Critical hardness of GCr15 in the hottest dry cutt

2022-08-26
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The critical hardness of hard dry cutting GCr15

preface

the final machining of hardened parts is usually completed by grinding, and the cutting fluid used in grinding brings pollution to the environment and damages the health of operators. Therefore, hard dry cutting technology has become a new research hotspot of cutting technology at present, which can avoid environmental pollution and conform to the mode of green manufacturing and cleaner production. The research results of foreign scholars show that the turning surface under hard conditions has more integrity than the grinding surface, the white layer formed on the surface can improve the service performance, and the turning method also has high machining efficiency. It has the advantages of low energy consumption and no pollution. The author's research shows that the cutting mechanism and chip formation mechanism under the conditions of high hardness and low hardness are different, and under the conditions of hard state (high hardness), its cutting mechanism does not conform to the general cutting theory. Therefore, there should be a critical hardness to distinguish between ordinary cutting and hard state cutting. This paper determines the critical hardness of PCBN tool when cutting bearing steel GCr15 in hard state through experimental research

1 cutting test

1) test methods and conditions

under dry cutting conditions, by systematically changing the cutting parameters (cutting speed, feed rate and back draft) and the hardness of the machined material, the cutting force, cutting temperature, hardness of the machined surface and the depth of the hardened layer of the machined surface are tested and studied to find out the influence law of the hardness of the machined material on the above quantities

test conditions: the machine tool is CA6140, equipped with frequency conversion and speed regulation device, and the cutting speed is detected with a repeater. The workpiece material is GCr15 bearing steel. The test materials with hardness of 30, 40, 50, 60 and 64hrc were obtained by heat treatment. The tool is PCBN cylindrical turning tool made of bn500 material, and its geometric parameters are shown in Table 1. The values of cutting parameters selected in the test are shown in Table 2. Table 1 test tool geometric parameters

rake angle go

(°) rear angle Ao

(°) blade inclination LS

(°) main deflection angle Kr

(°) tool tip arc re

(mm) chamfer width BGL

(mm) chamfer rake angle gbgl

(°) Value 0.8017 0.4 0.1 -25

Table 2 value of cutting amount

cutting amount cutting speed v

m/min feed rate f

mm/r back feed ap

mm value 75 110 160 200 0.08 0.15 0.24 0.10 0.25 0.50 0.80

2) cutting force test

the change law of main cutting force under different cutting conditions is shown in Figures 1, 2 and 3. It can be seen from figures 1 to 3 that within the range of test cutting parameters, the change law of the main cutting force basically conforms to the metal cutting theory, only after the workpiece hardness is higher than 50HRC, the processed material is in a high hardness state; The increase rate of main cutting force is slightly larger

f=0.24mm/r ap=0.5mm

Fig. 1 cutting force surface when v-hrc changes

v=110m/min ap=0.5mm

Fig. 2 cutting force surface when f-hrc changes

Fig. 3 cutting force when AP HRC changes from surface

3) cutting temperature test

cutting speed, feed rate, back feed rate and workpiece when the material hardness changes, the cutting temperature changes, and the white surface shows the characteristic that the workpiece material hardness is the 50HRC boundary, That is, the cutting temperature increases with the increase of workpiece material hardness. When the hardness of the workpiece is higher than 50HRC, the cutting temperature decreases with the increase of the hardness of the workpiece material, that is, the cutting temperature of the workpiece material with a hardness of 50HRC is the highest. When the workpiece hardness is lower than 50HRC, the change law of cutting temperature conforms to the general metal cutting theory, while when the workpiece hardness is higher than 50HRC, the change law of cutting temperature does not conform to the general metal cutting theory, which is the special cutting law of hard cutting

Figure 4 shows the comparison of surface roughness under different hardness

4) machined surface quality test

Figure 4 shows the comparison of surface roughness under different hardness, and figure 5 shows the comparison of hardened layer depth under different hardness. Ag600 large fire fighting/relief amphibious aircraft will make its maiden flight in the near future. It can be seen from Figure 4 that the surface roughness is the worst when the workpiece material hardness is about 50HRC. When the workpiece hardness is greater than 50HRC, the surface roughness value shows a downward trend with the increase of hardness; It can be seen from Figure 5 that the depth of the machined surface hardening layer increases with the increase of the hardness of the workpiece material. When the hardness of the workpiece reaches 50HRC, the depth of the machined surface hardening layer reaches the maximum, and then with the increase of the hardness of the workpiece, the depth of the hardened layer basically remains unchanged

Fig. 5 Comparison of hardening depth under different hardness

2 analysis and discussion of test results

1) relationship between hardness of processed material and chip morphology

when the hardness of workpiece changes, the chip morphology also changes. When the hardness of workpiece is lower than 50HRC, banded chips are generated, and when the workpiece reaches 50HRC; The chip shape changes to sawtooth cutting. Figure 6 shows the banded chips generated when cutting 40HRC, and Figure 7 shows the sawtooth chips generated when cutting 60HRC

cutting conditions: v=200m/min f=0.15mm/r ap=0.5mm 40hrc

Figure 6 ribbon chip photo × 120

when serrated chips are formed, due to the heat insulation effect of the cutting surface of the dividing serrations, a large amount of heat is taken away by the chips, which reduces the cutting temperature. Therefore, the workpiece hardness is higher than 50HRC; The cutting temperature shows a downward trend

cutting conditions: v=200m/min f=0.15mm/r ap=0.5mm 60hrc

Figure 7 sawtooth chip photo × 120

2) relationship between workpiece hardness and chip hardness and deformation

the change of chip hardness is also limited by 50HRC. When cutting GCr15 with different hardness, the comparison between workpiece material hardness and corresponding chip hardness is shown in Figure 8. It can be seen from figure 8 that the corresponding chip hardness of workpiece materials with hardness lower than 50HRC is increased, that is, the cutting heat hardens the chip; The corresponding chips of workpiece materials with hardness higher than 50HRC have reduced the hardness, that is, the cutting heat will temper the chips. Sometimes the cutting heat will make the chips melt

figure, but its emergence does bring more choices to people. 8. Comparison between the hardness of workpiece material and the corresponding cutting hardness

under the heat of higher cutting and innovative management reform carried out by the project team, the metal in the cutting layer is softened and the hardness is reduced, which makes the cutting process easier, and the tool also ensures a higher service life. This characteristic of cutting hardened materials is called metal softening effect. The action mechanism of PCBN tool cutting hard materials is the mechanism of softening the metal layer being cut

the deformation coefficient when cutting materials with different hardness is shown in Figure 9. It can be seen from Figure 9 that the deformation coefficient decreases with the increase of the hardness of the processed material, and when the hardness of the workpiece is higher than 50HRC; The deformation coefficient starts to be less than 1. Small chip deformation produces less heat, which is also one of the reasons why the cutting temperature decreases when the workpiece is high hardness

cutting conditions: v=200m/min f=0.15mm/r ap=0.5mm

Figure 9 deformation coefficient under various hardness

3 tool wear speed under different workpiece hardness

in the position of gently touching the mold cavity in the core, in order to find out the influence law of workpiece hardness on tool wear, the wear tests of tool flank wear and crescent width of front flank under five hardness were carried out. The comparison of flank wear under various hardness is shown in Figure 10, and the comparison of crescent pit width is shown in Figure 11. It can be seen from figures 10 and 11 that whether it is flank wear or crescent depression wear, when the hardness of the workpiece is at 40HRC and 50HRC, the relative wear of the tool is small at higher or lower hardness. This rule shows that the tool wear is also the largest in the range of 50HRC upper and lower workpiece hardness with high cutting temperature. Therefore, it is concluded that PCBN tool is not suitable for machining medium hardness materials, and it can give full play to its excellent cutting performance when machining high hardness materials

cutting conditions: v=200m/min f=0.15mm/r ap=0.5mm

Figure 10 flank wear under different hardness

cutting conditions: v=200m/min f=0.15mm/r ap=0.5mm

Figure 11 crescent width under different hardness

4 conclusion

1) when the workpiece hardness changes under different cutting parameters, the change law of cutting force conforms to the general metal cutting theory, that is, it increases with the increase of cutting parameters and workpiece material hardness, However, the change degree of workpiece hardness before and after 50HRC is different; The change law of cutting temperature does not conform to the general metal cutting theory, that is, with the increase of workpiece material hardness, it reaches the highest value at 50HRC hardness, and then with the increase of workpiece hardness, the cutting temperature shows a downward trend

2) the changes of chip morphology, machined surface roughness and hardened layer depth are all based on 50HRC, and the above quantities have qualitative changes at about 50HRC

3) from the above two points, it can be concluded that there is a critical hardness value to distinguish between ordinary cutting and hard cutting when cutting materials with different hardness, and the critical hardness of bearing steel GCr15 is 50HRC

4) when it is near the critical hardness, the tool wear is serious. Therefore, PCBN tools are not suitable for processing materials with medium hardness. (end)

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