Non metallic inclusion exceeding the standard leads to shaft fracture

20CrNiMo steel is a kind of low-carbon alloy steel, which is often used to manufacture gears in engines and transmission systems of small and medium-sized automobiles and tractors. The linkage shaft of cycloid hydraulic motor was broken during the motor life test. During the life test, the bearing torque of this type of linkage shaft is 600N · m, the rotating speed is 180r / min, and the life requirement is more than 60H. This life test has been tested for three times, and the fracture occurs in 6 ~ 8h. Therefore, the failure formation caused by fatigue wear of tooth surface is compared with that caused by previous fatigue wear. This paper analyzes the cause of one broken linkage shaft. The part is made of 20CrNiMo steel bar, and its production process is as follows: blanking → turning → hobbing → heat treatment → shot blasting → cleaning → final inspection.

Inspection process and results

(1) Macroscopic examination
The macroscopic fracture morphology of the linkage shaft is shown in Figure 1. Under the action of repeated torsional stress, there is a small amount of plastic deformation in the axial part of the linkage shaft, and the torsional fracture is 45 ° Combined with the failure time, it is preliminarily determined as torsional fatigue fracture.

Fig. 1 macroscopic fracture morphology of linkage shaft
(2) Analysis of chemical composition
The chemical composition of the linkage axis was analyzed, and the results are shown in Table 1. The chemical composition meets the requirements.
Table 1 test results of chemical composition (mass fraction) (%)

Item	C	Si	Mn	P	S	Cr	Ni	Mo	Cu
Spare parts	0.19	0.2	0.85	0.015	0.018	0.54	0.46	0.22	0.04
Standard	0.17~0.23	0.17~0.37	0.60~0.95	≤0.030	≤0.030	0.40~0.70	0.35~0.75	0.20~0.30	≤0.30

(3) Hardness test
The technical requirements of the parts: surface hardness 57 ~ 62Hrc, core hardness 30 ~ 45hrc, effective hardening depth of 0.6 ~ 0.9mm. The surface hardness of the linkage shaft is 59 ~ 60HRC, the core hardness is 35 ~ 37hrc, and the effective hardening depth is 0.79mm, which meets the technical requirements.
(4) Metallographic examination
The metallographic structure requirements of the part heat treatment: surface tempered martensite ≤ 5 grade, retained austenite ≤ 5 grade, carbide ≤ 3 grade. As shown in Figure 2, the surface tempered martensite of the linkage shaft is grade 5, the retained austenite is grade 5, and the carbide is grade 1, which meets the technical requirements. The test results of non-metallic inclusions at the fracture (see Fig. 3): a1.0, b3.0, c0.5, d3.0, ds0.5, a large number of class B and class D non-metallic inclusions were found near the fracture.

Fig. 2 metallographic structure

Fig. 3 detection of non metallic inclusions

Analysis and discussion

In conclusion, the chemical composition, hardness and metallographic structure of the 20CrNiMo steel linkage shaft meet the technical requirements, and the level of non-metallic inclusions is too high. The grade of tempered martensite and retained austenite is higher than the upper limit. The results show that a certain amount of retained austenite can relax the stress in the steel, absorb the impact energy, buffer the impact force of transformation martensite, reduce microcracks and enhance the fatigue resistance.
In addition, the influence of non-metallic inclusions on fatigue properties depends on the type, quantity, size, shape and distribution of inclusions; On the other hand, restricted by the structure and properties of the steel matrix, the large brittle inclusions and spherical non deformable inclusions with weak adhesion to the matrix are the most harmful. A large number of B-type and D-type non-metallic inclusions are found near the fracture surface of the shaft, which are brittle inclusions in nature. Because the brittle inclusions can not transfer the stress in the steel matrix, the fatigue crack will be induced, and the brittle inclusions will crack due to fragmentation under the action of stress, which is easier to produce fatigue crack. Under the action of repeated torsion stress, multiple fatigue sources and microcracks are produced near these brittle inclusions, and the cracks are 45 degrees along the axis ° Finally, a part of serrated section is formed, which leads to the abnormal fatigue fracture of the linkage shaft during the test.

Conclusions and suggestions

The reason for the early fracture of the 20CrNiMo steel linkage shaft in the life test is due to the high level of B and D brittle non-metallic inclusions in the steel. Therefore, in the future production process, we should strengthen the detection of non-metallic inclusions in raw materials and strictly implement the acceptance standard. Through the above measures, the problem of abnormal fracture of linkage shaft in life test can be solved.

Source: China Metal Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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