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/*728*90 created on 2018/5/16*/ var cpro_id = "u3440131"; Home>Bearing knowledge>Discussion on the wear mechanism of dry-cut hardened bearing steel Discussion on Wear Mechanism of Dry Cutting Hardened Bearing Steel
Polycrystalline Cubic Boron Nitride (PCBN) is a new type of tool that uses artificial methods to be second only to diamonds. It has been developed rapidly in the past two decades; it is widely used in aerospace, automotive, electronics, Precision industrial categories such as hydraulic components. Because polycrystalline cubic boron nitride has many excellent mechanical properties; it is not only used to process some high hardness and difficult to process materials; it can also be used to finish hardened steel instead of grinding process. So far; the processing skills of PCBN tool cutting hardened steel have not been widely used by related companies; the main reason is that the company has not thoroughly understood and grasped the processing mechanism, tool performance and application skills of cutting hardened steel with PCBN tools. In addition, some unstable factors in the cutting process and valuable tool cost also restrict the application and implementation of PCBN tools. In recent years, the research on PCBN tool wear has been strengthened at home and abroad; but the research content will be concentrated in the general wear mechanism. And in the processing of difficult processing materials under detailed processing conditions; and rarely reported the use of PCBN blade processing hardened steel Wear problems. /*250*250 was created on 2017/12/25*/ var cpro_id = 'u3171089';
In this paper, the dry-cut hardened GCr15 bearing steel of CB20 trademark PCBN tool of Sweden Sandvik Company is taken as the research target; the wear problem of PCBN tool is analyzed by the cutting experiment system; the relationship between chip shape and tool wear during the cutting process is discussed together; In order to expand the scale of the use of PCBN tools; the application of its application to the hard material processing provides experimental basis.
1 Experimental conditions
1) Experimental machine tool: C6132,
2) Workpiece: Workpiece data: hardened GCr15 bearing steel (hardness after quenching 61-63); workpiece standard: φ40×280mm,
3) Blade: SNMA120408 S01020E CB20 blade produced by Sweden Sandvik Co., Ltd. (this blade is a cubic boron nitride tip added with titanium nitride on the carrier of the carbide); the shape of the blade is shown in Figure #. Several parameters of the rear tool: go=-6°; ao=6°; ls=-4°; kr=75°; kr'=15°; the radius of the tool nose is re=0. 8mm; the negative chamfer is 26 ° × 0. 1mm (the scanning electron microscope image of the blade is shown in Figure 3a).
4) Cutting parameters: cutting speed vc=140m/min; back-feeding knife amount ap=0. 2mm; feed rate f=0.1mm/r; dry cutting is selected.
5) Measurement and investigation: in the cutting experiment; 3 min per distance; measure the main flank wear of the tool under a 40-fold object microscope and investigate the wear shape of the flank before and after the cutting stroke reaches 7800 m; use HITACHI S-570 Scanning electron microscopy (SEM) was used to investigate the wear of the tool surface; the cutting temperature was measured by thermocouple method; the surface roughness was measured by YCL type profiler.
2 Experimental results and analysis In the experiment of dry-cutting hardened GCr15 bearing steel with PCBN tool, the chip was dark red; it flowed out along the minor cutting edge. The cutting temperature measured by thermocouple method reached 1000 °C.
1) PCBN tool wear characteristics
When the CB20 blade is used to turn hardened GCr15 bearing steel, the cutting surface will wear out in the front and back of the tool. During the cutting process, the tip of the tool tip will wear first; then the blade face wears immediately after the attack; its primary feature is that it is near the chamfered surface. The crater is formed first. Following the cutting; the crater is deep in the forward flank, and the main blade and the secondary blade are extended. The SEM image of the crater formed when the cutting stroke reaches 7800 m is shown in Fig. 3b; the width and depth of the crater are compared. Uniform; however, the metallographic photographs near the tip of the knife are significantly different from the metallographic photographs that extend to the ends; there are particles falling and micro-cracking at the tip.
It is difficult to investigate the flank wear at the beginning of cutting; however, the wear near the tip is relatively significant. After 6 to 7 minutes of cutting, the main flank faces the edge of the tool tip and the wear surface appears. The cutting stroke arrives. The SEM image of the flank wear band formed at 7800 m is shown in Fig. 5b; the flank wear band and the conventional flank wear are significantly different; the wear of the secondary flank is much lower than that of the main flank Wear; the front and back flank wear areas are small.
2) Tool wear curve main flank wear; followed by cutting time; CB20 blade wear is gradually added evenly; there is a significant difference between initial wear and normal wear; while there is no significant difference between normal wear and severe wear boundary.
Analyzed; in the early stage of the cutting experiment; because the front and back of the new tool still have some rough and uneven localities in the microscopic; the touch stress on the surface of the workpiece in the cutting process is very large; so the tool wears faster, followed by the cutting moment After the new tool is run-in; the touch area of ​​the tool and the workpiece increases; the pressure is reduced. 至. When the experiment is completed, the cutting stroke reaches 7800 m; the wear amount of the main flank face reaches 0. 35 mm. The cutting process is still relatively stable; the surface roughness Ra is 1. 6.
3) Discussion and discussion of wear mechanism
a. The flank wear surface of the rake face will appear during the beginning of the cutting experiment; the first is the slight scratches and scratches caused by mechanical wear, followed by the cutting moment; the flank wear near the tip is crescent Shape; this is because the high temperature effect of the cutting zone causes the phase appearance of the tool to wear phase change; then it is embodied as “moon gingivalâ€. If the PCBN tool is properly cooled during the cutting process; for example, with inert gas (with N2 Good) to protect; can greatly reduce the wear of the tool rake face; then improve the tool life.
Scanning electron microscopy was used to analyze the crater near the micro-cracking area. It was compared with the metallographic image of the tool surface before wear. It can be clearly seen from Fig. 4 that the uncut CB20 tool has uniform grain spread on the rake face. Together, the worn tool presents a non-regular appearance of the pits. The appearance of this appearance is that the temperature of the cutting process is as high as 1000 °C or more; the PCBN tool must have an attack of oxidation and nitrogen release; its response equation for:
4BN(CBN)+3O2→2N2↑+2B2O3
The above formula indicates that the outer surface of the PCBN tool will form an oxide film on its outer surface after oxidation. However, due to the fierce conflict and thermal shock effect of the chip, this oxide film will be quickly taken away by the chip; it constitutes further oxidative wear. Phase change wear; Figure 4b is not significant.
b. The flank wear is at the beginning of the cutting process; the wear profile of the flank face is difficult to investigate; however, the blade tip is somewhat worn but more significant; this is because the blade edge arc re=0. 8mm; the back knife amount ap =0. 2mm; the flank of the tool and the workpiece are not touched when the cutting is just started; then the wear of the blade is only reflected in the arc of the tool tip and the flank; until the cutting is carried out 6~7 About a minute; the wear surface appears near the edge of the knife at the edge of the main.
The flank wear is somewhat pockmarked; the surface of the tool has particles falling. This is because under certain pressure and high temperature conditions; CBN activity is enhanced; the affinity with the processing data is continuously added; then the attack wear is attacked. The bonding condition of the secondary flank is better than that of the main flank. Because of the cutting process, the temperature of the main flank comes from the sharp conflict between the tool and the workpiece and the thermal impact of the chip on the outer surface of the tool; The temperature is mainly caused by the flow of high temperature chips; it is obviously lower than the cutting temperature of the main flank; thus it is proved that the bond wear is affected by temperature; the higher the temperature; the more severe the bonding condition.
(a) External flank surface description (100×) (b) Main flank surface appearance (100×) (c) Main flank surface micro arrangement (3000×)
The existence of another wear mechanism of the CB20 tool; that is, the phase change wear mentioned in section a. The metallographic arrangement of the main flank of CB20 can detect the papillary plaque; the CBN grain protrudes from the outside; the surrounding stick The knot has been removed. This is the typical feature of the phase change wear; that is, the CBN particles have a CBN→HBN conversion. Because the HBN hardness is very low; then the cutting ability is lost. Because the cutting of the rake face is generally considered The temperature is higher than the flank; the flank has a phase change wear; then the rake face must also have phase change wear.
c. Particle drop and micro-cracking surface particle drop and micro-cracking surface appearance are the unique types of wear of superhard polycrystalline diamond tool data. In this experiment; when cutting for about 25 minutes; CB20 tool cutting edge and machining workpiece The surface touch area (ie, the negative chamfer is close to the edge of the blade) has a micro-cracking appearance; the scanning electron microscope is used to investigate the CB20 tip of the micro-disintegration; some of the tips are missing. Because the PCBN tool is made up of many Fine CBN particles; the "impurity component" enriched at the grain boundary is equivalent to a "precision crack"; and there is uneven internal stress; thus greatly reducing the grain boundary strength. Therefore, when high temperature chips flow through When the tool tip is used, the micro-impact caused by the uneven material material or the presence of hard spots and the beating of the machine tool spindle constitute the CBN particle drop and micro-cracking.
4) Analysis of the shape change of the chip: Professor Qiu Qiusheng's research thought: in the condition that the cutting speed is high and the processing data is still at a lower temperature; the orientation of the tool tip is now satisfied with the condition of the brittle shear of the excised data; thus the satisfactory cutting The plastic shear shear stress condition of the zone data constitutes the crack of the sawtooth chip and generates the sawtooth chip. The same amount of cutting is used in the experiment; the chip will have some changes in the same period. During the initial period of cutting (see Figure 7a); It can be clearly seen that the chips are composed of layered sheets kneaded and stacked. This is illustrated in connection with Figure 8: the initial stage of cutting; the touch surface of the tool and the workpiece. The 棺饔 棺饔 冉 冉 è¡Œ è¡Œ è¡Œ è¡Œ è¡Œ è¡Œ 鹘 艘I would like to ask about the appearance of the èž… èž… å€ ; ; ; ; ; ; ; ; ; ; ; ; ; 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削 切削It is lubricated; then it is suitable for the outflow of chips; the shear deformation and discharge resistance of the chips are reduced. The rush is rushed (see Figures 7b, 7c).
(a) Chip shape at the beginning of cutting of a new tool (100×)
(b) Chip shape at lower temperature after tool wear (100×)
(c) Chip cutting after cutting and cutting at high temperature (80×)
Figure 7b and 7c: Figure 7b is the starting chip shape of the blade after the wear measurement and the second cutting (lower workpiece temperature); the chip is straight and elongated, followed by the cutting time; after the cutting temperature rises; It becomes the spiral zigzag shape of Figure 7c. Because the cutting temperature has changed in this process; the cutting parameters have not changed; therefore, I think that the temperature change has a certain influence on the chip shape: followed by the increase of cutting temperature The shear deformation of the chip increases; the pitch and the tooth height of the sawtooth chip will be changed. The detailed influence of the temperature element on the chip remains to be further studied.
3 Conclusion
1) Hardened steel with CB20 tool (HRC>60); the cutting process is relatively smooth and the tool has long service life; it is the augmentation tool for hard and brittle materials such as hardened steel. It is recommended to use the following cutting amount: cutting speed Vc ≥140m/min; back-feeding knife amount ap=0. 2mm; feed rate f=0.1mm/r.
2) The CB20 tool has a small area of ​​wear before and after the tool face and is mainly located near the tool tip and the secondary chamfer; the wear edge is mainly mechanical wear, oxidative wear, phase change wear and bond wear; their effect together constitutes the tool abrasion.
3) CB20 tool has particle drop and micro-cracking edge in the wear area; therefore, it is necessary to select the cutting amount reasonably in the processing; improve the raw material of the workpiece data; select the machine tool with good rigidity; pay attention to the oscillation of the tool workpiece process system.
4) Machining is a dynamic process; under the same cutting parameters; the shape of the chip will change with the wear of the tool; the reason and mechanism of the influence on the shape change of the chip need to be further studied.
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Discussion on Wear Mechanism of Dry Cutting Hardened Bearing Steel
Source: Bearing network time: 2018-04-10