Schematic diagram of spindle grinding position 2 High temperature stress removal test and stress analysis The conventional quenching and tempering treatment process of Cr17Ni2 spindle is: 980 heat preservation 4h oil cooling, 7206h oil cooling twice tempering. The tempering is based on the secondary hardening phenomenon of the steel, that is, the ferrite is unchanged after the first tempering, the martensite is transformed into tempered sorbite, and a small amount of retained austenite is transformed into martensite; Two temperings transform this martensite into tempered sorbite, which tends to make the tissue more stable. The stress-removing process was 580 for 6 h air cooling. According to the above process, the deformation is unstable when trial grinding, and it is good or bad. In the investigation, it was found that several shafts were still unqualified after repeated treatment, and the stress removal process was changed several times, but the effect was not very good.
In order to completely eliminate the stress, the author removes the stress by re-increasing the temperature of a Cr17Ni2 main shaft which has been subjected to multiple stress removal treatments and whose deformation is still unqualified. First, two test pieces were cut at the end of the shaft and cooled at 740 and 760 for 2 hours.
It can be seen that the performance is slightly reduced after tempering again, but it is much higher than the design requirements. To this end, continue to do the stress removal test, the stress removal process is: 750 insulation 6h, when the temperature is greater than 350, the heating rate is less than 70 / h, cooling using furnace cooling to accurately grasp the true temperature of the various positions on the workpiece in the stress removal, will 9 thermocouples were spot-welded to the upper, middle and lower positions of the shaft. Each of the three thermocouples was separated by 120. The 9 thermocouples were respectively connected to 9 digital display temperature instruments. The shaft was suspended vertically. In the well type resistance furnace. The upper, middle and lower temperature control potentiometers in the furnace were manually adjusted throughout the process. The results show that during the heat preservation period, the temperature difference of the three sections on the shaft is less than 15, the temperature difference of the three points in the circumferential direction is less than 5, and the temperature difference of the three sections during the cooling period is less than 23. After this treatment, the radial runout amount during the trial grinding is 002 mm, which is still unqualified.
In order to understand the residual stress state of the shaft after heat treatment and stress removal, the 35CrMoV spindle of the chlorine press that has been subjected to multiple times of stress removal but the test grinding has not been qualified is sent to the Institute of Metal Materials of the Chinese Academy of Sciences for stress measurement. The measurement was carried out by means of an X-ray stress meter, and the distribution of the axial stress along the depth was measured by a layer-by-layer etching peeling method. The measurement position and stress along the depth distribution are as shown.
Stress measurement position and stress along the depth distribution curve, where A, B, and C3 are subjected to trial grinding before measurement, and 0 and 180 sides are tested respectively; E and F are not machined, and only 180 sides are measured. It can be seen that the stress-affected layers on both sides of 0, 180, A, B, and C3 are composed of a compressive stress region and a tensile stress region. The compressive stress zone depth is about 002004mm, and the maximum compressive stress on the surface is 120280MPa. The stress distribution on both sides of 0 and 180 is about the same, the maximum tensile stress is about 120MPa, and the stress influence layer depth is 055mm (from the surface to the depth of the stress value close to 0). The tensile stress on the 180 side of B is slightly higher than the tensile stress on the 0 side; the stress distribution on the sides of 0 and 180 is larger, the maximum tensile stress of the former is 233 MPa, and the depth of the stress influence layer is about 045 mm. The latter has no tensile stress zone, and the depth of the stress-affected layer is only about 005 mm. The residual stresses of the unprocessed parts E and F are basically constant along the depth direction, which is about -30-60 MPa, which can be considered as no heat treatment residual stress.
From the results of stress measurement, the shaft did not produce significant residual stress after heat treatment, but generated stress after grinding. The inconsistent stress distribution of each grinding point was caused by different grinding conditions.
The improvement of the trial grinding process is observed from the trial grinding process. After each stress removal process, the shaft will have a certain amount of deformation, and the tip hole will also change, which will affect the trial grinding of the spindle on the grinding machine: First, the top hole is not round. It will affect the reliability of the detected value. Secondly, due to the deformation of the shaft, the grinding force in the circumferential direction will be different when grinding, and the new grinding stress will be generated. Third, sometimes the amount of the knife is large during grinding, which will cause the top hole to be damaged. Deformed due to uneven force. It has been found that the radial runout value after grinding near the tip hole is 0.010.015mm, indicating that the tip hole itself is not round, and it is inaccurate to check the radial runout value of the spindle with such a tip hole as a reference.
As a test method, it is necessary to have an accurate test basis and a strict grinding process in order to avoid too much detection error in the trial grinding process or to generate new grinding stress. To this end, Jinxi Chemical Machinery Co., Ltd. has specially formulated the spindle trial grinding regulations, which have strict regulations on the accuracy of the top hole and the amount of cutting during grinding to ensure the validity of the test grinding test.
The test verified that the spindle deformation problem was studied. Three Cr17Ni2 shafts were used as test shafts, and the numbers were T05, T06 and T07, and the chemical composition was found. T05, T06 shaft quenching and tempering treatment is carried out according to the above conventional process, and the mechanical properties after heat treatment.
The two shafts of T06 and T07 were subjected to trial grinding before the stress was removed after roughing. The results show that the radial runout of T06 axis is not more than 0.01mm, which is qualified; the radial runout of T07 axis is 0.015mm, which is unqualified. The T07 shaft was insulated by 650 for 6 hours, and then sent to the Institute of Metal Research, Chinese Academy of Sciences for testing residual stress. Detection location.
Chemical composition of Cr17Ni2 test shaft %CSPMnSiCrNi0.170.0060.0290.290.3017.172.08 Mechanical properties after quenching and tempering shaft number b/MPas/MPa5/%Ak/JT06878821145671, 70T07746667195877, 76T06 shaft residual stress measurement position diagram T06 shaft residual stress measurement value Position 123456 stress / MPa-19+10-19-43-77-48 From the test results in the table, it can be seen that T07 only has a stress value of -5 MPa at position 5, and the stress values ​​at other positions are small. After several times of repairing the top hole, the radial runout is not more than 0.01mm, which is qualified once.
For the T05 shaft test, a new quenching and tempering treatment process was adopted, namely, 9604h oil cooling, 7306h two oil cooling tempering, and 7205h air cooling once. The quenching is 20 lower than the original, the first two tempering temperatures are increased by 10, and a 720 air-cooling temper is added. In this way, the first is to reduce the retained austenite in the steel and make the structure more stable. The second is to investigate the influence of multiple high temperature tempering on the performance of Cr17Ni2, and accumulate experience for the subsequent heat treatment. See the mechanical properties of the T05 shaft after quenching and tempering treatment.
The mechanical properties of the T05 shaft after quenching and tempering treatment b/MPas/MPa5/%Ak/J7646022161102,106 are visible, and the mechanical properties of the T05 shaft fully meet the requirements. The shaft is bent and deformed by about 10mm due to improper operation, and is straightened by cold pressing, and the stress treatment is eliminated after 6708h. The radial runout after trial grinding is not more than 0.01mm.
The above three test axes are all qualified once and put into production.
Conclusion Cr17Ni2 steel has secondary hardening phenomenon, and high temperature tempering at 740760 after quenching and tempering treatment does not significantly reduce the mechanical properties. After the Cr17Ni2 shaft is normalized and tempered, the residual stress can be reduced to the required level. For the Cr17Ni2 main shaft which is bent and deformed by cold pressing, after 6708h stress relief treatment, the cold compressive stress can be eliminated and the residual stress can be reduced. Inappropriate grinding can create new grinding stresses, so the spindle grinding requirements must be strictly enforced during the trial grinding process.
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