Research on Cutting Engineering Technology of High Hard Cast Iron

Research on Cutting Engineering Technology of High Hard Cast Iron

• Admin
• 2024/12/7

ABSTRACT There are two typical high hardness cast iron, including chilled cast iron and anti-wear alloy cast iron. Withcomprehensive properties of high temperature resistance, wear resistance, corrosion resistance, low temperature resistanceand demagnetization, the selection of the cutting tool for the cast iron is greatly restrained.

However, it is imminent to realizethe high quality processing of these two typical high hardness materials with the development of high efficiency cuttingtechnology for parts and components.The research status of cast iron processing technology was analyzed, It is found that theresearch of cast iron cutting technology mainly focuses on the cutting or grinding technology of cemented carbide, PCBN,ceramics and other cutting tools.

Then, from the three aspects of blade structure, blade material and cutting parameters, thetechnical measures to achieve high efficiency processing of high hardness cast iron were discussed. The recommendedworking angle rake of the cemented carbide cutting tools is 0° ~5° , cutting edge angle is less than or equal to 45° andinclination angle is less than or equal to -6°. At the same time, a special chip breaking groove was used in combination with medium cutting parameters (the judgment standard is the processing form). Finally, the matching geometric parameters andcutting parameters were proposed for different blade selection. The research can provide a reference for high efficiencyprocessing of high hardness cast iron.

Both chilled cast iron and anti-wear alloy cast iron are typical high-hardness, difficult-to-machine materials, and are commonly used as anti-wear materials for industrial parts. Chilled cast iron, also known as “chilled cast iron”, is characterized in that the whitening structure can be obtained by adjusting the chemical composition during the casting process, and the rest is gray iron and tinplate structure, so it not only has High hardness and good wear resistance, but also has excellent fracture toughness characteristics. Typical chemical composition of chilled cast iron is: 3% to 3.5% C, 0.5% to 0.7% Si, 0.5% to 0.7% Mn, and P content ≤0.4%, S content ≤0.07%. In industrial production, chilled cast iron is often used to make rolls, train wheels, running wheels and shredder components.

Anti-wear alloy cast iron, also known as “special performance cast iron”, refers to cast iron with excellent anti-wear properties by adding other element additives during the casting process. Among them, typical element additives are: Si, Mn, P, Ni, Cr, Mo, Cu, Al, B, V, Ti, Ba, Sn, etc. Additives can make the cast iron matrix change in structure during the casting process, thereby obtaining excellent comprehensive properties such as high temperature resistance, wear resistance, corrosion resistance, demagnetization and low temperature resistance. In terms of industrial production, wear-resistant alloy cast iron is widely used in the manufacture of key components in the aerospace, land and sea transportation equipment and energy industries.

1 Machinability of High-Hardness Cast Iron

In the actual production process, the machining of chilled cast iron and wear-resistant alloy cast iron workpieces is usually mainly rough machining, and its processing characteristics are mainly reflected in the following two aspects:

(1) High hardness and strength:

chilled cast iron and wear-resistant alloy cast iron are typical hard and brittle materials, and their surface hardness is relatively high (usually up to HRC50~60). During the cutting process, the hard points of cast iron and casting defects are prone to form irregular impact loads, which can easily cause premature blade failure.

(2) Heavy mechanical load during cutting:

the cutting pressure of chilled cast iron and wear-resistant alloy cast iron during cutting can reach more than 3 000 MPa, and the load during the cutting process will continue to change, and the blade is always subjected to alternating frequency loads Impact, so the blade is prone to premature fatigue.

2 Overview of Research on High-Hardness Cast Iron Cutting

At present, Chinese and foreign scholars have rich experience and data on the machining of cast iron, especially in the processing technology and the failure mechanism of CNC tools.

In China, Su Guosheng and Guo Yan studied the effect of cutting vermicular graphite cast iron on the tool wear under high-pressure cutting fluid. The research shows that different jet directions spray high-pressure coolant on the flank face than on the rake face. Anti-wear performance is more beneficial;

Lin Yongchuan, He Fang et al. proposed measures to improve the machinability of vermicular graphite cast iron by its difficult-to-machine characteristics and the limitations of traditional cutting methods; Du Maohua and Wang Junhua studied the cutting parameters of high-speed milling cast iron alloys The cutting force and the influence mechanism of the surface finish of the workpiece can obtain the optimal cutting parameter combination with milling instead of grinding;

Ruan Jingkui, Gong Aihong and others conducted high-speed compression experiments on alloy cast iron and analyzed their mechanical properties in the “three high” state, thereby studying the blade-chip friction model and heat transfer equation, and finally established a high-speed molybdenum-chromium alloy cast iron Finite element model of cutting;

Zhang Peng, Du Jin, Yi Mingdong and others studied and analyzed the influence of coating materials and matrix materials on the cutting performance of ceramic tools during the cutting process; In addition, Song Zongyi and Zhao Heng studied the carbide tools through cutting experiments The failure mechanism of the tool in the process of machining the anti-wear alloy cast iron MT-4.

Foreign scholar Adilson José de Oliveira and others studied the effects of PCBN tool grades and cutting methods on high-chromium white cast iron hard turning tools, and determined that two grades of PCBN tools (high CBN and low CBN content, with ceramic phase added) are continuous and intermittent The performance of cutting high chromium white cast iron. An evaluation method for tool life, tool edge wear mechanism, roughness and residual microstructure on the machined surface was proposed; Rosemar Batista Da Silva and Mariana Landim Silveira Lima used two SiC wheels to match three cast iron grades (gray cast iron, ductile iron, Vermicular graphite cast iron), the outer peripheral surface was subjected to the grinding test, and the surface and sub-surface quality evaluation techniques of the three cast iron grades ground under different conditions were obtained;

Luiz Roberto Muñoz Dias and Anselmo Eduardo Diniz studied the impact of gray cast iron microstructure on milling cutter life and cutting force. The two scholars used two different materials as samples, one containing 100% pearlite and the other containing 50% pearlite and 50% ferrite. The results show that, compared with 50% ferrite alloys, 100% pearlite alloy milling tools wear faster and have higher cutting forces. Ceramic tools have a longer life than carbide tools. The observed wear mechanisms are diffusion, wear and thermal cracking [15]; Nicolae-Valentin Ivan, Cristina Gavrus, Gheorghe Oancea proposed a new method to determine the cutting depth related to roughing, semi-finishing and finishing, in order to achieve The purpose of gradually eliminating machining surface errors to ensure the required dimensional accuracy.

The analysis of related research at home and abroad shows that the machining of cast iron is mainly concentrated on cemented carbide tools, PCBN tools, CBN tools and ceramic tools. The turning, milling or grinding process and its failure mechanism are other aspects. Hard cast iron and anti-wear alloy cast iron are not perfect enough in the systematic exploration of engineering technology for efficient cutting inserts. Therefore, it is of great practical significance to discuss this aspect.

3 Engineering Technology

According to the cutting characteristics of the two workpiece materials of chilled cast iron and wear-resistant alloy cast iron, the three aspects of the blade structure, blade material and cutting parameters are discussed, so as to summarize the key engineering technology to realize the efficient cutting of these two materials.

3.1 Blade structure

Because of the high mechanical strength of chilled cast iron and wear-resistant alloy cast iron, in order to improve the overall strength of the blade, a negative working rake angle and a small working rake angle are usually used. According to the principle of workpiece-knife-material matching, different blade materials correspond to different working rake angles, such as: the working rake angle of carbide GK05 is recommended to be 0°~5°, and the working rake angle of ceramic blades is recommended to be -5° ~-10°, the working rake angle of PCBN blade is recommended to be 0°~-10°. Similarly, in order not to affect the working strength of the blade, after actual production and application research: it is recommended to use a negative design for the indexable insert for cutting (the blade has no back angle).

The so-called negative design means that the front face and back face of the blade are at an angle of 90° (as shown in Figure 1), which can maximize the working wedge angle of the blade and indirectly make up for the insufficient performance of the blade material. Therefore, not only a small working clearance angle is ensured, but also the mechanical strength of the blade itself. In order to reduce the mechanical impact load and thermal load that the blade bears during the cutting process and avoid cutting vibration, a smaller main declination angle and negative working blade inclination angle are used. Generally, the main declination angle Kr≤45° and the blade inclination angle λ≤-6° are selected.

For PCBN and ceramic inserts, the cutting edge prefers the negative chamfer type and the chamfering structure parameters are recommended: chamfering angle -20°~-30°, chamfering width 0.2~0.4 mm. In the process of cast iron cutting, although the chips are pop-up type, in order to reduce the instantaneous elastic effect during the chip breaking process, the chip should be designed with a special chipbreaker to improve the chip breaking performance of the blade and the surface finish of the parts.

Figure 2 is a practical example of the structural design of the indexable carbide cast iron car blade. It can be seen from the enlarged view of the cutting function area: for the processing characteristics of chilled cast iron and wear-resistant alloy cast iron, the blade adopts a combination design method of wide edge, large chip flute and shallow groove depth, which can realize high feed processing and stable cutting, optimization Chip evacuation performance; In addition, the design size of the cutting edge is larger, the strength of the blade tip has been improved, and it is more suitable for intermittent cutting and descaling (as shown in Figure 2(a)).

Mark the blade CNMG120408-HK GK1115 shown in Figure 2(a) as “Insert A” and the conventional cast iron turning blade (flat type blade) CNMA120408-GK1115 shown in Figure 2(b) as “Blade B” performs numerical simulation of chilled cast iron cutting under the same conditions, where the cutting conditions are as follows:

• Workpiece material: LTCrMoR;
• Cutting parameters: vc=400 m/min, ap=1.5 mm, f=0.25 mm/r;
• Tool brand: GK1115;
• Cutting method: dry turning.

Fig. 3, Fig. 4 and Fig. 5 are turning simulation analysis cloud diagrams of two kinds of inserts, which are mainly compared and analyzed from three aspects of cutting pressure, cutting temperature and chip shape.

From Fig. 3(a) cutting pressure cloud, we can see that the cutting pressure distribution in the cutting area of ​​insert A is more uniform than that of insert B and the stress field extension area is distributed along the cutting edge, which shows the decomposition of cutting pressure of insert A groove shape. The effect is better than the tablet. It is proved that structural optimization can increase the mechanical strength of the tool to a certain extent, thereby making up for the lack of mechanical properties of the tool material.

Figure 3(b) is the cutting stress curve of the tool. The cutting stress growth trend of insert A and insert B is consistent, but the cutting stress of insert A is about 1 000 MPa lower than that of insert B, which further illustrates the three-dimensional geometry. Plays a good role in improving the structural strength of the tool.

Figure 4 shows a comparison of the cutting temperatures of the two inserts. From the analysis of Fig. 4(a), it can be seen that the temperature distribution of the cutting area of ​​insert A is smaller than that of insert B and the transition of the temperature difference zone of the former is more obvious than that of the latter, that is, the temperature fluctuation is smaller. In addition, the high temperature area of ​​insert A is evenly distributed along the cutting edges involved in cutting, which proves that the three-dimensional groove structure plays a certain role in promoting heat conduction and can effectively weaken the small-scale accumulation of cutting heat. Fig. 4(b) shows the cutting temperature distribution curve of the two inserts in the steady cutting process. The growth trend of this curve further illustrates the stability of the temperature distribution of the cutting area of ​​insert A.

Figure 5 is a comparison chart of the chip morphology of the two blades (due to the material database of the simulation system, the chips generated are not powder or short chip).

It Can Be Seen From the Graph Analysis:

• Under the same cutting conditions, the chip shape produced by insert A is short and tightly curled, with a long curvature and a C shape;
• However, the chip shape produced by the blade B is a short piece, and the squeeze wrinkle inside the curved surface is very obvious. The main reason for the obvious difference in chip morphology is that the blade A has a three-dimensional chip breaking groove to promote the curling and bending of the chip, so the generated chip is longer and is mainly wound off;
• Because insert B does not have a chip breaker structure, the generation of chips is mainly extrusion, so the chip deformation is very serious, there are obvious compression wrinkles and it is shorter than the chips generated by insert A. Among them, short tight curling chips are the ideal form in the cutting process. During the generation of such chips, the cutting force of the tool is small, the difference in cutting temperature fluctuation is small, and the surface finish of the workpiece is high.

Based on the comprehensive simulation analysis results, the blade designed with the three-dimensional groove structure has less cutting stress, lower cutting temperature fluctuation and better chip breaking performance when cutting the chilled cast iron than the traditional flat blade. Therefore, it is more suitable for the efficient and high-quality processing of chilled cast iron and wear-resistant alloy cast iron.

3.2 Blade Material

In view of the cutting characteristics of chilled cast iron and wear-resistant alloy cast iron, the material of the special blade should have excellent resistance to high temperature and wear, high bending strength, large thermal conductivity, and strong anti-bonding. Performance. At present, the blade materials suitable for efficient cutting of these two materials are mainly: cemented carbide, PCBN and ceramics. Figure 6 shows the life curve of inserts of different materials when cutting chilled cast iron LTCrMoR under their optimal cutting parameters.

It can be seen from the analysis of the wear curve in Figure 6 that the uncoated GK05 cemented carbide blade has the largest slope of the wear curve, proving that the uncoated cemented carbide blade has the fastest wear failure when cutting chilled cast iron; lm≤1× at the same cutting distance In the case of 106 mm, the order of wear rate and wear is: uncoated cemented carbide GK05>composite Al₂O₃>pure high temperature ceramic Si₃N₄>composite high temperature ceramic Si₃N₄.

In recent years, in order to further reduce the limitations of tool materials for efficient cutting of chilled cast iron and wear-resistant alloy cast iron, tool materials with higher matching are introduced, such as PCBN and hard alloy with special coating. Compared with ceramic tools, PCBN has high red hardness, and its applicable cutting speed vc is higher, and it can obtain better processed surface quality. But when cutting chilled cast iron and wear-resistant alloy cast iron, not all PCBN can match it, the main reason is the size and content of CBN particles in the composition of PCBN, the type of binder (ceramic, Co or Ni-Ti Alloy, etc.) and the synthesis conditions have a great influence on the performance of the blade.

Studies have shown that: when the volume content of CBN in PCBN material is 40%~60%, it is suitable for cutting high-hard cast iron and hardened steel, and when the volume content of CBN is increased to about 90%, it is suitable for high-temperature alloy and cemented carbide, etc. Material cutting, but inserts with high CBN content have poor cutting performance.

Carbide with special coating mainly includes MC5015 of Mitsubishi of Japan, CA315 of Kyocera, GC3210 of Sandvik and GK1115 of Xiamen Jinlu. Among them, MC5015 of Mitsubishi of Japan is the most advanced, and GK1115 of Xiamen Jinlu is second. Taking GK1115 as an example (as shown in Figure 7), the substrate uses fine-grained WC cemented carbide, which has higher hardness and strong wear resistance. Therefore, when cutting cast iron, it is suitable for higher cutting speed vc; the transition layer uses TiCN to achieve a high-strength combination between the outer layer and the substrate, so that the blade has excellent resistance to wear and impact damage; the outermost layer is added Thick Al₂O₃ not only further improves the high temperature resistance of the blade, but also reduces the friction coefficient of the blade surface, and realizes the self-lubricating function of the blade during the cutting process.

With the rapid development of tool technology, the choice of cutting tools for chilled cast iron and wear-resistant alloy cast iron is increasing. Therefore, in the selection of tool material matching, the economic benefits of parts processing should also be considered, so as to achieve the goal of high efficiency performance of cutting processing, that is: high production efficiency, low processing cost and low energy consumption, and low pollution.

3.3 Cutting Parameters

The cutting parameters are selected according to the service life of the blade and the processing requirements of the workpiece. The feed rate f is the main factor to ensure the surface finish of the workpiece. Usually, f ≤ 0.5 mm/r is used when cutting high-hard cast iron; the cutting depth ap is the main factor to ensure that the number of passes is reduced as much as possible during roughing and semi-finishing. Cast iron cutting usually a_p ≤ 3 mm; cutting speed v_c is the main factor of cutting temperature during the cutting process is also one of the factors that need to be considered when applying ceramic blades and PCBN blades. The choice of cutting speed under normal conditions is usually v_c ≤ 400 m/min.

During finishing, the surface finish and geometric accuracy of the parts are the primary considerations for the selection of cutting parameters. Therefore, usually choose a small feed f, a higher cutting speed v_c, and a mid-lower cutting depth a_p; When finishing and roughing, the cutting efficiency needs to be considered, so on the premise of ensuring the strength of the blade, it is recommended to use large cutting parameters. Taking the grade of GK1115 indexable carbide insert as an example, the recommended cutting parameters are shown in Table 1.

In summary, in order to achieve efficient cutting of chilled cast iron and wear-resistant alloy cast iron, the first thing to consider is the matching of the blade structure, blade material, and cutting parameters with the workpiece structure, workpiece material, and precision requirements. Of course, in addition to the above three main factors, the factors that affect the efficient cutting of these two materials include machine tool system characteristics, cooling conditions, cutting paths, and cutting techniques. In the actual production process, due consideration should be given to the actual processing requirements.

4 Conclusion

In view of the processing characteristics of chilled cast iron and wear-resistant alloy cast iron, the engineering technology of indexable inserts was discussed. The main conclusions are as follows:

(1) Structural Aspects

In view of the characteristics of high hardness and strength of chilled cast iron and anti-wear alloy cast iron, and large cutting impact load, the overall structure of the indexable insert is recommended to adopt a negative design. At the same time, the appropriate working angle should also be selected according to the material of the blade, such as: the working rake angle of the carbide is recommended to be 0°~5°, the working rake angle of the ceramic blade is recommended to be -5°~-10°, the main The deflection angle Kr≤45° and the blade inclination angle λ ≤ 6°; the working front angle of the PCBN blade is recommended to be 0°~-10°, and the chamfering angle is -20°~-30°.

(2) Material

Efficient cutting of chilled cast iron and wear-resistant alloy cast iron requires that the blade material should have excellent performance in high temperature and wear resistance, high flexural strength, large thermal conductivity, and strong anti-bonding. Therefore, the high matching and good general performance are mainly hard alloy with special coating and PCBN with CBN volume content of 40% to 60%.

(3) Cutting Parameters

In the field of high-hardness cast iron processing, the comprehensive performance of the specially coated cemented carbide and PCBN is relatively similar, usually the cutting parameters are: a_p ≤ 3 mm, f ≤ 0.5 mm/r, v_c ≤ 400 m/min.

The actual processing should be selected according to the requirements. For example, when finishing, the surface finish and shape accuracy of the parts are mainly considered. It is advisable to choose a small feed f, a higher cutting speed v_c, and a mid-to-lower cutting depth a_p; In semi-finishing and roughing, the cutting efficiency and blade strength are mainly considered, and a larger cutting speed v_c, feed amount f and medium cutting depth a_p can be selected.

• TPKT Insert
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• VNGA160404S01525 A66N