1. Deformation factors of quenched parts
1. Causes of deformation of quenched parts:
1), carbon content and its influence on quenching deterioration;
2) Influence of alloying elements on quenching deformation;
3) Influence of original structure and stress state on heat treatment deformation;
4) The stress state of the workpiece itself before quenching has an important influence on the deformation. Especially for the workpiece with complex shape and large feed, the residual stress will have a great influence on quenching deformation if it is not eliminated. ;
5) Influence of workpiece geometry on heat treatment deformation;
6) Influence of process parameters on heat treatment deformation.
Second, the problems encountered in the processing of hardened steel
1, Rough machining of hardened steel:
In the production process of turning heat-treated gears and gear rings, the hardness of some gears and gear rings after quenching or carburizing quenching is generally above HRC55, and some hardness reaches HRC60 or even HRC65. Common gear materials: 20Cr, 20CrMnTi, 40Cr, 42CrMo, 35CrMo. Some gears are seriously deformed after heat treatment, especially the large gear ring after carburizing and quenching, such as high-speed rail gear, large gear ring of construction machinery and large gear ring for heavy industry. The deformation of these large gear rings after quenching is very large, which involves the rough machining of hardened steel. Similarly, in the production process of die steel, rough machining of hardened steel is often seen, but many manufacturers take various measures, some cut off the large allowance first, some chew it slowly with cemented carbide tools, and some finish it several times with CBN tools. Workers feel that rough machining of hardened steel is very difficult and there is no way.
2. Intermittent processing of hardened steel;
Intermittent cutting has always been a difficult problem, not to mention the hardened steel of HRC60. Especially when turning hardened steel at high speed, if the workpiece is cut intermittently, the tool will have to complete more than 100 times of impact machining every minute when cutting hardened steel intermittently, which is a great challenge to the impact resistance of the tool. Taking automobile gear processing as an example, it has become a trend to replace grinding with turning for hard gear. It is understood that as one of the three major markets in the gear industry, automobile gears account for 62% of the entire gear market, of which automobile gears account for 62% of the automobile gear market share. In other words, automobile gears account for nearly 40% of the whole gear market, which shows the importance of gears to the automobile industry. Although turning hardened steel instead of grinding and hard turning has become very popular, in fact, there are still many problems in the process of machining hardened gears in automobiles, such as oil holes in some automobile gears, which leads to intermittent cutting problems, and many CBN tools are prone to edge collapse when encountering oil holes at high speed, and the position tolerance of gears is difficult to guarantee.
3. Grooving hardened steel:
For a simple example, the hard turning of the meshing groove of the synchronizer gear sleeve after quenching, although the CBN tool manufacturer has developed a special CBN grooving tool for the synchronizer gear sleeve, the service life of the CBN tool is still not satisfactory.
4. Smoothness of hardened steel turning;
For example, the turning of bearing steel after quenching usually requires a good surface finish. Gcr 15 steel is a common bearing steel, and the hardness after quenching is generally around HRC62. In production and weighing, because of the high precision and smoothness of bearings, it is not impossible for the turning smoothness of hardened steel to reach Ra0.4 if the cutting edge of CBN tool is designed and adjusted.
Three, coated cemented carbide processing hardened steel
The coated cemented carbide tool is coated with one or more layers of TiN, TiCN, TiAlN and Al2O3 with good wear resistance. The thickness of the coating is 2 ~ 18 micron, and the coating usually has the following two functions: on the one hand, it has much lower thermal conductivity than the tool matrix and workpiece material, which weakens the thermal effect of the tool matrix; On the other hand, it can effectively improve the friction and adhesion in the cutting process and reduce the generation of cutting heat. Compared with cemented carbide tools, coated cemented carbide tools have been greatly improved in strength, hardness and wear resistance. For turning workpieces with dry hardness between HRC 45 and 55 HRC, low-cost coated cemented carbide can realize high-speed turning. In recent years, some manufacturers have greatly improved the performance of coating tools by improving the coating materials and proportions. For example, some manufacturers in the United States and Japan produce blades with Swiss AlTiN coating material and patented new coating technology, and the hardness is as high as 4500 ~4900HV. When the turning temperature is as high as 1500℃~ 1600℃, the hardness will not decrease and will not be oxidized. The service life of the blade is four times that of the common coated blade, but the cost is only 30%, and the adhesion is good. It can process die steel with hardness of 47 ~ 52 HRC at the speed of 498.56 m/min.
Four, ceramic materials processing hardened steel
Ceramic tools have the characteristics of high hardness (9 1 ~ 95 HRA), high strength (bending strength is 750 ~ 1000 MPa), good wear resistance, good chemical stability, good adhesion resistance, low friction coefficient and low price. In normal use, its durability is extremely high, and its speed is 2-5 times higher than that of cemented carbide. It is especially suitable for processing high-hardness materials, finishing and high-speed machining, and can process all kinds of hardened steel and hardened cast iron with hardness of 62HRC. Commonly used are alumina-based ceramics, silicon nitride-based ceramics, cermets and whisker toughened ceramics. In recent years, the bending strength and toughness of ceramic materials have been greatly improved through a lot of research, improvement and adoption of new manufacturing processes. For example, the new cermet NX2525 developed by Mitsubishi Metal Company in Japan and the new cermet blade CT series and coated cermet blade series developed by Kolaman Company in sandvik, Sweden, the grain size is as small as 1μm, and the bending strength and wear resistance are much higher than ordinary cermet, which greatly broadens the application scope of ceramic materials. The silicon nitride ceramic tool successfully developed in Tsinghua University has also reached the international advanced level.
Five, cubic boron nitride tool (CBN tool) processing hardened steel.
The hardness and wear resistance of cubic boron nitride are second only to that of diamond, and it has excellent high temperature hardness. Compared with ceramic tools, its heat resistance and chemical stability are slightly worse, but its impact strength and crushing resistance are better. It is widely used for cutting hardened steel (above 50HRC), pearlite gray cast iron, chilled cast iron and superalloy, and its cutting speed can even be increased by one order of magnitude compared with cemented carbide tools.
CBN tool with high cubic boron nitride content has high hardness, good wear resistance, high compressive strength and good impact toughness, but its disadvantages are poor thermal stability and low chemical inertia, and it is suitable for cutting heat-resistant alloys, cast iron and iron-based sintered metals. The content of cubic boron nitride particles in the composite P cubic boron nitride tool is low, and the hardness of the ceramic binder is low, but it makes up for the shortcomings of the former material, such as poor thermal stability and low chemical inertia, and is suitable for cutting hardened steel.
In the application field of cutting gray cast iron and hardened steel, ceramic tools and cubic boron nitride tools can be selected at the same time, so it is necessary to analyze the cost-effectiveness and processing quality to determine which material is more economical. When dry cutting Si3N4 hardened steel, the cutting performance of P cubic boron nitride tool material is better than that of Al2O3, and the cost of Al2O3 ceramics is lower than that of cubic boron nitride material. Ceramic tools have good thermochemical stability, but their toughness and hardness are not as good as cubic boron nitride tools. Ceramic tools are a good choice when cutting workpieces with hardness below 6OHRC and small feed. Cubic boron nitride tool is suitable for the case that the hardness of workpiece is higher than 60HRC, especially for automatic machining and high precision machining. In addition, under the same amount of flank wear, the residual stress on the workpiece surface after cutting by cubic boron nitride tool is relatively stable than that of ceramic tool.
Dry cutting hardened steel with cubic boron nitride cutter should also follow the following principles: when the rigidity of the machine tool allows, the cutting depth should be selected as large as possible, so that the heat generated in the cutting area can locally soften the metal in the front area, which can effectively reduce the wear of cubic boron nitride cutter. In addition, cubic boron nitride tools should be used as much as possible for small cutting. Because of the poor thermal conductivity of cubic boron nitride tool, the heat in the cutting area can not be diffused, and the shear area can also produce obvious metal softening effect, reducing the wear of the cutting edge.
Six, the cutting amount of hardened steel
The cutting parameters for cutting hardened steel are mainly selected according to the tool material, the physical and mechanical properties of the workpiece material, the shape of the workpiece, the rigidity of the process system and the machining allowance. When selecting the three elements of cutting parameters, we should first consider the reasonable cutting speed, cutting depth and feed.
(1) Wire cutting speed of quenched steel: generally, the heat resistance of quenched steel is 200℃ ~ 600℃, that of cemented carbide is 800℃ ~ 1000℃, and that of ceramic tool is1100℃ ~1200℃. Except for high-speed steel, the hardness of hardened steel generally begins to decline when it reaches about 400℃, while the above tool materials still maintain their original hardness. Therefore, when cutting hardened steel, we should make full use of the above characteristics, and the cutting speed should not be too low or too high to keep the tool durable. From the current experience, the cutting speed of different tool materials for cutting hardened steel is VC = 30 ~ 75 m/min; Ceramic tool VC = 60 ~120m/min; Cubic boron nitride cutter VC =100 ~ 200m/min. Intermittent cutting, when the hardness of workpiece material is too high, the cutting speed should be reduced, which is generally about 1/2 of the upper and lower cutting speed. The best cutting speed of continuous cutting is that the chips are dark red.
(2) Cutting depth: generally, it is selected according to the machining allowance and the rigidity of the process system. In general, α p = 0. 1 ~ 3 mm. At present, the tool material for cutting hardened steel with large allowance is KBN700, and the cutting depth reaches 7- 10 mm. In view of the problems that the machining allowance of hardened steel is difficult to control and needs annealing and reprocessing, a satisfactory tool selection is given.
(3) Feed rate: generally 0.05 ~ 0.4 mm/r. When the hardness of the workpiece material is high or cutting intermittently, in order to reduce the unit cutting force, the feed rate should be reduced to prevent edge collapse and cutting; For the selection of machining feed, it is suggested to select the tool angle according to the shape and size of the workpiece, which can improve the machining efficiency and effectively improve the service life of the tool.
7. Innovation of cubic boron nitride tool for processing hardened steel.
With the increasing application of hardened steel parts, there are many different processing conditions, such as continuous cutting, medium intermittent cutting and strong intermittent cutting, especially the strong intermittent cutting of hardened steel parts is even more difficult. The tool is easy to collapse during cutting, which affects the surface quality of hardened steel parts. In order to better meet the different processing conditions of hardened steel, superhard tool enterprises continue to develop and innovate. Finally, a whole set of machining scheme of grinding hardened steel by turning instead of turning was worked out. According to different working conditions of hardened steel, three materials, KBN 100, KBN 150 and KBN200, were developed for continuous cutting, moderate intermittent cutting and strong intermittent cutting respectively. The surface smoothness reaches Ra0.8, which improves the production efficiency.
The above three materials belong to composite CBN blades, and the delay depth should be controlled within 0.5 mm For example, KBN700 material can be selected for processing large hardened steel parts (wind turbine bearings, etc.). ) In the rainstorm. This material belongs to fully polycrystalline CBN blade, and the cutting depth can reach 1- 10mm/ time.
Eight, cubic boron nitride tool processing hardened steel shell parameters.
1 and KBN 100 cutting parameters for continuous cutting of hardened steel;
Processing material: 20CrMnTi gear, HRC58-62,
Blade selection: KBN 100 CNGA 120408,
Cutting parameters: ap=0. 1mm, Fr=0. 1mm/r, Vc= 180m/min, dry cutting,
Machining effect: the tool life is 1.45 times that of a CBN tool of a certain brand.
2, KBN 150 medium-sized intermittent cutting hardened steel box parameters:
Processing material: 20CrMnTi gear, HRC58-62,
Select blade: kbn150cccgw09t304,
Cutting parameters: ap=0.25mm, Fr=0.08mm/r, Vc= 135m/min, dry cutting,
Machining effect: the tool life is 1.7 times longer than that of a European brand CBN tool, and the efficiency is 22%.
3, KBN200 material strong intermittent cutting hardened steel case parameters:
Processing material: 20CrMnTi gear, HRC58-62,
Select blade: KBN200 WNGA080404,
Cutting parameters: ap=0. 15mm, Fr=0. 1mm/r, Vc= 1 17m/min, dry cutting,
Machining effect: the tool life is 6 times that of a brand CBN blade.