It was found in production that there were serious internal defects in the bearing rings. The defects are irregular inside holes (some are cracked and the most serious are slag). By ultrasonic flaw detection, the defect is distributed in a raceway of the ring, and is not covered with circumference. After the artificial break, no inclusions were found. From the defect distribution morphology, it also excludes the possibility that the center of the material is loose and the general porosity is caused. After analysis, the confirmation is due to the local over-burning, temperature control improper overheating caused by excessive heat.
In order to ensure the quality of the bearings, we use the defective part of the bearing sampling qualitative, the defect point sample Chemical analysis method, the component analysis of the bearing ring. This paper mainly chooses the methods of optical microscope and SEM to analyze the internal defects of gcr15simn steel bearing ring, and shows that the main reason causing the inner defects of the ring is the high temperature of forging process and final forging, The existence of carbide segregation in the ring tissue and the mesh distribution of carbides in the material lead to a significant increase in the brittleness of the material. In the process of quenching, because of the thermal stress and the effect of phase change stress, there are black spots and holes in the inner wall of the bearing rings, and cracks will occur when the external force of the grinding process is serious.
2. Bearing Ring Defect Type
The macro defect of bearing ring is mainly through observing the phenomenon of hole, crack, scratch, scar or slag which can be found on the surface of bearing ring, and also includes using low concentration pickling to check, the found shrinkage cavity, bubble, over-burning, loose, surface decarburization and other phenomena, different kinds of bearing steel standard has strict stipulation to these defects.
Microscopic defects mainly include in the use of optical microscope amplification test, there are a variety of non-metallic inclusions in the bearing ring and carbide, but also include the strip of annealed tissue, these organizations exhibit uneven characteristics, these defects affect the bearing ring purity and uniformity, strictly control the occurrence of defects, As far as possible to avoid the emergence of various types of bearing steel processing technology is an important research direction.
3. Chemical composition Analysis
The use of mechanical compression method to make the ring along the small hole, first of all, the surface of the surface of the material is observed by the naked eye, and then cut at the fracture of the specimen by wire cutting a piece, such a specimen is a hole in the source area of the expansion zone, the pressure zone of the hole specimen. Then the sample is washed with alcohol, cleaned and placed in acetone, and then washed with ultrasonic 20 minutes after the removal of dry treatment, followed by scanning electron microscope to observe the original hole morphology. In the surface perpendicular to the source section of the hole as the end face of the specimen, after mechanical polishing by the concentration of 4% nitrate alcohol for solution leaching, 10 minutes after the penetration of the optical microscope to observe the microstructure of the specimen, and the use of a scanning electron microscope spectrometer and chemical analysis method to analyze the chemical composition of the specimen. Along the vertical section of the crack (metallographic surface) by mechanical grinding after the measurement of its composition, the sample for chemical composition analysis, see table 1.
SEM analysis of inclusions, in the absence of defects in the substrate also has a small hole, along the crystal melting hole phenomenon, holes and overheating over-burning defects, indicating that the hole is the other side temperature control improper overheating caused. As shown in Figures 1 and 2.
4. Test results and analysis
From the above test results, the pore near the oxidation of the composition of the SEM analysis, near the hole oxidation, mainly iron oxide, no non-metallic inclusions, holes and overheating over-burning defects, indicating that the hole is the other side of the temperature control improper overheating caused.
From the above analysis results can be obtained, GCR15SIMN steel bearing ring chemical composition requirements, hardness in line with the requirements, the presence of oxidation near the hole, mainly iron oxide, no non-metallic inclusions, holes and overheating over-burning defects coincide, which shows that the crack is formed after quenching, rather than before the formation. From the hole morphology can be seen, the grain showed a small uniform characteristics, no obvious non-metallic inclusions and other defects, and carbide particles and fine hidden needle martensite is very small, but from the hole in the morphology from the wear to the microstructure of the coarse and semi-closed mesh carbide, and there is a serious segregation of carbide structure, These phenomena indicate that the main causes of ring holes are:
(1) In the processing of steel, carbide fragmentation is not complete, resulting in a serious segregation of carbide structure phenomenon;
(2) because the final forging temperature is high, especially after the final forging temperature is high, in the process of slow cooling, the relatively coarse semi-closed carbide is precipitated along the grain boundary, and the distribution of the mesh is presented.
(3) The hole and overheating over-burning defects are in agreement, indicating that the hole is due to improper temperature control overheating.
5. Concluding remarks
For this reason, it is recommended to reduce the start-forging temperature of gcr15simn steel bearing ring, cold steel for 2-5 minutes before forging, reduce the heating temperature by 10-20 degrees Celsius, strictly control the final forging temperature between 800 ℃ and 850 ℃, and accelerate the cooling rate by means of dispersed forgings and enhanced ventilation. This preventive measure is adopted to improve the yield.
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