Bearing Parts Made of High Carbon Chromium Bearing Steel: Analysis of Quenching Structure Defects

After martensite quenching and tempering, the structure of high carbon chromium steel bearing parts shall be cryptocrystalline, fine crystal or small acicular martensite, evenly distributed fine residual carbide and a small amount of residual austenite.

Except for micro bearings, a small amount of acicular or massive troostite is allowed (as shown in Fig. 1).

The microstructure after quenching and tempering shall be in accordance with the provisions of clause 3.2.2 microstructure in JB / T1255-2014 technical conditions for heat treatment of high carbon chromium bearing steel parts of rolling bearings.

The high carbon chromium bearing steel with this structure has good hardness, strength, wear resistance and fatigue resistance.

After tempering, the bearing steel can also obtain some good comprehensive properties such as elasticity, toughness and dimensional stability.

Bearing Parts Made of High Carbon Chromium Bearing Steel: Analysis of Quenching Structure Defects 1
Fig. 1 microstructure of martensite quenching and tempering (500 ×)

During the heat treatment of high carbon chromium steel bearing parts, due to the defects of bearing steel materials, heat treatment process, processing equipment and human factors, overheating metallographic microstructure (coarse needle martensite), underheating metallographic microstructure (troostite exceeds the standard), coarse grain carbides and serious network carbides and other microstructure defects are caused.

Some of these metallographic microstructure defects directly cause product scrapping, such as overheated metallographic microstructure (coarse needle martensite).

Although some defects will not cause the product to be scrapped, they will affect the service life.

For example, the under-heated metallographic microstructure (troostite exceeds the standard) will affect the service life of the bearing, resulting in early ring breakage of the bearing, which seriously affects the quality of the bearing product.

1. Quenching superheated metallographic microstructure (coarse needle martensite)

Fig. 2 and Fig. 3 are the metallographic microstructure of quenching superheat, and the structural characteristics are obvious coarse acicular martensite.

This structure will reduce the toughness and impact resistance of the bearing, shorten the service life of the bearing, and even cause quenching cracks in case of severe overheating.

Bearing Parts Made of High Carbon Chromium Bearing Steel: Analysis of Quenching Structure Defects 2
Fig. 2
Bearing Parts Made of High Carbon Chromium Bearing Steel: Analysis of Quenching Structure Defects 3
Fig. 3

(1) Cause

It is mainly caused by too high quenching heating temperature or too long holding time at the upper limit of quenching heating temperature, which causes too much dissolution of secondary carbides;

The austenite grain also has the opportunity to grow, so the effect of hindering the growth of martensite is weakened, resulting in the possibility that martensite may grow larger.

Under the 500 times (or 1000 times) metallographic microscope, the coarse needle martensite, that is, the overheated metallographic microstructure, can be seen.

It may also be that the carbides in the raw material are seriously banded or the size distribution of the carbides in the annealed structure is uneven, and there is fine flake pearlite in the annealed structure.

Even under the normal quenching process, it is easy to form coarse acicular martensite at the places where the carbides are distributed sparsely or the particles are fine, and there are few obstacles to the growth of martensite.

There is no or only a small amount of carbides at the surface decarburization, which has a small effect on hindering the growth of martensite.

When the cooling conditions are good, martensite also has the opportunity to grow and form coarse acicular martensite.

(2) Measures

Reasonably select quenching heating temperature and heating time.

Select according to the material standard and strictly control the carbide banding.

To improve the annealing quality, pay close attention to the furnace temperature in the production process, and take effective measures in time in case of power failure and equipment failure.

2. Metallographic microstructure of quenching under heating (troostite exceeds the standard)

Troostite is the structure produced by undercooling or poor cooling.

It is the product of pearlite transformation of austenite during cooling.

It is an extremely fine pearlite structure.

The troostite in bearing steel can be divided into massive troostite (see Fig. 4), acicular troostite (see Fig. 5), acicular and massive mixed structure (see Fig. 6) and banded troostite (see Fig. 7) according to the metallographic morphology.

Bearing Parts Made of High Carbon Chromium Bearing Steel: Analysis of Quenching Structure Defects 4
Fig. 4 the metallographic microstructure has large lump troostite
Bearing Parts Made of High Carbon Chromium Bearing Steel: Analysis of Quenching Structure Defects 5
Fig. 5 metallographic microstructure with obvious acicular troostite
Bearing Parts Made of High Carbon Chromium Bearing Steel: Analysis of Quenching Structure Defects 6
Fig. 6 metallographic microstructure: acicular troostite and massive troostite mixed
Bearing Parts Made of High Carbon Chromium Bearing Steel: Analysis of Quenching Structure Defects 7
Fig. 7 metallographic microstructure: banded troostite

Troostite structure exists in quenched bearing steel, which will cause the decrease of hardness and strength of the steel, which is also unfavorable to wear resistance and fatigue resistance, and greatly reduces the rust resistance of bearing steel.

The hardness of the part is qualified, but a small amount of acicular and massive troostite meets the metallographic microstructure requirements specified in JB / T1255-2014 technical conditions for heat treatment of high carbon chromium bearing steel parts for rolling bearings.

However, the massive and reticular troostite exceeds the standard provisions, which is an unqualified structure, which may lead to low hardness of the part and easy to find soft spots after pickling.

(1) Cause

Massive troostite is produced due to insufficient heating (low heating temperature or short holding time).

That is to say, insufficient heating or holding time results in low and uneven austenitic alloying concentration, low hardenability in local areas, and pearlite transformation in this area during normal cooling;

Acicular troostite is caused by poor cooling (insufficient cooling rate of quenching medium).

The cooling rate is not enough, and even under normal heating, some areas in the steel do not reach the critical cooling rate;

The zonal troostite is caused by the zonal carbides in the raw material of bearing steel and distributed in the strip shape in the carbon poor area.

(2) Measures

If troostite appears in production, its metallographic microstructure shall be inspected, the causes shall be analyzed and corresponding measures shall be taken.

If the troostite is massive, the quenching heating temperature should be properly increased and the holding time extended;

If it is acicular troostite, the cooling rate should be increased.

If the heating temperature, heat preservation and cooling are normal and troostite occurs, check the raw material problems, temperature control problems, equipment failures, etc., find out the causes in time and take measures.

3. Severe network carbide

Fig. 8 shows the severe network carbide after deep corrosion with 4% nitric acid alcohol solution.

This structural defect is not formed in the quenching process, but is formed by improper rolling, forging or annealing, which is only found after inspection after quenching.

Bearing Parts Made of High Carbon Chromium Bearing Steel: Analysis of Quenching Structure Defects 8
Fig. 8 network carbide

(1) Cause

Reticulated carbides will increase the inhomogeneity of chemical composition in steel, which is easy to cause great structural stress during heat treatment and quenching, resulting in deformation and cracking of parts.

The existence of reticulated carbides greatly weakens the relationship between the matrix grains, and reduces the mechanical properties, especially the impact properties of steel.

With the increase of reticulated carbides, the impact properties continue to decrease.

The network carbide also has a significant effect on the bending strength and tensile strength.

In addition, the contact fatigue strength decreases with the increase of the level of reticulated carbides in the steel, and the contact fatigue strength of the longitudinal specimen with coarse reticulated carbides decreases by about 30%.

The service life of parts will be reduced by about 1 / 3 for each grade increase of reticulated carbide.

The severe network carbide can not be eliminated in the subsequent spheroidizing annealing process, and the structure of the network carbide can be eliminated or improved only by normalizing process.

If the network carbide is light, part of the network can be broken and spheroidized during spheroidizing annealing.

However, the carbide particles are larger and the carbide particles in the spheroidized annealed structure are not uniform.

(2) Measures

The reticulated carbides in the raw materials of bearing steel shall be strictly controlled so that the level of reticulated carbides shall not exceed the level specified in GB / T18254-2016 high carbon chromium bearing steel.

Control the cooling rate during the forging process of bearing forgings to avoid the generation of reticulated carbides due to the slow cooling rate.

If necessary, use air cooling to accelerate the cooling rate of forgings and prevent the generation of reticulated carbides.

4. Conclusion

The causes of the main defects in the microstructure of high-carbon chromium bearing steel parts after quenching have been analyzed in depth, and the preventive and corrective measures have been put forward, hoping to play a role in improving the quenching quality of high-carbon chromium bearing steel parts.

In the production practice, the situation is very complex, and it is necessary to make a specific analysis of different situations.

Only in this way can the quenching quality of high carbon chromium bearing steel parts be guaranteed, and the internal quality of bearing products be guaranteed reliably.

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