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Abstract: The twin carbide in 40Cr15Mo2VN bearing steel are invested with optical microscope and SEM. The results show that the twin carbide changes the fracture mode，increasing brittleness，and isn’t eliminated by quenching and tempering，being regarded as defect.
Key words: rolling bearing; 40Cr15Mo2VN bearing steel; twin carbide; fracture appearance
At present, the main materials for stainless bearing steel in China are 9Cr18 and 9Cr18Mo high carbon chromium stainless steel. Due to the high carbon and chromium content of these two steels, large eutectic carbides will be produced during the processing. Due to the uneven distribution of carbides, most of them precipitate at grain boundaries and cannot be eliminated during heat treatment, which can have adverse effects on the grinding and ultra precision machining of bearing rings. When the bearing bears a large load, it is easy to cause stress concentration at the eutectic carbides, resulting in fatigue crack sources, which can damage the bearing's performance and contact fatigue life. A large amount of research has been conducted by domestic and foreign researchers on this topic, and different types of nitrogen containing stainless bearing steels have been developed. This type of material is uniformly distributed with small granular carbon nitrogen compounds formed by nitrogen and carbon, similar to the spheroidized annealing structure of high carbon chromium bearing steel, but without coarse eutectic carbides and needle like eutectic carbides in high carbon chromium stainless steel.
The domestically developed 40Cr15Mo2VN nitrogen-containing stainless bearing steel is one of them, but there is relatively little research and reporting on this steel in China. Due to the high alloy element content of 40Cr15Mo2VN nitrogen-containing stainless bearing steel, the forging heating temperature range is narrow, and the processing difficulty is high. It is prone to coarse forging structure and even twinned carbide structure caused by high forging temperature and long insulation time. The following text mainly analyzes the twin carbides that appear during the forging process of 40Cr15Mo2VN and their influence on the fracture morphology.
1. Experiment
1.1 Test materials
The experimental material is 40Cr15Mo2VN bearing steel, which is smelted using the double vacuum method,. The nitrogen element was determined using an oxygen nitrogen analyzer, while the remaining elements were determined using direct reading spectroscopy and evaluated according to the AMS 5925A-2006 standard.
The material undergoes heat treatment after forging and annealing, and the microstructure of the samples after forging and annealing and heat treatment is observed; Take quenched samples that have been forged and annealed, and those that have been found to have twinned carbides, and manually break them to obtain the fracture surface.
1.2 Test methods
Normal samples are forged at 1100 ℃, while twin samples are forged at 1250 ℃. The forged ring is quenched using a vacuum quenching furnace, cold treated using a freezer, and tempered using a vacuum tempering furnace. The heat treatment process is: 1049 ℃ x 30 min quenching+-73 ℃ x 120 min cold treatment+177 ℃ x 60 min tempering. The microstructure of the forged annealed and quenched specimens was observed using a Japanese OLYMPUS inverted metallographic microscope and image analysis system, and the fracture morphology was observed using a scanning electron microscope.
2. Experimental results and analysis
2.1 The normal microstructure of 40Cr15Mo2VN nitrogen-containing stainless bearing steel after forging and annealing should be composed of uniform spherical and fine-grained pearlite, as well as a small amount of primary carbides and ferrites, as shown in Figure 1a. After long-term high-temperature heating, nitrogen containing stainless bearing steel forms coarse grains with clear grain boundaries. Each grain contains one or more parallel twin carbides, which almost run through the entire grain. The morphology is shown in Figure 1b. After measurement, the length of twin carbides is about 50-150 μ M.
(a) normal tissue
(b) Coarse twin carbide structure
Figure 1 Photo of Forged Annealed Structure
2.2 Structure of twinned carbides after quenching and tempering
The 40Cr15Mo2VN nitrogen-containing stainless bearing steel with twin carbides after forging and annealing was treated according to the quenching and tempering process in the test method, and its microstructure is shown in Figure 2. From the figure, it can be seen that twin carbides still exist after quenching and tempering, indicating that the twin carbides produced by forging and annealing cannot be eliminated in the subsequent quenching and tempering process.
Figure 2 Photos of Quenched and Tempered Structures
2.3 Effect of twinned carbide structure on mechanical properties
2.3.1 Effect of twinned carbide structure on fracture morphology
Due to the small size of both the sample and the product, it is not possible to make an impact specimen for impact toughness testing. Therefore, the sample was manually broken to observe its fracture morphology. By comparing the artificial fracture morphology of nitrogen containing stainless bearing steel with and without twinned carbides, the influence of twinned carbides on its properties is determined. When there are no twin carbides present, the fracture surface of nitrogen containing stainless bearing steel shows no obvious plastic deformation on a macroscopic level, and the fracture surface is flat with brittle fracture characteristics. The scanning electron microscopy morphology of the fracture surface is shown in Figure 3, which is a typical quasi cleavage fracture with short and discontinuous river patterns on the quasi cleavage surface. There are tearing edges around the quasi cleavage surface, and its microstructure is ductile dimples.
Figure 3 Fracture morphology of twin free carbides
The microstructure of the fracture surface of nitrogen containing stainless bearing steel in the presence of twin carbides is shown in Figure 4. From the figure, it can be seen that in addition to river patterns on the quasi cleavage surface, there are also tongue shaped patterns similar to those in cleavage fractures at the fracture surface. At the same time, there are tearing edges and dimples at the fracture surface, and the length of the tongue shaped pattern is about 50-150 μ m. And it is arranged in a parallel manner, and the length and arrangement of the "tongue shaped" pattern are consistent with the twin structure of carbides in the optical microstructure, indicating that the "tongue shaped" pattern is caused by the presence of twin carbides in the microstructure. During the fracture process, when the material fractures along the quasi cleavage plane and encounters twin carbides in the structure, the cleavage surface will tilt and continue to propagate forward along the twin plane until fracture occurs. So the fracture mode in the presence of twin carbides has shifted from quasi cleavage fracture to partial cleavage fracture, increasing the brittleness of the material and belonging to a defect structure. Materials containing this type of organization used in bearing parts can cause early failure of the bearing, and even lead to fracture failure without any prior signs. Therefore, the appearance of twinned carbide structures should be avoided during bearing processing.
Figure 4: Fracture morphology with twin carbides
2.3.2 Effect of twinned carbide structure on hardness
The microstructure of twinned carbides has no significant effect on the hardness after annealing, but the hardness of samples with twinned carbides after quenching and tempering is about 1.5 HRC lower than that of samples without twinned carbides.
3. Conclusion and solutions
(1) The twinned carbide structure generated during the forging process cannot be eliminated in the subsequent quenching and tempering process. (2) The twinned carbide structure generated during the forging process changes the fracture mode, increases the brittleness of the material, and belongs to the defect structure. (3) The twinned carbide structure has little effect on the hardness of the sample after annealing, but reduces the hardness of the sample by about 1.5 HRC after quenching and tempering. The twinned carbide structure that appears after forging annealing can usually be eliminated by normalizing process, and the specific process method will be discussed in subsequent work.
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2024-03-29