The key product’s bearing ring is made of 20CrNi2MoA carburized steel, and its processing process involves the following steps: incoming inspection of raw materials, forging, turning, carburizing and secondary quenching and tempering, primary grinding, acid pickling and spot check, fine grinding, nondestructive testing, and phosphating treatment.
It is well known that pickling inspection can reveal soft spots, burns, carbon deficiency, and other defects on the surface of bearing parts. Under normal conditions, the surface of carburized steel bearing parts should be uniformly gray after the pickling process.
However, during the pickling inspection of this batch of products, it was discovered that the outer diameter surface of the inner and outer rings of the bearing mainly showed white spots. This phenomenon also existed on the raceway surface of the rings. As a result, macro and micro inspections were conducted on white spot samples to identify the causes of this issue. Subsequently, preventive measures were proposed to address such defects.
1. Physical and chemical inspection
(1) Macroscopic examination
After the initial ring grinding, the acid cleaning inspection is mainly conducted to detect any burn defects in the grinding process of the product. However, during this inspection, a significant number of white spot defects were found, which are distinct from conventional grinding burn defects.
Based on the parts produced, the pickling inspection of both the outer ring and inner ring from the same batch displayed this phenomenon. The outer ring’s white spots were primarily located on the outer diameter and raceway surfaces, while the inner ring’s white spots were predominantly on the inner diameter and raceway surfaces.
The appearance inspection revealed that the white spots were present on the surface of the ferrule and were distributed in large patches or blocks. The morphology of the white spots is presented in Fig. 1. Fig. 1a shows the white spot morphology on the outer diameter of the outer ring, while Fig. 1b illustrates the white spot morphology on the raceway surface of the inner ring.
Fig. 1 Appearance of ferrule white spot
(2) Component analysis
Samples were collected from the normal sections of the ferrule. The chemical composition analysis of the raw materials is presented in Table 1, which indicates that their primary chemical composition complies with the technical requirements for G20CrNi2MoA electroslag remelting bearing steel in TB/T 2235-2016 Rolling Bearings for Railway Vehicles.
The electron probe analysis was used to evaluate the chemical composition of the white area and the normal area of the outer ring. Table 2 summarizes the results of this analysis. It is apparent from the table that the carbon content in the 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|
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|
(3) Non metallic inclusion inspection
To evaluate the content of non-metallic inclusions, take a metallographic sample of the defective part and observe the non-metallic inclusions in its longitudinal profile. The actual inspection method to be used is Method A, as specified in GB/T10561-2005/ISO4967:1998 (E).
The results are presented in Table 3. It is evident that the non-metallic inclusion level of the bearing parts complies with 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|
|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|
(4) Profile of hardened layer
To investigate the defective ferrule, obtain a metallographic sample along the height direction of the white spot ferrule. It is necessary to observe the metallographic structure of both the white spot area and the normal area.
After corroding the longitudinal section with 4% nitric acid alcohol, Figure 2 shows the profile of the hardened layer on the bearing inner and outer rings.
Figure 2a illustrates the profile distribution of the hardened layer on the longitudinal section of the outer ring with a white spot defect on the outer diameter surface. In contrast, Figure 2b shows the profile distribution of the hardened layer on the longitudinal section of the inner ring with a white spot defect on the raceway surface.
It is evident that the hardening layer profile corresponding to the white spot area is significantly different from the normal area. Moreover, 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.
Fig. 2 Profile of Hardened Layer
(5) Metallographic examination
Please observe the microstructure of the infiltration layer on the outer and inner ring sections under a microscope, as illustrated in Fig. 3.
Figures 3a and 3b demonstrate 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 visible.
The surface structure consists of cryptocrystalline or fine acicular martensite, uniformly distributed fine-grained carbide, and a small amount of retained austenite.
According to the fifth level diagram of JB/T8881-2011 Technical Conditions for Carburizing Heat Treatment of Rolling Bearing Parts, the surface structure of both inner and outer rings is Grade 2, which is acceptable.
The surface structure does not contain coarse carbide or network carbide.
Figures 3c and 3d display the metallographic morphology of the white spot area and the normal area of the inner race raceway section at 500 times, respectively, and no significant differential structure is evident.
It has been assessed according to Technical Conditions for Carburizing Heat Treatment of Rolling Bearing Parts (JB/T8881-2011) and found to be qualified.
(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 utilized for assessing the surface hardness of the longitudinal section of the bearing’s inner and outer rings, as well as the gradient distribution of the hardened layer. Testing is conducted separately for the white spot area and the normal color area of the bearing’s inner and outer rings.
Table 4 shows the comparative results of the surface hardness and the depth of the hardened layer in the white spot area and the normal area.
The gradient curves of the hardened layer in the white spot area and the normal area of the outer ring are compared in Fig. 4.
Fig. 4a displays the hardness gradient curve of the hardened layer in the white spot area of the outer ring, whereas Fig. 4b illustrates 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|
|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|
Fig. 4 Gradient Curve Comparison of Hardened Layer
2. Analysis and discussion
To achieve high-quality carburizing results, most large bearing rings are carburized using push rod continuous gas carburizing furnaces. These gas carburizing furnaces not only ensure uniform furnace temperature within the effective heating zone, but also utilize oxygen probes and various carbon controllers to regulate the carburizing cycle.
This controls the furnace gas carbon potential, as well as the carbon content, depth, and surface carbon concentration gradient of the carburizing layer, ensuring the uniformity of layer depth and surface hardness in the metallographic structure of the carburizing layer meets the technical requirements of the product.
During the heat treatment production line of pushrod-type continuous gas carburizing furnaces, carburized steel ferrules undergo the following processes in sequence: pre-cleaning, pre-oxidation, heating zone, carburizing, diffusion cooling, primary quenching, post-cleaning, and low-temperature tempering.
The ferrules are placed in the carburizing atmosphere of the carburizing furnace, heated, carburized, and diffused so that active carbon atoms are adsorbed onto and infiltrate the surface of the workpiece.
After the diffusion of the carburized layer to a certain depth and surface carbon concentration, secondary quenching and tempering are carried out to achieve the desired performance of the product.
Uniform surface carburizing quality can be achieved in products that undergo a well-controlled carburizing process of bearing rings.
Both the inner and outer rings can achieve the required surface structure after secondary quenching and tempering, as evaluated according to the JB/T8881-2011 Technical Conditions for Carburizing Heat Treatment of Rolling Bearing Parts, ensuring a qualified organization.
Furthermore, the surface hardness and carburizing uniformity can meet the standard values required in Table 4.
From a metallographic structure perspective, there are no features of troostite structure, decarburized structure, or structural changes caused by grinding burns, compared to structures in the normal area and white spot area.
Thus, 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 identical, and no features of surface decarburization are observed during the carburizing process and subsequent secondary quenching heating.
Additionally, there is no burn tissue in the grinding process, indicating that the grinding process is normal and has no adverse effect on the surface hardness.
The influence of the secondary quenching process and grinding process on the surface hardness and carburizing uniformity can be eliminated.
The Vickers hardness method was utilized to measure the surface hardness and depth of the hardened layer in various areas of the same part, located 0.10 mm from the workpiece surface.
However, during the processing of this batch of products, the surface hardness of some physical samples failed to meet the requirements, and the hardening depth of the same part was also deemed unqualified (see Table 4 for measured values).
According to the JB/T10175-2008 Quality Control Requirements for Heat Treatment, the limit deviation of carburizing layer depth for normal gas carburizing of push rod furnace should meet the requirement of ≤± 0.15 mm.
It is evident from Table 4 that the limit deviation of carburizing uniformity of the same part surpasses the standard value.
As the inner and outer ring sleeves are employed in mounting bearing rings and are subjected to one carburizing furnace, white spots appear on the inner and outer rings, which are related to uneven carburizing layers during carburizing.
Non-uniformity of the carburizing layer can be observed 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 result in 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 exhibit both cases.
Carburizing is a dynamic process.
It includes two stages:
The carburizing process involves two steps:
First, carbon atoms are transferred from the gas phase to the surface of the part.
Secondly, carbon atoms diffuse from the surface to the interior.
During the carburizing process, it is evident that the transfer of carbon atoms from the gas phase or the diffusion of carbon atoms on the surface in the ferrule white spot area is slower than the carburizing speed in the normal area.
As a result, 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. Consequently, the contact fatigue life of the bearing is significantly reduced, and it is prone to peeling during use, resulting in premature failure.
(1) Based on the analysis above, the white spot on the bearing ring is a defect caused by uneven carburization during the 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 shallower. This greatly reduces the contact fatigue life of the bearing, making it easier for the bearing to peel off during use, causing early failure.
(3) The process investigation of the field carburizing process shows that there are two reasons that lead to uneven carburization: the surface quality of the workpiece before it enters the furnace, and the abnormal atmosphere of the carburizing furnace.
Therefore, product quality control of the carburizing process should be strengthened in these two areas to avoid such defects.