1. What is a hardened steel?
Hardened steel is the metal after quenching, martensite, hardness greater than HRC50 steel. Hardened steel cutting in hard materials occupies a large proportion.
Machining hardened steel is the traditional method of grinding. However, in order to improve processing efficiency, but can not solve the complex shape of the workpiece after grinding and quenching the shape and position error resulting problems often require the use of turning, milling, boring, drilling and reaming and other cutting methods. Hardened steel cutting has the following characteristics:
(1), high hardness, high strength, almost no plastic: This is the main cutting characteristics of hardened steel. When the hardness of hardened steel to HRC50 ~ 60, its strength up to σb = 2100 ~ 2600MPa, according to classification by processing material processing requirements, the hardness and strength of hardened steel are 9a-class, are the most difficult to cut materials.
(2) hardened steel cutting power, cutting temperatures are high: from the high hardness and high strength of the workpiece cut chips, the unit cutting force of up to 4500MPa. In order to improve the cutting conditions, increase heat dissipation area, the smaller the main tool selection and vice declination angle. At this time will cause vibration, required to have a good technology system rigidity.
(3) hardened steel is not easy to BUE: hardened steel, high hardness, brittleness, cutting is not easy to BUE, get a lower surface to be machined surface roughness.
(4) collapse easily broken blade, wear: Because of the brittleness of hardened steel, cutting short the chip and blade contacts, cutting force and cutting tool edge heat concentrated in the near collapse of broken blade easy to make and wear.
(5) Low thermal conductivity: the thermal conductivity is generally hardened steel is 7.12W / (m • K), about 45 steel, 1 / 7. Machinability of the material level is 9a-level, are difficult to cut materials. As low thermal conductivity of hardened steel, the cutting heat is difficult to pass chip away, cutting temperature is high, accelerated tool wear.
2. How to Select hardened steel cutting tool material?
Reasonable choice of cutting tool material is an important condition of hardened steel. According to the characteristics of hardened steel cutting tool materials not only have high hardness, wear resistance, heat resistance, and to have a certain strength and thermal conductivity.
(1) carbide: To improve the performance of cemented carbide, the choice of carbide, should give priority to adding the right amount of TaC or NbC carbide ultrafine particles. Because WC-Co cemented carbide in the class, adding TaC after the original can be improved high temperature strength of 800 ℃ 150 ~ 300MPa, room temperature hardness HV40 ~ 100. After adding NbC, high-temperature strength increased 150 ~ 300MPa, room temperature hardness HV70 ~ 150. And TaC and NbC grain refinement can improve the crater wear resistant carbide ability. TaC can also reduce the friction coefficient, lower cutting temperature, enhanced carbide thermal cracking and thermoplastic deformation capacity, but also the WC grain refinement to 0.5 ~ 1μm, the hardness HRA1.5 ~ 2, bending strength can increase 600 ~ 800MPa, high temperature hardness than carbide.
Used to cut hardened steel, carbide grades are: YM051, YM052, YN05, YN10, 600,610,726,758,767,813 and so on.
(2) hot pressing silicon nitride ceramics and ceramic composites: TiC added in Al2O3 and other metal elements and the use of hot pressing process, to improve the ceramic density, improve the performance of alumina-based ceramics, it's hardness to HRA95.5, bending strength can reach 800 ~ 1200MPa, heat resistance up to 1200 ℃ ~ 1300 ℃, can be used to reduce wear and diffusion bonding. Its main brands are AG2, AG3, AG4, LT35, LT55, AT6 and so on. Si3N4 silicon nitride based ceramics are adding TiC and other metal elements, and its hardness HRA93 ~ 94, the bending strength is 700 ~ 1100MPa. Its main brands are HS73, HS80, F85, ST4, TP4, SM, HDM1, HDM2, HDM3. Both ceramics for turning, milling, boring, planing hardened steel.
(3), cubic boron nitride composite film (PCBN) tool: its hardness HV8000 ~ 9000, composite flexural strength of 900 ~ 1300MPa, relatively high thermal conductivity, heat resistance to 1400 ℃ ~ 1500 ℃, the tool material the highest. It is very suitable for hardened steel, semi-finishing and finishing.
The best hardened steel cutting tool material is cubic boron nitride, followed by composite ceramics, and then followed by the new carbide grades.
3. How to choose the cutting tool geometry parameters of hardened steel?
When cutting hardened steel, having a good tool material, tool geometry without reasonable parameters, they can not achieve satisfactory results. Therefore, according to the specific tool material, workpiece material and cutting conditions, a reasonable choice of tool geometry, in order to effectively play the cutting tool material should have.
(1) front: cutting the size of the anterior horn of the great influence of hardened steel. Since the hardness of hardened steel, high strength, cutting power, and concentrated near the edge of the tool. In order to avoid chipping and playing knife, rake angle should be zero and negative selection, the general γ0 =- 10 ° ~ 0 °. Workpiece material hardness, intermittent cutting, should be selected large negative rake angle, γ0 =- 10 ° ~ -30 °. If you use positive rake angle indexable inserts should grind bγ = 0.5 ~ 1mm wide, γ01 =- 5 ° ~ -15 ° larger negative chamfering, to enhance the edge strength.
(2) horn: hardened steel cutting tools should be higher than normal posterior horn of the posterior horn of the large number of tools to reduce the friction flank, generally α0 = 8 ° ~ 10 ° as well.
(3) the main and sub-declination angle: the tip in order to enhance strength and improve heat dissipation, the main angle κr = 30 ° ~ 60 °, Vice-angle κ'r = 6 ° ~ 15 °.
(4) inclination: blade angle is negative, you can increase tip strength. But the negative is too large, will force Fp increases system rigidity is poor in the process, causing the vibration. So under normal circumstances, λs =- 5 ° ~ 0 °; intermittent cutting, λs =- 10 ° ~ -20 °; hardened hob scraper, it's blade angle λs =- 30 °.
(5) tip radius: it is the size of the impact of tip strength and machined surface roughness. Rigidity due to the impact of process systems, tip radius γε = 0.5 ~ 2mm appropriate.
Hardened steel cutting tools, in a reasonable, based on the selected geometric parameters must be carefully grinding and research Bei, the surface of the grinding tool to improve the quality of the tool life is improved.
4. How to choose when cutting hardened steel cutting?
Machining hardened steel cutting, mainly based tool material, physical and mechanical properties of workpiece material, workpiece shape, rigidity and allowance process system to choose. Three elements in the choice of cutting parameters, first consider selecting a reasonable cutting speed, followed by the depth of cut, and then followed by the feed.
(1) cutting speed: a general heat-resistant hardened steel at 200 ℃ ~ 600 ℃, while the carbide heat resistance of 800 ℃ ~ 1000 ℃, the heat resistance of ceramic tool 1100 ℃ ~ 1200 ℃, cubic nitrogen of the heat resistance of boron 1400 ℃ ~ 1500 ℃. In addition to high-speed steel, in general, hardened steel to about 400 ℃, its hardness began to decline, and the tool material remained its original hardness. So when cutting hardened steel, full use of the above this feature, select the cutting speed should not be too low or too high to maintain the tool has some durability. From the current experience, a different cutting tool materials, cutting hardened steel, carbide cutting tools Vc = 30 ~ 75m/min; ceramic tool Vc = 60 ~ 120m/min; cubic boron nitride cutting tools Vc = 100 ~ 200m / min. In intermittent cutting and workpiece material hardness is too high, should reduce the cutting speed, generally above the lowest cutting speed of about 1 / 2. The best in cutting speed continuous cutting, to cut the chip Chenganhongse appropriate.
(2) depth of cut: general allowance and process systems based on rigid selection, under normal circumstances, αp = 0.1 ~ 3mm.
(3) feed: generally 0.05 ~ 0.4mm / r. The workpiece material hardness or intermittent cutting, in order to reduce the unit cutting force, feed rate should be reduced to prevent chipping and play knife.
5. How hardened steel with a ceramic cutting tool?
Ceramic cutting tool material hardened steel, carbide cutting than hardened steel, has a significant effect. This was reflected in the hardness of ceramic cutting tools and heat resistance than carbide. Use it to create the turning, milling, thread cutting tools, can be successful in cutting hardened steel, its durability is higher than YT05, 643, YM052 carbide.
(1) full ceramic tool material carbide hardness and heat resistance than the high of this feature, select the cutting speed should be higher than the hardened steel cutting carbide cutting speed, generally 50% higher. If the cutting speed 50m/min, the ceramic tool flank wear and carbide close. When cutting speed increased to 95m/min, its wear resistance is much higher than YT05 carbide. Such as the use made of ceramic blade cutter blade on three sides, the cutting speed to 102m/min end milling depth 5.2mm, width 16mm, length 700mm hardened steel keyway, the tool is basically no wear.
(2) ceramic tool in cutting under impact load, the tool should be selected small main angle, or use a large circular arc blade tip to increase tip strength, to avoid tool damage. If using a circular blade made of ceramic machine clip end milling cutter, milling hardened steel, Vc = 120 ~ 150m/min, Vf = 230 ~ 290mm/min, αp = 1 ~ 2mm.
(3) should be negative with negative rake angle and blade angle, in order to facilitate and increase the blade tip strength. Blade and flank roughness Ra is less than 0.4μm.
(4) cutting without cutting fluid generally, to use, throughout the full supply to, or blade cracks due to thermal expansion and contraction.
(5) lower than the bending strength of ceramic cutting tools carbide bending strength, in order to reduce the force per unit area of the tool, cutting feed rate to be smaller, typically f = 0.08 ~ 0.15mm / r.
6. How to use cubic boron nitride cutting tool hardened steel?
Cubic boron nitride cutting tools (CBN) is not only a good material for manufacture of abrasives and grinding it easy (free diamond grinding wheel) is used to make turning tools, boring tools, milling cutters, gun drill, reamer, gear tools such good material. CBN cutting of hardened steel is mainly used, can also be used to cut other difficult to cut materials. It not only has a high metal removal rate, and have a good surface quality. Cutting of hardened steel can be effectively replaced by grinding to reduce the manufacturing processes and improve productivity. Sold on the market of CBN inserts, mostly in the composite film with the form of carbide, indexable inserts made of or tool, its purpose is to increase the flexural strength of CBN inserts.
Because CBN tools, high hardness (HV8000 ~ 9000), high heat resistance (1400 ℃ ~ 1500 ℃), when used several times to allow the cutting speed is higher than the carbide cutting hardened steel, and durability is the hardness of alloy several times to several times. CBN tools when cutting hardened steel and carbide tools compared.
CBN tools suitable for low speed cutting, CBN cutting tool by cutting heat generated in the cutting area within a small softening of workpiece material to cutting.
In the cutting hardness of HRC55 ~ 65 of the material, CBN tool cutting speed should be 50 ~ 120m/min. Milling of the Vc = 100 ~ 160m/min, feed rate per minute, Vf = 70 ~ 160mm/min; reaming when Vc = 60 ~ 130m/min, ap = 0.1 ~ 0.2mm, f = 0.07 ~ 0.2mm / r . CBN tool is mainly used for hardening steel, semi-finishing and finishing. The machined surface is not generated as grinding burns, as the phenomenon is about ten times more efficient than grinding.
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