Why the Bearing Ring Has White Spots: Cause Analysis

The bearing ring of a key product is made of 20CrNi2MoA carburized steel.

Its processing process is as follows: incoming inspection of raw materials → forging → turning → carburizing and secondary quenching and tempering → primary grinding → acid pickling and spot check → fine grinding → nondestructive testing → phosphating treatment.

As we all know, if there are soft spots, burns, carbon deficiency and other defects on the surface of bearing parts, they can be displayed through the pickling inspection process.

Under normal conditions, the surface of carburized steel bearing parts shall be uniformly gray after the pickling process;

However, in the pickling inspection of this batch of products, it was found that the outer diameter surface of the inner and outer rings of the bearing mostly showed white spots, and this phenomenon also existed on the raceway surface of the rings.

Therefore, the macro and micro inspection and analysis of its white spot samples were carried out to find out the causes of this phenomenon, and the preventive measures for such defects were proposed.

Why the Bearing Ring Has White Spots: Cause Analysis 1

1. Physical and chemical inspection

(1) Macroscopic examination

After the initial grinding of the ring, the acid cleaning inspection is mainly to check whether there is any burn defect in the grinding process of the product.

However, this inspection found a large proportion of white spot defects, which are completely different from the conventional grinding burn defects;

From the point of view of the generated parts, the pickling inspection of the outer ring and inner ring of the same batch all showed this phenomenon, and the outer ring was mostly distributed on the outer diameter surface and raceway surface, while the inner ring was mostly distributed on the inner diameter surface and raceway surface.

The appearance inspection shows that the white spots are located on the surface of the ferrule and are distributed in large pieces or blocks.

The morphology is shown in Fig. 1. Fig. 1a shows the white spot morphology on the outer diameter of the outer ring and Fig. 1b shows the white spot morphology on the raceway surface of the inner ring.

Why the Bearing Ring Has White Spots: Cause Analysis 2
(a) Outer ring
Why the Bearing Ring Has White Spots: Cause Analysis 3
(b) Inner ring

Fig. 1 Appearance of ferrule white spot

(2) Component analysis

The samples were taken from the normal parts of the ferrule.

The chemical composition analysis results of the raw materials are shown in Table 1.

It can be seen that their main chemical composition meets the technical requirements for the composition of G20CrNi2MoA electroslag remelting bearing steel in TB/T 2235-2016 Rolling Bearings for Railway Vehicles.

The chemical composition of white area and normal area of outer ring was analyzed by electron probe. The results are shown in Table 2.

 It is obvious that the carbon content in white area is lower than the standard value specified in JB/T8881-2011 Technical Conditions for Carburizing Heat Treatment of Rolling Bearing Parts.

Table 1 Main Chemical Composition of Bearing Race (Mass fraction) (%)

Element

Outer ring

Inner ring

Standard value

Measured value

Measured value

C

0.2

0.21

0.17~0.23

Si

0.27

0.3

0.15~-0.40

Mn

0.49

0.49

0.40~0.70

Cr

0.52

0.5

0.40-0.60

Ni

1.6

1.7

1.60~2.00

Mo

0.23

0.22

0.20-0.30

P

0.012

0.011

≤0.020

S

0.002

0.002

≤0.020

Cu

0.09

0.09

≤0.20

Al

0.03

0.03

0.01-~0.05

Table 2 Main Chemical Composition of Outer Circle White Spot Area and Normal Area(Mass fraction) (%)

Element

Measured value of white spot area

Measured value in normal area

Standard value

C (surface)

0.42

0.82

0.801.06

C (heart)

0.2

0.21

0.17~0.23

Si

0.27

0.3

0.15~0.40

Mn

0.49

0.49

0.40~0.70

Cr

0.52

0.5

0.40~0.60

Ni

1.6

1.7

1.60~2.00

Mo

0.23

0.22

0.20~0.30

P

0.012

0.011

≤0.020

S

0.002

0.002

≤0.020

Cu

0.09

0.09

≤0.20

Al

0.03

0.03

0.01~0.05

(3) Non metallic inclusion inspection

Take the metallographic sample of the defective part, observe the non-metallic inclusions in its longitudinal profile, and evaluate the content of non-metallic inclusions according to the actual inspection method A in GB/T10561-2005/ISO4967:1998 (E).

The results are shown in Table 3. It can be seen that the non-metallic inclusion level of the bearing parts meets the technical requirements of TB/T2235-2016 Rolling Bearings for Railway Vehicles.

Table 3 Levels of non-metallic inclusions

Project

Category A

Category B

Category C

Class D

fine

fine

fine

crude

fine

crude

fine

crude

Outer ring

Measured value

1.0

1.0

1.0

0.5

0

0

0.5

0.5

Inner ring

Measured value

1.0

1.0

1.0

0.5

0

0

0.5

0.5

Standard value

2.0

1.5

2.0

1.0

0.5

0.5

1.0

1.0

(4) Profile of hardened layer

Take the metallographic sample of the defective ferrule along the height direction of the white spot ferrule;

It is required to observe both the white spot area and the metallographic structure of the normal area.

After the longitudinal section is corroded by 4% nitric acid alcohol, the profile of the hardened layer on the longitudinal section of the bearing inner and outer rings is shown in Fig. 2.

Fig. 2a shows the profile distribution of the hardened layer on the longitudinal section of the outer ring with white spot defect on the outer diameter surface, and Fig. 2b shows the profile distribution of the hardened layer on the longitudinal section of the inner ring with white spot defect on the raceway surface;

It can be seen that the hardening layer profile corresponding to the white spot area is obviously different from the normal area, and the depth distribution of the hardening layer profile in the white spot area is lower than that in the normal area, whether it is the outer ring or the inner ring.

Why the Bearing Ring Has White Spots: Cause Analysis 4
(a) Outer ring
Why the Bearing Ring Has White Spots: Cause Analysis 5
(b) Inner ring

Fig. 2 Profile of Hardened Layer

(5) Metallographic examination

Observe the microstructure of the infiltration layer on the outer ring and inner ring sections under the microscope, as shown in Fig. 3.

Figures 3a and 3b represent the metallographic morphology of the white spot area and the normal area on the outer diameter section of the outer ring at 500 times, respectively, and no differential structure is observed.

The surface structure is composed of cryptocrystalline or fine acicular martensite+uniformly distributed fine-grained carbide+a small amount of retained austenite.

Evaluated according to the fifth level diagram of JB/T8881-2011 Technical Conditions for Carburizing Heat Treatment of Rolling Bearing Parts, the surface structure of inner and outer rings is Grade 2, acceptable;

The surface structure has neither coarse carbide nor network carbide.

Figures 3c and 3d represent the metallographic morphology of the white spot area and the normal area of the inner race raceway section at 500 times, respectively, and no obvious differential structure is seen.

It is evaluated as per Technical Conditions for Carburizing Heat Treatment of Rolling Bearing Parts (JB/T8881-2011) and is qualified.

Why the Bearing Ring Has White Spots: Cause Analysis 6
(a) White spot area of outer circle (b) Normal area of outer circle
Why the Bearing Ring Has White Spots: Cause Analysis 7

(c) White spot area of inner circle           (d) Normal area of inner circle

Fig. 3 Structural Morphology of Inner and Outer Race Raceway Section

(6) Surface hardness and depth of hardened layer

The Vickers hardness tester is used to test the surface hardness of the longitudinal section of the bearing inner and outer rings and the gradient distribution of the hardened layer, and the white spot area and the normal color area of the bearing inner and outer rings are tested respectively.

The comparison results of the surface hardness and the depth of the hardened layer in the white spot area and the normal area are shown in Table 4.

The comparison of the gradient curves of the hardened layer in the white spot area and the normal area of the outer ring is shown in Fig. 4.

Fig. 4a is the hardness gradient curve of the hardened layer in the white spot area of the outer ring, and Fig. 4b shows the hardness gradient curve of the hardened layer in the normal zone of the outer ring.

Table 4 Comparison of Surface Hardness and Hardened Layer Depth

Project

Surface hardness HRC (0.1mm from the raceway surface)

Hardening layer depth/mm (550HV position distance)

Uniformity of carburized layer of the same part/mm

Leukoplakia

Normal area

Leukoplakia

Normal area

Outer ring

57

60

0.95

1.75

±0.40

Inner ring

56

60

1.37

2.10

±0.37

Standard value of finished product

59~63

1.5~2.3

Limit deviation value ≤± 0.15

Standard value after secondary quenching and tempering

59.5~63

2.0~2.5

Why the Bearing Ring Has White Spots: Cause Analysis 8

Fig. 4 Gradient Curve Comparison of Hardened Layer

2. Analysis and discussion

In order to obtain good carburizing quality, most of the large bearing rings are carburized in push rod continuous gas carburizing furnace.

In addition to meeting the requirements of furnace temperature uniformity in the effective heating zone, the gas carburizing furnace uses oxygen probes and various types of carbon controllers to adjust the carburizing cycle, so as to control the furnace gas carbon potential, control the carbon content on the workpiece surface, the depth of the carburizing layer, the surface carbon concentration gradient, and meet the metallographic structure of the carburizing layer uniformity of layer depth and surface hardness shall meet the technical requirements of the product.

When the carburized steel ferrules enter the pushrod type continuous gas carburizing furnace production line for heat treatment, they need to enter the following processes in turn: pre cleaning, pre oxidation, heating zone, carburizing, diffusion cooling, primary quenching, post cleaning, low temperature tempering, which are placed in the carburizing atmosphere in the carburizing furnace and heated, carburized, diffused, so that the active carbon atoms are adsorbed on the surface of the workpiece and infiltrated into the surface.

After diffusion to a certain depth of carburized layer and surface carbon concentration, secondary quenching and tempering are carried out to obtain the expected performance of the product.

The products with well controlled carburizing process of bearing ring can obtain uniform surface carburizing quality.

Both inner ring and outer ring can reach the required surface structure after secondary quenching and tempering, that is, the qualified organization evaluated according to JB/T8881-2011 Technical Conditions for Carburizing Heat Treatment of Rolling Bearing Parts.

At the same time, the surface hardness and carburizing uniformity can reach the standard values required in Table 4.

From the perspective of metallographic structure, compared with the structures in the normal area and the white spot area, there are no features of troostite structure, decarburized structure and the structural changes caused by grinding burns.

From this point of view, after the secondary quenching process of the ring, the heating and cooling conditions of the normal area and the white spot area of the bearing ring are the same, and no features of surface decarburization are found in the carburizing process and subsequent secondary quenching heating;

There is no burn tissue in the grinding process, which indicates that the grinding process is normal and has no adverse effect on the surface hardness.

Therefore, the influence of the secondary quenching process and grinding process on the surface hardness and carburizing uniformity can be ruled out.

The Vickers hardness method was used to measure the surface hardness and the depth of the hardened layer at different areas of the same part 0.10 mm from the workpiece surface.

However, during the processing of this batch of products, the surface hardness of the physical sample partially failed to meet the requirements and the hardening depth of the same part was also unqualified (see Table 4 for measured values);

According to JB/T10175-2008 Quality Control Requirements for Heat Treatment, for normal gas carburizing of push rod furnace, the limit deviation of carburizing layer depth shall meet the requirement of ≤± 0.15 mm.

It can be seen from Table 4 that the limit deviation of carburizing uniformity of the same part obviously exceeds the standard value.

Since the inner and outer ring sleeves are used in the mounting of bearing rings, and both of them are of one carburizing furnace, white spots appear on the inner and outer rings, which is related to the uneven carburizing layer during carburizing.

The non-uniformity of the carburizing layer is manifested in two ways: the non-uniformity of the depth of the carburizing layer and the non-uniformity of the composition of the carburizing layer.

This defect will cause the inconsistency of the hardness, wear resistance and fatigue resistance of the parts, and will cause early damage during the service life of the workpiece.

The defective samples of this batch have both cases.

Carburizing is a dynamic process.

It includes two stages:

First, carbon atoms are transferred from the gas phase to the surface of the part;

The second is the diffusion of carbon atoms from the surface to the interior.

Obviously, during the carburizing process in the ferrule white spot area, the gas phase transfer from carbon atoms or the diffusion of carbon atoms on the surface are lower than the carburizing speed in the normal area.

The surface hardness of the raceway in the white spot area of the bearing ring is low, and the depth of the carburized layer is shallow, so the contact fatigue life of the bearing will be greatly reduced, and it is easy to peel off during the use of the bearing, causing early failure.

3. Conclusion

(1) Based on the above analysis, the white spot of bearing ring is a defect caused by uneven carburization in ring carburizing process.

(2) The hardness of the raceway surface in the white spot area of the bearing ring is lower than that in the normal area, and the depth of the carburized layer is shallow, which will greatly reduce the contact fatigue life of the bearing, and the bearing is easy to peel off during use, causing early failure.

(3) From the process investigation in the field carburizing process, it can be seen that there are two reasons that affect the uneven carburizing:

One is the surface quality of the workpiece before it is put into the furnace, and the other is the abnormal atmosphere of the carburizing furnace.

Therefore, the product quality control of the carburizing process should be strengthened from these two aspects to avoid such defects.

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