Research on Characteristics and Application of Hard Turning
Foreword
Turning is one of the most basic, broadest and most important processes in the machinery industry. It directly affects production efficiency, cost, energy consumption and environmental pollution. Due to the development of modern science and technology, various high-strength and high-hardness engineering materials are increasingly used. Traditional turning technology is difficult or impossible to realize the processing of certain high-strength and high-hardness materials. Modern Hard turning technology makes this possible and delivers significant benefits in production.
1 Hard turning and its characteristics
1) Definition of hard turning
The so-called hard turning refers to the turning of hardened steel as the final machining or finishing process, so as to avoid the grinding technology currently in common use. Hardened steel generally refers to a workpiece material having a martensite structure after quenching, high hardness, high strength, and almost no plasticity. When the hardness is HRC>55, the strength sb=2100-2600 MPa. Usually, the workpiece is roughed before the heat treatment is hardened, and only the finishing is performed in a hardened state. Fine grinding is the most commonly used processing technology for finishing, but its processing range is narrow, investment is large, production efficiency is low, and it is easy to cause environmental pollution, which has been plaguing the economical and efficient processing of hardened steel. With the development of processing technology, hard turning has become possible instead of grinding, and has achieved significant benefits in production. At present, the hardened steel (hardness HRC55-65) is machined on a lathe or turning center using polycrystalline cubic boron nitride (PCBN) tools, ceramic tools or coated carbide tools. The machining accuracy is up to 5 ～10 μm, the root mean square value of the surface roughness is less than 20 μm on average.
　 2) Features of hard turning
·High processing efficiency
Hard turning has higher processing efficiency than grinding, and it consumes 1/5 of the energy of ordinary grinding. Hard turning often uses large cutting depths and high workpiece rotation speeds, and the metal removal rate is usually 3-4 times that of grinding. In the turning process, a variety of surface processing (such as outer circle, inner hole, and trough) can be completed in one clamping, and grinding requires multiple installations. Therefore, the auxiliary time is short and the positional accuracy between surfaces is high.
·Hard turning is a clean processing process
In most cases, hard turning does not require coolant. In fact, the use of coolant can adversely affect tool life and surface quality. Because hard turning is performed by annealing the material of the shearing portion to soften, if the cooling rate is too high, the effect caused by the cutting force is reduced, thereby increasing mechanical wear and shortening tool life. At the same time, hard turning can save the coolant-related devices, reduce production costs, simplify the production system, and form chips that are clean and easy to recycle.
· Low equipment investment, suitable for flexible production requirements
When the productivity is the same, the lathe investment is 1/3 to 1/20 of the grinding machine, and the cost of the auxiliary system is also low. For small batch production, hard turning does not require special equipment, while high-volume machining of high-precision parts requires CNC machines with good rigidity, positioning accuracy and high repeatability.
The lathe itself is a flexible processing method with a wide processing range. The workpiece clamping is fast. It is easy to realize the conversion between two different workpieces by using a modern CNC lathe with multiple tool turntables or magazines. Hard turning is especially suitable for this. Class processing. Therefore, hard turning is better suited to flexible production requirements than grinding.
·Hard turning enables good overall machining accuracy of parts
Most of the heat produced in hard turning is carried away by the chips, which does not cause surface burns and cracks like grinding. It has excellent surface quality and precise roundness, which ensures high positional accuracy between the machined surfaces. .
2 Conditions for hard turning
1) Hard turning tool materials and their selection
·Coated carbide
The coated cemented carbide tool is coated with one or more layers of wear-resistant TiN, TiCN, TiAlN and Al2O3 on the toughness of the hard alloy tool. The thickness of the coating is 2-18 μm. The coating is usually It plays the following two aspects: 1 has a much lower heat transfer coefficient than the tool base and the workpiece material, which weakens the thermal action of the tool base; 2 can effectively improve the friction and the bounce effect of the cutting process, and reduce the generation of cutting heat . Compared with cemented carbide tools, coated carbide tools offer significant improvements in strength, hardness and wear resistance. For turning HRC 45-55 workpieces, low-cost coated carbide tools enable high-speed turning. In recent years, some manufacturers have greatly improved the properties of coated tools by improving the coating materials and proportions. For example, some manufacturers in the United States and Japan use Swiss AlTiN coating materials and new coating patented technology to produce blades with HV hardness of 4500 to 4900. When the turning temperature is as high as 1500 to 1600 °C, the hardness is not reduced or oxidized, and the blade life is generally The coating blade is 4 times, and the cost is only 50%, and the adhesion is good. It can process die steel with a hardness of HRC 47-52 at a speed of 498.56 m/min.
·Ceramic material
Ceramic tools have high hardness (hardness HRA91 ~ 95), high strength (bending strength of 750 ~ 1000MPa), good wear resistance, good chemical stability, good anti-knot performance, low friction coefficient and low price. When used normally, the durability is extremely high, and the speed can be increased by 2 to 5 times than that of cemented carbide. It is especially suitable for high hardness material processing, finishing and high speed machining. It can process all kinds of hardened steel and hardened cast iron with hardness HRC62. Commonly used are alumina-based ceramics, silicon nitride-based ceramics, cermets, and whisker toughened ceramics. In recent years, through a large number of research, improvement and adoption of new production processes, the flexural strength and toughness of ceramic materials have been greatly improved, such as the new cermet NX2525 developed by Mitsubishi Metal Corporation of Japan and developed by Sandvik, Sweden. The new CT series and coated cermet blade series of cermet inserts have a grain structure diameter of less than 1μm, and the bending strength and wear resistance are much higher than ordinary cermets, which greatly expands the application range of ceramic materials. The silicon nitride ceramic material tool successfully developed by Tsinghua University has also reached the international advanced level.
·CBN
CBN is second only to diamond in hardness and wear resistance. It has excellent high temperature hardness. Compared with ceramic tools, its heat resistance and chemical stability are slightly worse, but impact strength and crush resistance are better. It is widely used for the cutting of hardened steel (HRC50 or higher), pearlitic gray cast iron, chilled cast iron and superalloy, and its cutting speed can be increased by an order of magnitude compared with cemented carbide tools.
PCBN with high CBN content has high hardness, good wear resistance, high compressive strength and good impact toughness. Its shortcomings are poor thermal stability and low chemical inertness. It is suitable for the cutting of heat-resistant alloys, cast iron and iron-based sintered metals. . The composite PCBN cutter has a low content of CBN particles and uses ceramic as a binder. Its hardness is low, but it compensates for the shortcomings of poor thermal stability and low chemical inertness of the former material, and is suitable for the cutting of hardened steel.
In the field of cutting gray cast iron and hardened steel, ceramic tools and CBN tools are available for simultaneous selection, so cost-effective and processing quality analysis is necessary to determine which material is more economical. Ceramic tools are a good choice by analyzing workpieces with cutting hardness below HRC60 and small feed rates. PCBN tools are suitable for workpiece hardnesses higher than HRC60, especially for automated machining and high precision machining. In addition, the residual surface stress of the workpiece after cutting the PCBN tool is relatively stable compared to the ceramic tool under the same flank wear.
The use of PCBN tools for dry-cut hardened steel should also follow the following principle: Select a large depth of cut as far as possible under the rigidity of the machine tool, so that the heat generated in the cutting zone softens the metal in the front of the blade, which can effectively reduce the wear of the PCBN tool. PCBN tools should also be considered for small depth of cut. The poor thermal conductivity makes the heat in the cutting zone less than possible. The shear zone can also produce a significant metal softening effect and reduce the wear of the cutting edge.
2) Determination of blade structure and geometric parameters
Proper determination of blade shape and geometry parameters is critical to maximizing tool cutting performance. According to the tool strength, the blade tip strength of various blade shapes is from high to low: circular, 100° diamond, square, 80° diamond, triangle, 55° diamond, 35° diamond. After the blade material is selected, the blade shape with the highest possible strength should be used. Hard turning inserts should also be selected with the largest possible tool nose arc radius, roughed with round and large radius inserts, and the tool nose radius during finishing is 0.8 to 1.2 μm.
The hardened steel chips are red and soft forging strips, which are brittle, easy to break and not erect, generally do not produce built-up edge on the cutting surface, and the surface quality of the processing is high, but the hardened steel has a large cutting force, especially The radial cutting force is larger than the main cutting force, so the tool should adopt a negative rake angle (γ0≥-5°) and a large relief angle (α0=10～15°). The lead angle depends on the rigidity of the machine tool. Take 45 to 60° to reduce workpiece and tool flutter.
3) Selection of cutting parameters
The higher the hardness of the workpiece material, the smaller the cutting speed should be. The suitable cutting speed for hard turning finishing is 80-200m/min, and the common range is 10-150m/min. The cutting depth should be kept at 80-100m/min with large depth or strong intermittent cutting of high hardness material. In general, the depth of cut is 0.1 to 0.3 mm. For processing surface roughness, a small cutting depth can be selected when high, but it should not be too small and suitable. The feed rate can usually be selected from 0.05 to 0.25 mm/r, depending on the surface roughness value and productivity requirements. When the surface roughness is Ra 0.3 to 0.6 μm, hard turning is much more economical than grinding.
4) Requirements for the process system
In addition to selecting a reasonable tool, hard turning has no special requirements for lathes or turning centers. If the lathe or turning center is rigid enough and the required precision and surface roughness can be obtained when machining soft workpieces, it can be used for hardened steel. Processing. In order to ensure smooth and continuous turning operations, the usual method is to use rigid clamping devices and medium rake angle cutters. It is generally accepted that hard turning requires a highly rigid lathe, that is, the key to hard turning is that the machine has sufficient rigidity, while the tool, workpiece, and clamping device are compact and equally rigid. If the workpiece is positioned, supported and rotated under the cutting force, it can be kept fairly stable, and the existing equipment can be used for hard turning.
3 Application of hard turning technology
After 10 years of development and promotion, hard turning technology has achieved great economic and social benefits. The following is an example of the industry such as roll processing to illustrate the promotion and application of hard turning technology in production.
1) Roll processing industry
More than a dozen large-scale roller enterprises in China have used hard turning technology to carry out cutting and roughing, roughing and finishing of various types of rolls such as chilled cast iron and hardened steel, and all have achieved good results. The average processing efficiency is increased by 2 to 6 times, and significant benefits of saving processing time and power of 50% to 80% have been achieved. For example, in the rolling mill of Wuhan Iron and Steel Company, when the chilled cast iron rolls with hardness of HS60-80 are rough and semi-finished, the cutting speed is increased by 3 times, and one roll per car saves electricity and labor costs by more than 400 yuan, saving tool costs. Nearly 100 yuan, has achieved huge economic benefits. For example, the Electromechanical Experiment Center of Weifang University used the FD22 cermet tool to turn the 86CrMoV7 hardened steel roll of HRC58~63 (v=60m/min, f=0.2mm/r, cutting depth ap=0.8mm), single-blade continuous cutting roll The path is up to 15000m (VCmax=0.2mm), which meets the requirements of fine grinding.
2) Industrial pump processing industry
At present, 70% to 80% of domestic pulp pump production plants have adopted hard turning technology. The slurry pump is widely used in mining, electric power and other industries. It is an urgently needed product at home and abroad. Its sheath and shield are Cr15Mo3 high-hard iron castings with hardness HRC6367. In the past, it was difficult to turn it by various tools, so it was necessary to use a process of annealing, softening, and then quenching. After the hard processing technology, the problem of hardening processing was solved smoothly, and the two processes of annealing and quenching were eliminated, saving a lot of man-hours and electricity.
3) Automotive processing industry
In the high-volume production industries such as automobiles and tractors, the machining problems of quenching hardware are often encountered in the crankshaft, camshaft and drive shaft, knife measuring industry and equipment maintenance. For example, in a locomotive and vehicle factory in China, the inner ring of the bearing needs to be processed in the maintenance of the equipment. The hardness of the inner ring of the bearing (material GCr15) is HRC60, the diameter of the inner ring is 285mm, and the grinding process is used. The grinding allowance is uneven, requiring 2h. It can be ground well; with hard turning, an inner ring is machined in only 45 minutes.
4 Conclusion
After years of research and exploration, China's hard turning technology has made great progress, but the application of hard turning technology in production is still not extensive. The main reasons are: (1) manufacturers and operators have insufficient understanding of the effect of hard turning. It is generally believed that hard materials can only be ground; (2) the cost of tools is too high. The initial tool cost of hard turning is more expensive than ordinary cemented carbide (such as CBN is more than ten times more expensive than ordinary hard alloy), but its cost on each part is lower than that of grinding, and the effect is harder than ordinary The quality of the alloy is much better; (3) insufficient research on the mechanism of hard turning; (4) the specification of hard turning is not sufficient to guide production practice. Therefore, in addition to the in-depth study of the hard turning mechanism, it is necessary to strengthen the training of hard turning machining knowledge, successful experience demonstration and strict operating specifications, so that this efficient and clean processing method is more applied to production practice. At present, if hard turning and fine grinding are combined, the cost of processing a general part will be 40% to 60% lower than the cost of roughing and finishing the grinding machine.
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