Characteristics and cutting application of polycrystalline cubic boron nitride cutting tools

Published: 2020-06-20 Publisher: FUNIK R&D Center

1.Characteristics of polycrystalline cubic boron nitride (PCBN) cutting tool

PCBN cutting tool is made of CBN grains by high temperature and high pressure sintering with or without binder. Its structure is composed of integral sintering type and composite sheet with cemented carbide matrix. Because PCBN is mainly made of CBN, PCBN cutting tool has all kinds of excellent properties of CBN. Its performance index is shown in Table 1, and its main characteristics are as follows:

Table 1 Mechanical properties of various cutting tool materials

 

Material properties

Cemented carbide (K10)

Vitrifieds

PCBN

PCD

Mechanical properties

Hardness HV

1500~1700

1800~2500

3000~5000

4000~6000

Young's modulus

590~630

300~400

580~680

680~810

Breakage toughness MPa

10.8

2.0~3.0

3.7~6.3

6.8~8.8

Thermal characteristics

Heat resistance ℃

800~1000

>1200

1400-1500

700-800

thermal conductivity W/(K·M)

17-125

20-29

40~100

500-2000

10-6 J/℃

Coefficient of thermal expansion 10-6 J/℃

4-9

6.3-9

2.1-4.8

0.9-4.8

(1)High strength and wear resistance

The crystal structure of polycrystalline cubic boron nitride is similar to that of diamond, and its hardness and compressive strength are close to that of diamond. In cutting with PCBN cutting tool, its wear resistance is much higher than that of cemented carbide cutting tools and vitrified cutting tools, and can be used for machining tough cast iron and steel or other alloy materials with high strength, high hardness and high thermal sensitivity.

(2)Excellent chemical stability

CBN has high chemical stability and will not react with iron materials when the temperature reaches 1200-1300 ℃, and CBN will not be oxidized at 1000 °C, so the chemical wear of PCBN cutting tools is smaller than that of other tool materials in cutting, so it can be used to process many kinds of materials.

(3)Good thermal conductivity and low friction coefficient

The thermal conductivity of CBN is much higher than that of high speed steel, vitrifieds and cemented carbide, and the thermal conductivity of PCBN cutting tools increases with the increase of temperature. This makes the temperature of cutting tip not too high in cutting, which is beneficial to improve the tool durability and prolong the service life of the tool. When CBN interacts with other materials, the friction coefficient is about 0.1-0.3, which is much smaller than that (0.4-0.6) between cemented carbide and other materials, so that the cutting force and cutting temperature are reduced, the tool sticking phenomenon is reduced in cutting, the chip accumulation is effectively avoided, and the surface machining quality of the workpiece is improved.

2.Application of polycrystalline cubic boron nitride (PCBN) cutting tool

PCBN cutting tools are mainly used for machining difficult-to-machining materials and high-hardness materials (> HRC35), such as tool steel, high-speed steel, hardened steel, austenitic soft iron, wear-resistant materials such as gray cast iron, sintered iron, etc. Difficult-to-machining materials such as surface coating material, surface hardening alloy, nodular cast iron, etc., and the PCBN cutting tool can replace the traditional grinding process, and the precision after machining can be as high as IT5-IT6, surface roughness can reach Ra0.8~ 0.2 μm, which can achieve the precision of grinding.

2.1Machined gray cast iron

High-speed cutting gray cast iron material is one of the typical applications of PCBN cutting tools. When high-speed cutting gray cast iron is machined by PCBN cutting tools, the tool life can reach 20-30 times that of vitrified cutting tools. For different grades of PCBN cutting tools, the higher the CBN content, the higher the hardness, wear resistance and thermal conductivity of PCBN cutting tools. When the ferrite content in the gray cast iron material is less than 10%, it is more suitable for the cutting of the PCBN cutting tool, and the life of the PCBN cutting tool is longer, because when there is too much ferrite, it will react with the boron element in the CBN at high temperature during cutting, thus increasing the chemical wear of the PCBN cutting tool and reducing the service life of the tool. In the cutting of gray cast iron, one aspect that needs extra attention is timeliness, that is, time from casting to cutting with PCBN cutting tool; this is because during casting, gray cast iron will produce a lot of oxygen and nitrogen, which will be harmful to the durability of PCBN cutting tools, so it takes a period of time to let oxygen and nitrogen volatilization, to ensure the normal service life of the tool. The general aging days are 10 days.

2.2Machined hardened steel

Hardened steel is a typical difficult-to-machining and wear-resistant material. Grinding has been used to finish the finishing of hardened steel parts. When hardened steel is machined with PCBN cutting tools, the machining efficiency is faster than grinding, and it is more suitable for cutting parts of different shapes and more flexible than grinding. Because PCBN cutting tools often use dry cutting, it can also avoid the pollution of cutting waste liquid and save energy. The quality of the machined surface is the same as that of grinding, or even higher than that of grinding. In the process of high-speed cutting hardened steel, the high heat generated by the PCBN cutting tool softens the cutting area of the workpiece material continuously, so as to carry out cutting. Compared with the cemented carbide cutting tool, the efficiency is greatly improved, and the surface roughness of the workpiece can be improved and the machining accuracy can be improved.

2.3Difficult-to-machining material

Relatively speaking, the application and advantages of PCBN cutting tools in machining nodular or vermicular cast iron are relatively small. In the process of machining nodular or vermicular cast iron, PCBN cutters are easy to produce serious chemical wear and reduce tool life. Therefore, when PCBN cutting tools have to be used to ensure the dimensional accuracy and geometric tolerance of parts, they are usually coated to protect PCBN cutting tools and increase their durability. In addition to the above typical applications, PCBN cutting tools can also be used for cutting nickel-base superalloys, surface hardening alloys, cemented carbide, vitrifieds, powder metallurgy and other materials. It has good advantages in machining efficiency, workpiece dimensional accuracy, surface roughness and tool durability, and has been widely used.

3.Conclusion

PCBN cutting tool has the advantages of high machining efficiency, long tool life and low machining cost. It has been widely used in the field of ferrous metal processing of gray cast iron and hardened steel. At the same time, it can also carry out high-speed machining of superalloy, thermal spraying materials, cemented carbide and other difficult-to-machining materials, which can realize turning instead of grinding, and greatly improve the machining efficiency. The application of PCBN cutting tool solves the difficult problem of hard cutting of ferrous metal, which is of great significance for promoting high-speed machining technology.

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