and analyzed.

1. Physical and chemical testing and analysis

(1) Macro observation

The overall appearance of the failed gear is shown in Fig. 1.

Some gears are broken from the root, and the number of broken teeth exceeds half of the total number of teeth.

The macro morphology of the fracture is shown in Fig. 2.

According to the fracture morphology, most of the fractures show obvious fatigue fracture characteristics.

The fracture source is located at the root of the tooth.

The fatigue expansion area is smooth and radiates outward, accounting for 1/3~1/2 of the total root area.

The fracture surface of the transient fracture area is rough and dark gray.

Except for the fatigue fracture, some gear fractures have no fatigue fracture characteristics, which are one-time overload fracture.

Analysis of Tooth Fracture of Carburized and Quenched Gears 1

Fig. 1 Overall View of Broken Teeth

Analysis of Tooth Fracture of Carburized and Quenched Gears 2

Fig. 2 Fracture morphology

(2) Fractography

After sampling, the fracture morphology was observed under scanning electron microscope.

The appearance of the crack source is shown in Fig. 3.

The crack source is located at the root of the tooth. It can be observed from the figure that the crack source of the fracture does not converge to a point, but is linear.

The surface of the crack source is smooth after repeated friction and extrusion.

It is further observed that there are black abnormal structures at the crack source (see Fig. 4);

When magnified to 1000 times, observe the microscopic morphology of the fatigue growth zone as shown in Fig. 5.

At high magnification, fatigue striations and radial prisms can be observed;

The appearance of the transient fracture zone is dimple+quasi cleavage fracture (see Fig. 6), indicating that the toughness of the gear center is good.

Analysis of Tooth Fracture of Carburized and Quenched Gears 3

Fig. 3 Crack Source

Analysis of Tooth Fracture of Carburized and Quenched Gears 4

Fig. 4 Black Structure of Crack Source

Analysis of Tooth Fracture of Carburized and Quenched Gears 5

Fig. 5 Fatigue Propagation Fracture

Analysis of Tooth Fracture of Carburized and Quenched Gears 6

Fig. 6 Morphology of dimple+quasi cleavage in transient fracture zone

(3) Gear material inspection

Take samples from the failed gear for chemical composition analysis, and the results are shown in Table 1.

The chemical composition of the gear meets the technical requirements of SAEJ1268 standard for 8620H steel.

Table 1 Test Results of Chemical Composition (Mass Fraction) (%)

ElementCSPSiMnCrNiMoCu
Standard value0.17~0.23≤0.040≤0.0300.15~-0.350.60~0.950.35~0.650.35~0.750.15~0.25≤0.35
Detection value0.220.0170.0100.280.870.580.450.180.086

(4) Quality inspection of gear heat treatment

Take the unbroken gear near the broken gear to test the heat treatment quality.

The surface hardness is 61HRC, and the core hardness is 45HRC;

The surface structure is martensite and retained austenite, the content of retained austenite is about 15%, and the center is lath martensite and a small amount of bainite;

The effective hardened layer depth at 1/2 tooth height is 1.01mm.

The gear has been carburized and quenched, and all the heat treatment indexes meet the design requirements of the drawing.

Further cut the sample along the middle of the tooth width with a precision cutting machine for sample preparation, and then observe the metallographic structure of the tooth root on the cutting surface under a metallographic microscope.

Without corrosion (see Fig. 7), it can be seen that there are serious black tissues at the root of the tooth, which are distributed in a network, with an average depth of about 20 μm. Individual black tissue depth reaches 30 μ m.

A straight crack forms from the black tissue of the tooth root and extends inwards along the direction perpendicular to the tooth root;

The observation after corrosion (see Fig. 8) shows that the normal carburized and quenched structures are on both sides of the crack;

The metallographic observation of the root of the two end faces of the sample shows that no cracks are found.

According to the above inspection, it is believed that the observed cracks are generated in the process of use, indicating that the tested teeth have generated fatigue cracks and expanded, and the test has stopped before the fracture occurs.

If the test continues, it is expected that the fracture will occur;

According to the metallographic analysis, the crack is closely related to the black tissue at the root of the tooth.

Analysis of Tooth Fracture of Carburized and Quenched Gears 7

Fig. 7 Black tissue and crack at tooth root (500 ×) No corrosion

Analysis of Tooth Fracture of Carburized and Quenched Gears 8

Fig. 8 Structure on both sides of crack (50 ×) 4% nitric acid alcohol solution

2. Analysis and discussion

Most of the broken teeth of the failed gear are fatigue fracture, and the cracks start at the root of the middle part of the tooth width.

From the metallographic observation and scanning electron microscope observation results of the tooth root, it can be seen that the black tissue becomes the source of crack initiation during the use of the gear.

With the increase of the number of operations, the crack source expands, which eventually leads to the fracture failure of the gear.

After carburizing, the surface microstructure of alloy steel often appears dotted, reticulated or banded black microstructure distributed along the grain boundary.

The reason for this kind of structure is that the oxygen in the carburizing medium diffuses into the steel, forming the oxides of chromium, manganese, titanium, silicon and other elements on the grain boundary, which makes the alloy elements at the grain boundary depleted, resulting in a decrease in local hardenability, so that black austenitic decomposition products (such as troostite) appear.

Research at home and abroad shows that the existence of black tissue significantly reduces the surface hardness, bending fatigue strength and contact fatigue strength of parts, and seriously affects the service life of parts.

Therefore, most well-known vehicle manufacturers at home and abroad have clearly specified the depth of black tissue.

For example, German Benz and BMW require that the depth of black tissue must be less than 3μm.

FAW Group also plans to reduce the depth of black tissue from less than 20μm to less than 3μm.

3. Suggestions for improvement

It can be seen from the above inspection and analysis that the depth of black structure in the surface metallographic structure of carburized and quenched parts must be strictly controlled.

According to its formation principle, the control of black tissue mainly starts from the following two aspects:

① Improve the purity of carburizing gas and reduce the content of oxygen.

For example, strictly control the purity and water content of carburizing agents such as methanol and acetone, and strictly control the amount of air.

② More intense quenching and cooling methods shall be adopted, such as using quenching medium with stronger quenching and cooling performance or adopting faster quenching, cooling and stirring.

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