Abstract: many rollers with diameter no more than Φ 11mm cracked after heat treatment, with analyzing the organization before heat treatment, it is proved that the reason of causing cracks is the excessive thermal stress when hardening rollers, by way of improving the technical parameter of heat treatment and decreasing the thermal stress, hardening crack was avoided. In this way, the aim of increasing production and practicing economy were realized.is
Key words: heat treatment; crack; thermal stress; martensite
1. Preface
Roller is an important part of rolling bearing, and its quality will directly affect the accuracy and service life of the whole bearing. About 10% of the small and medium-sized rollers processed with raw materials supplied by a steel plant have cracks after heat treatment and quenching in CT drum furnace, which seriously affects the quality of the rollers and causes great waste, as shown in the table below.
Table. proportion of roller cracks before process improvement
Specifications | 3 0 2 0 3 0/ 4 | 3 0 2 0 5/0 4 | 3 0 3 04 /04 | 32 0 10/ 0 4 |
Crack ratio | 9% | 11% | 8% | 5% |
2. Analyze the Current Situation and Find Out the Problems
First, analyze whether the equipment is normal and whether there is a high temperature zone in the furnace. After repeated temperature calibration and testing, it is determined that there is no high temperature zone in the furnace and the equipment is normal. After eliminating the causes of the equipment, the generation mechanism of the cracks is analyzed. It is found that these rollers have cracks after quenching. Therefore, it is inferred that there are two possible causes of the cracks of the rollers, that is, the causes of raw materials or the causes of heat treatment and quenching process. Before feeding in the steel plant, this type of roller is quenched in CI - drum furnace. The process is as follows: the temperature of quenching heating zone is 850± 5 ℃, the temperature of quenching insulation zone is 850 ± 5 ℃, the cycle is 130s, the frequency is 2, the heating time is 30 - 35 min, the quenching medium is rapid quenching oil, the temperature is 60 - 70 ℃, and the product has no cracks after quenching. See Fig. 1. The small and medium-sized rollers processed with raw materials supplied by a steel plant are still quenched in the CT drum furnace. The process is still the original process, and cracks appear after quenching. Through the inspection of the roller before quenching, it is found that the raw material is not heated enough during annealing, resulting in insufficient dissolution of carbides, uneven internal structure and low carbon content on the surface of the roller. The surface of the roller is prone to overheating during heating and cracks during quenching.
Figure 1 original process
3. Solution
Due to equipment reasons, only martensite quenching can be adopted, and the cracks of rollers can be reduced by changing the quenching process parameters of rollers. Firstly, the temperature of quenching heating and insulation area is reduced from 850 ± 5 ℃ to 845 ± 5 ℃, the cycle and frequency remain unchanged, and the roller still has cracks after quenching, and the processing results are not improved. Heat the quench and heat the holding area The temperature continued to drop to 840 ± 5 ℃, and the crack proportion of the roller decreased significantly. When the temperature drops to 810 ℃, there is no crack after quenching, but its hardness and microstructure can not meet the requirements of the inspection standard. According to the test results, try to pull a certain gradient between the heating zone and the holding zone of the quenching furnace, which not only reduces the thermal stress, but also ensures the quenching temperature of the roller. Under the condition that the equipment capacity allows, after many tests, we finally found a quenching process to eliminate the crack phenomenon of small and medium-sized rollers during quenching in CT drum furnace, and the hardness and microstructure meet the requirements of the inspection standard, that is, the temperature of quenching heating zone is 810 ℃, the temperature of quenching holding zone is 840 ± 5 ℃, the cycle is 180 s, the frequency is 3, and the heating time is 42 - 47min, The quenching medium is quick quenching oil, and the temperature is 60 - 70 ℃, as shown in Figure 2.
Figure 2 improved process
From the comparison between the original process and the improved process, it is easy to see that the improved process reduces the heating temperature of the workpiece and prolongs the holding time. When the workpiece is heated to 810 ℃, the overheating of the workpiece is avoided. The heat preservation is about 20 min, so that the structure can be uniform and the stress can be relieved. Then, the heat preservation is about 20 min from 810 ℃ to 840 ± 5 ℃. While ensuring the quenching temperature, the workpiece structure can be further stabilized, which can effectively make up for the defects of raw materials and greatly reduce the tissue stress in the cooling process.
4. Conclusion
30203/04, 30205/04, 30210/4, 30304/04 and 32010/04 were selected for quenching test. No crack was found on the roller after quenching. The production practice from July 2002 to December 2004 proved that this process can eliminate the crack phenomenon of small and medium-sized rollers during quenching in CT drum furnace The hardness and microstructure after tempering meet the requirements of the inspection standard. It shows that the process is stable and reliable.
More about Sunspeed Cylindrical roller bearings:
Sunspeed cylindrical roller bearings are available in a wide range of designs, series, variants and sizes. The bearings can meet the challenges of applications faced with heavy radial loads and high speeds. Accommodating axial displacement (except for bearings with flanges on both the inner and outer rings), they offer high stiffness, low friction and long service life.
The bearings can meet the challenges of applications faced with heavy radial loads and high speeds. Accommodating axial displacement (except for bearings with flanges on both the inner and outer rings), they offer high stiffness, low friction and long service life.
Cylindrical roller bearings are also available in sealed or split designs. In sealed bearings, the rollers are protected from contaminants, water and dust, while providing lubricant retention and contaminant exclusion. This provides lower friction and longer service life. Split bearings are intended primarily for bearing arrangements which are difficult to access, such as crank shafts, where they simplify maintenance and replacements.
SUNSPEED’s products are as follows: deep groove ball bearing, tapered roller bearing, self-aligning ball bearing, spherical roller bearing, cylindrical roller bearing, angular contact bearing, thrust ball bearing, thrust roller bearing, needle roller bearing, joint bearing, etc. Bearings are widely used in motor, machine tool, automobile, wind power, mining and other fields.
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