After martensite quenching and tempering, the structure of high carbon and chromium steel bearing parts should be cryptocrystalline or fine crystalline with small acicular martensite. In addition, it should have evenly distributed fine residual carbide and a small amount of residual austenite.
For microbearings, a small amount of acicular or massive troostite is allowed, as shown in Figure 1.
The microstructure after quenching and tempering must meet clause 3.2.2 of JB/T1255-2014, the technical specifications for heat treatment of high chromium and chromium steel parts for bearings.
This high carbon and chromium bearing steel structure features good hardness, strength, wear resistance and fatigue resistance.
After tempering, bearing steel can also acquire excellent comprehensive properties such as elasticity, toughness and dimensional stability.
During the heat treatment of high carbon and chromium steel bearing parts, various defects may occur due to problems with bearing steel materials, heat treatment process, processing equipment and human factors. Such defects may include overheating of the metallographic microstructure (coarse needle martensite), underheating of the metallographic microstructure (troostite exceeding the standard), coarse-grained carbides, severe lattice carbides and other microstructural defects.
Some of these metallographic microstructure defects can directly cause product scrap, such as overheated metallographic microstructure (coarse needle martensite). However, other defects may not result in the product being discarded, but may still negatively impact the product's useful life.
For example, underheated metallographic microstructure (troostite exceeding the standard) can affect bearing life, leading to early bearing ring breakage and significantly impacting bearing product quality.
1. Quenching of superheated metallographic microstructure (coarse needle martensite)
Figures 2 and 3 represent the metallographic microstructure resulting from quenching at superheating temperature, showing coarse acicular martensite with distinct structural characteristics. This type of structure is known to decrease bearing toughness and impact resistance, leading to shorter lifespan and even extinguishing cracks in cases of severe overheating.
(1) Cause
This problem is mainly caused by excessively high quenching heating temperature or prolonged holding time at the upper limit of quenching heating temperature, which results in excessive dissolution of secondary carbides. The austenite grain also has the opportunity to grow, weakening the hindering effect of martensite growth and increasing the possibility of further martensite growth.
When viewed under a 500x (or 1000x) metallographic microscope, the superheated metallographic microstructure is evident as coarse needle martensite.
Another possible cause is the presence of severely banded carbides in the raw material or irregular size distribution of carbides in the annealed structure, resulting in thin flakes of pearlite in the annealed structure.
Even during normal quenching, coarse acicular martensite can form in areas with sparsely distributed carbides or fine particles, with few obstacles to martensite growth.
Surface decarburization results in little or no carbide and therefore has a minimal effect on inhibiting martensite growth.
If cooling conditions are ideal, martensite still has a chance to grow and form coarse acicular martensite.
(2) Measurements
It is important to choose an appropriate quenching temperature and heating time. These parameters must be selected in accordance with the material standards, and it is necessary to strictly control the formation of carbide bands.
To improve annealing quality, it is crucial to closely monitor the furnace temperature during production. In case of power or equipment failure, timely and effective measures must be taken to avoid any negative impact on the process.
2. Metallographic microstructure of quenching under heating (troostite exceeds the standard)
Troostite is a structure that forms due to undercooling or insufficient cooling during the cooling process. It is the result of the pearlite transformation of austenite.
Troostite has an exceptionally fine pearlite structure. In bearing steel, troostite can be categorized based on its metallographic morphology into four types: massive troostite (see Fig. 4), acicular troostite (see Fig. 5), a combination of acicular and massive structures (see Fig. 6 ), and banded troostite (see Fig. 7).
The troostite structure can be found in hardened bearing steels and can lead to a decrease in both the hardness and strength of the steel. This structure is also unfavorable to wear resistance and fatigue resistance, and greatly reduces the rust resistance of bearing steel.
Although the hardness of the part is within the qualified range, the presence of a small amount of acicular and massive troostite meets the metallographic microstructure requirements specified in technical conditions JB/T1255-2014 for heat treatment of high-carbon steel parts and chrome for lamination. bearings.
However, the presence of massive and reticular troostite exceeds what is foreseen in the standard, making it an unqualified structure. This can result in lower hardness of the part and make it easier to identify weak points after pickling.
(1) Cause
Massive troostite forms when there is inadequate heating (either the temperature is too low or the retention time is too short). This results in uneven austenitic alloy concentration and poor hardenability in certain areas of the steel, leading to pearlite transformation during normal cooling.
Acicular troostite forms due to poor cooling, where the quenching medium is not able to cool the steel at a sufficient rate. Even with normal heating, certain areas of the steel may not reach the critical rate of cooling necessary for proper hardening.
Zonal troostite forms when there are carbides present in the raw material for bearing steel that are distributed in a strip format in areas with low carbon concentration.
(2) Measurements
If troostite appears during production, its metallographic microstructure must be inspected and the causes analyzed to take appropriate measures.
If the troostite is in massive form, the quench heating temperature must be increased accordingly and the retention time extended.
If the troostite is in acicular form, the cooling rate must be increased.
If the heating, heat preservation and cooling temperatures are within the normal range, but troostite still occurs, it is necessary to check for raw material problems, temperature control problems, equipment malfunctions and other potential causes. It is important to identify the cause in a timely manner and take the necessary measures.
3. Severe network carbide
Figure 8 shows the severe formation of network carbides resulting from deep etching using a 4% nitric acid alcohol solution.
This structural defect does not arise during the tempering process, but results from inadequate rolling, forging or annealing. It can only be detected through post-extinction inspection.
(1) Cause
The presence of cross-linked carbides in steel increases the heterogeneity of its chemical composition. This can lead to significant structural stresses during heat treatment and quenching, which in turn can cause warping and cracking of parts.
Cross-linked carbides weaken the relationship between matrix grains and reduce the mechanical properties of the steel. In particular, they can significantly reduce the impact properties of steel. Furthermore, as the level of cross-linked carbides increases, the impact properties of the steel continue to decrease.
Network carbides also have a significant effect on the flexural and tensile strength of steel. Furthermore, the contact fatigue resistance of steel decreases with increasing level of cross-linked carbides. In fact, the contact fatigue resistance of longitudinal samples with coarse cross-linked carbides decreases by approximately 30%.
Each increase in cross-linked carbide grade reduces part life by approximately one third. Severe cross-linked carbides cannot be eliminated in subsequent spheroidization annealing processes, and the structure of the carbides can only be eliminated or improved through a normalization process.
In cases where the crosslinked carbide is light, part of the network may be broken and spheroidized during spheroidization annealing. However, if the carbide particles are larger, the carbide particles in the spheroidized annealed structure may not be uniform.
(2) Measurements
Strict control must be implemented on the cross-linked carbides present in bearing steel raw materials. The level of cross-linked carbides should not exceed the limit specified in GB/T18254-2016 for bearing steel with high carbon and chromium content.
During the forging process of forged bearings, it is important to regulate the cooling rate to avoid the formation of cross-linked carbides resulting from a slow cooling rate.
If necessary, air cooling can be used to accelerate the cooling rate of forgings and prevent the occurrence of cross-linked carbides.
4. Conclusion
An in-depth analysis of the causes of primary defects in the microstructure of bearing steel parts with high carbon and chromium content after quenching was carried out, and preventive and corrective measures were proposed to improve the quenching quality of bearing steel parts with high carbon and chromium content.
Given the complexity of production practices, it is crucial to carry out a specific analysis of different situations to ensure the quenching quality of high carbon and chromium bearing steel parts and ensure the reliable internal quality of bearing products.
