Hardened steel cutting

1. What is quenched steel? What cutting characteristics does it have?

Hardened steel is a steel whose hardness is greater than HRC50 after the metal has been quenched. It occupies a considerable proportion in hard-to-cut materials.

The traditional method of machining hardened steel is grinding. However, in order to improve the machining efficiency and solve the problem that the shape of the workpiece is complex and it is not possible to grind and quench the shape and position, it is often necessary to use cutting methods such as turning, milling, boring, drilling, and reaming. Hardened steel has the following characteristics when cutting:

(1) High hardness, high strength, and almost no plasticity: This is the main cutting characteristic of hardened steel. When the hardness of hardened steel reaches HRC50-60, its strength can reach σb=2100～2600MPa. According to the grading rules of the processability of the processed material, the hardness and strength of the hardened steel are all 9a grade, and it belongs to the hardest cutting material.

(2) High cutting force and high cutting temperature: To cut chips from high hardness and high strength workpieces, the unit cutting force can reach 4500 MPa. In order to improve the cutting conditions and increase the heat dissipation area, the tool selects the smaller main and auxiliary angles. This will cause vibration and require a good process system rigidity.

(3) It is difficult to produce a built-up edge. Hardened steel has high hardness and brittleness. It is not easy to produce built-up edge when cutting, and the surface to be processed can obtain a low surface roughness.

(4) The blade is easy to disintegrate and wear: Due to the brittleness of the hardened steel, the chip and blade contact with each other during cutting is short. The cutting force and cutting heat are concentrated near the cutting edge of the tool, and the blade is easily disintegrated and worn.

(5) Low thermal conductivity: Generally, the thermal conductivity of hardened steel is 7.12W/(m•K), which is about 1/7 of that of No. 45 steel. The material's machinability rating is 9a and it is a hard-to-cut material. Due to the low thermal conductivity of the hardened steel, the cutting heat is difficult to carry away through the chips, the cutting temperature is high, and tool wear is accelerated.

2. How to choose cutting hardened steel tool material?

Reasonable choice of tool material is an important condition for cutting hardened steel. According to the cutting characteristics of hardened steel, the tool material must not only have high hardness, wear resistance, heat resistance, but also have certain strength and thermal conductivity.

(1) Cemented Carbide: In order to improve the performance of cemented carbide, when selecting a cemented carbide, it is preferable to add an appropriate amount of ultra-fine particle cemented carbide of TaC or NbC. Because in the WC-Co type cemented carbide, after the addition of TaC, its original high temperature strength of 800°C can be increased by 150-300 MPa, and its normal temperature hardness can be increased by HV 40-100. After adding NbC, the high temperature strength increased by 150-300 MPa, and the normal temperature hardness increased by HV 70-150. Moreover, TaC and NbC can refine grains and improve the ability of cemented carbides to resist crater wear. TaC can also reduce the friction coefficient, reduce the cutting temperature, enhance the ability of the hard alloy to resist thermal cracking and thermoplastic deformation. At the same time, it also refines the grain size of WC to 0.5~1μm, its hardness increases by HRA1.5~2, bending strength. Can increase 600 ~ 800MPa, high temperature hardness higher than the average hard alloy.

Commonly used to cut hardened steel grades are: YM051, YM052, YN05, YN10, 600, 610, 726, 758, 767, 813 and so on.

(2) Hot-pressed composite ceramics and hot-pressed silicon nitride ceramics: The addition of TiC and other metal elements in Al2O3 and the use of hot-pressing techniques improve the compactness of the ceramics, improve the performance of alumina-based ceramics, and increase its hardness. To HRA 95.5, bending strength can reach 800 ~ 1200MPa, heat resistance up to 1200 °C ~ 1300 °C, in use can reduce adhesion and diffusion wear. Its main brands are AG2, AG3, AG4, LT35, LT55, AT6 and so on. Silicon nitride-based ceramics is added to Si3N4 TiC and other metal elements, its hardness is HRA93 ~ 94, bending strength of 700 ~ 1100MPa. Its main brands are HS73, HS80, F85, ST4, TP4, SM, HDM1, HDM2, HDM3. These two ceramics are suitable for car, milling, boring, and planing hardened steel.

(3) Cubic boron nitride compound sheet (PCBN) cutter: Its hardness is HV8000-9000, its composite bending strength is 900-1300MPa, its thermal conductivity is relatively high, and its heat resistance is 1400°C-1500°C. It is a tool material. Highest. It is very suitable for semi-finishing and finishing of hardened steel.

In summary, the best tool material for cutting hardened steel is cubic boron nitride, followed by composite ceramics, followed by new grade cemented carbide.

3. How to choose the geometric parameters of cutting hardened steel cutters?

When cutting hardened steel, the light has good tool materials, and without reasonable tool geometry parameters, it can not achieve satisfactory results. Therefore, according to the specific tool materials, workpiece materials and cutting conditions, reasonable choice of tool geometry parameters, in order to effectively play the cutting material should have the cutting performance.

(1) Front angle: The size of the rake angle has a great influence on the quenched steel. Due to the hardness and strength of the hardened steel, the cutting force is large and concentrated near the cutting edge of the tool. In order to avoid chipping and knife cutting, the front corner should be selected for zero and negative values, generally γ0 = -10° ~ 0°. When the workpiece material has high hardness and intermittent cutting, a larger negative rake angle should be selected, γ0 = -10° to -30°. If a positive rake angle indexable insert is used, a negative chamfer with b γ = 0.5 to 1 mm wide and γ 01 = -5° to -15° should be ground to enhance the blade strength.

(2) Back angle: The back angle of the cutting hardened steel tool should be larger than the back angle of the general tool to reduce the friction of the back surface, generally α0=8°~10° is better.

(3) Main and auxiliary declination angles: In order to enhance the strength of the blade tip and improve the heat dissipation conditions, the main declination angle κr = 30° to 60° and the declination angle κ'r = 6° to 15°.

(4) Blade angle: When the blade angle is negative, the blade tip strength can be increased. However, when the negative value is too large, the force of Fp will increase, and when the rigidity of the process system is poor, vibration is caused. Therefore, under normal conditions, λs = -5° to 0°; during interrupted cutting, λs = -10° to -20°; hardened surface scraping hob, its blade inclination λs = -30°.

(5) Tool nose arc radius: its size affects the blade tip strength and the machined surface roughness. Due to the influence of the rigidity of the process system, the radius of the tip radius γε = 0.5 ~ 2mm is appropriate.

Cutting hardened steel cutters must be carefully sharpened and ground on the basis of reasonable selection of geometrical parameters, so as to improve the sharpening quality of each surface of the cutter and to increase the durability of the cutter.

4. How to choose the amount of cutting when cutting hardened steel?

The cutting amount of the hardened steel for cutting processing is mainly selected based on 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 factors of cutting amount, first consider the choice of a reasonable cutting speed, followed by the depth of cut, followed by the amount of feed.

(1) Cutting speed: The general heat resistance of quenched steel is 200°C～600°C, while the heat resistance of cemented carbide is 800°C～1000°C, the heat resistance of ceramic cutting tool is 1100°C～1200°C, cubic nitrogen. The heat resistance of boron is 1400°C to 1500°C. In addition to high-speed steel, when the hardened steel reaches approximately 400°C, its hardness begins to decrease, and the above-mentioned tool material still maintains its original hardness. Therefore, in the cutting of hardened steel, make full use of this feature, the cutting speed should not be chosen too low or too high, in order to maintain the knife has a certain degree of durability. From the current experience, different tool materials cut the cutting speed of hardened steel, carbide tool Vc = 30 ~ 75m/min; ceramic tool Vc = 60 ~ 120m/min; cubic boron nitride tool Vc = 100 ~ 200m /min. When interrupted cutting and workpiece material hardness is too high, the cutting speed should be reduced, typically about 1/2 of the minimum cutting speed above. The optimum cutting speed for continuous cutting is that the cut chips are dark red.

(2) Depth of cut: Normally, it is selected based on the machining allowance and the rigidity of the process system. Under normal circumstances, αp = 0.1 to 3 mm.

(3) Feed rate: Generally 0.05-0.4mm/r. When the workpiece material hardness is high or intermittent cutting, in order to reduce the unit cutting force, the feed should be reduced to prevent chipping and knife.

5. How to cut hardened steel with a ceramic tool?

The cutting of hardened steel with a ceramic tool material has a significant effect compared to cutting a hardened steel with a hard alloy. This is mainly reflected in the hardness and heat resistance of ceramic tools is higher than that of cemented carbide. The turning tools, milling cutters, and thread cutters that are used to make them can all successfully cut hardened steels, and their durability is higher than that of YT05, 643, YM052, and other cemented carbides.

(1) Bringing the hardness and heat resistance of the ceramic tool material into full play is higher than that of the cemented carbide. The cutting speed should be higher than the cutting speed of the hard alloy cutting hardened steel, generally 50% higher. For example, at a cutting speed of 50 m/min, the flank wear of the ceramic tool is close to that of cemented carbide. When the cutting speed is increased to 95m/min, its wear resistance is much higher than that of YT05 and other cemented carbides. For example, with a three-blade cutter made of ceramic inserts, a 5.2 mm deep, 16 mm wide, and 700 mm long hardened steel keyway was machined at a cutting speed of 102 m/min, and the tool was essentially free of wear.

(2) When the ceramic tool is subjected to impact load during cutting, the tool should select a small primary angle, a large tip radius, or use a circular blade to increase the strength of the blade tip to avoid tool damage. For example, a milling cutter made of a round ceramic blade and milling a hardened steel, Vc=120 to 150 m/min, Vf=230 to 290 mm/min, and αp=1 to 2 mm.

(3) The negative rake angle and the negative rake angle should be adopted so as to increase the blade edge and tool nose strength. The roughness Ra of the blade and the knife face is less than 0.4 μm.

(4) Cutting fluid is generally not used for cutting. If it is used, it must be fully supplied from beginning to end. Otherwise, the blade will crack due to thermal expansion and contraction.

(5) The bending strength of ceramic tools is lower than that of cemented carbide. In order to reduce the force per unit area of ​​the tool, the feed amount during cutting is smaller, generally f=0.08-0.15mm/r.

6. How to use cubic boron nitride tool to cut hardened steel?

The cubic boron nitride tool (CBN) is not only a good material for the manufacture of abrasive tools, but also it is easy to grind (can use diamond grinding wheel sharpening), but also for the manufacture of turning tools, boring tools, milling cutters, gun drills, reamers, gears Good materials such as knives. CBN is mainly used to cut all kinds of hardened steels, and it can also be used to cut other hard-to-cut materials. It not only has a high metal removal rate, but also has a very good surface quality. Cutting all kinds of hardened steels can effectively replace grinding, reducing processing steps and increasing productivity. Most of the CBN blades sold on the market today are made of indexable inserts or knives in the form of composite sheets with cemented carbide, the purpose of which is to increase the bending strength of CBN blades.

Because CBN tools have high hardness (HV8000～9000) and high heat resistance (1400°C～1500°C), they can be used to cut hardened steel at a cutting speed several times higher than that of cemented carbide. The durability is hardness alloy. Several times to several times. Comparison of CBN cutting tools and carbide cutting tools for cutting hardened steel.

The domestic manufacturers of CBN inserts are Chengdu Tool Research Institute, whose production grade is LDP-J, and DLS-F1, DLS-F2, and DLS-F3 produced by the sixth wheel factory. There are also many manufacturers of CBN indexable inserts and welding knives. LDP-J and DLS-F1 are mainly used for cutting various hardened steels. DLS-F2 is mainly used for cutting various cast irons. DLS-F3 is mainly used for cutting high temperature alloys and titanium alloys.

The CBN tool is not suitable for low-speed cutting. The CBN tool cuts the cutting heat generated by cutting and softens the workpiece material within a tiny area of ​​the cutting area.

When cutting materials with hardness of HRC 55-65, the cutting speed of CBN tools should be 50-120 m/min. Milling Vc = 100 ~ 160m/min, feed per minute Vf = 70 ~ 160mm/min; Reaming Vc = 60 ~ 130m/min, ap = 0.1 ~ 0.2mm, f = 0.07 ~ 0.2mm/r . CBN tools are mainly used for semi-finishing and finishing of hardened steel. The machined surface does not burn like grinding, and its efficiency is about ten times higher than that of grinding.

The geometric parameters of CBN cutting tool when cutting hardened steel are γ0=-15°～-5°, α0=α′0=10°～15°, κr=30°～60°, κ'r=5°～15°, Λs = 0° to 10° and γε = 0.3 to 1 mm.

7. When cutting a hardened steel with a CBN tool, under what conditions is it most effective to replace grinding?

(1) Cutting complex surfaces and several complex surfaces on a CNC machine, instead of grinding, can reduce 1/3 to 2/3 of the amount of work, and can ensure high positional accuracy.

(2) Intricate inner holes or small holes. If grinding is used, the shape of the grinding wheel is also required to be complex. Sometimes it cannot be ground. Turning is most advantageous at this time.

(3) Several surfaces of a part (external circle, inner hole, end face, step, groove) need to be ground. At this time, turning is used to complete one process, and the grinding tooling can be subtracted.

(4) After the parts are easily deformed after quenching and the remaining margin is small, it can cause waste products. At this time, a large margin can be left. After quenching, remove excess margin with CBN tool, and then grinding, to reduce the large deformation caused Waste products.

(5) The surface high-frequency parts used under the conditions where the load variation is large and difficult are processed by CBN tools. The surface microstructure and physical and mechanical properties of the workpiece are better than that of grinding, and the service life of the parts can be extended.

8. How to Turn the Thread of a Hardened Steel Roller?

The threads of the roller after quenching are generally ground using a thread grinder. However, in order to improve the processing efficiency and solve the problem of no thread grinding, it can also be processed by turning.

(1) Technical requirements of the rolling wheel: The material of the blank is Cr12MoV alloy tool steel, the hardness after quenching is HRC59, the tooth pitch is 1.5mm, the number of thread heads is 10, the half-width angle α/2=30o±25', tooth depth It is 0.922 mm, the cusp height h1 = (0.435±0.042) mm, and the root height h2 = (0.487 (+0.010/-0.029) mm.

(2) Machine tools and tools: The machine tool is a type 8955 290mm shovel tooth lathe produced in Dalian. The machine tool accessories are indexing disks. The tool is divided into rough turning tools and fine turning tools. The tool material is YM052 hard alloy. The geometry of the tool is: γ. =-3o,α. = 5o, α' = 0o to 2o, εr = 60o20', λs = -5o, the rake is inclined 3o in the spiral direction. After all the tools have been ground and ground, the diamond whetstone is used to grind each angle and a negative chamfer is made at the main and auxiliary blades and at the tip of the tool.

(3) Cutting amount: cutting speed 26.2m/min, rough thread aP=0.1-0.15mm, precision car aP=0.05-0.08mm. In turning, each time the knife is taken, the head is divided once to ensure that each tooth is evenly removed.

(4) Precautions: In order to prevent the tool tip from collapsing when cutting in and out, the two ends of the thread are inverted at an angle of 30o. In order to accurately control the depth of each knife, put a meter in the lateral feed.

9. How to drill out the tap broken in the threaded hole?

When tapping a thread with a tap, a thread that is slightly inadvertently or difficult to attack the cutting material often breaks the tap in the hole and is not easy to remove, causing the workpiece to become a waste product. In order to reduce waste, use sparks to break the tap broken in the hole or drill with a carbide drill. The use of hard alloy drills is simple and easy to master.

(1) Tool: Use a carbide rod to grind into a square-edge diamond as shown in Figure 2-1. The material is YG6X or YM052, 813, etc. The diameter of the drill should be greater than the diameter of the tap core.

(2) Handling Precautions: After correcting the position of the drill and hole, fix the drill and the workpiece. The head of the tap broken in the tapped hole is not even and must be slow when starting to drill so as not to damage the screw hole and drill the hole. When drilling, the cutting speed is 20-25m/min. Observe the drilling condition at any time and remove the chips and chips of the tap in time until the tap broken in the hole is drilled.

10. How to use high-speed steel drills to drill hard materials?

The hard material mentioned here refers to a material whose hardness is HRC 38 to 42 after heat treatment, or a hardened layer surface after carburizing, cyanidation, and chrome plating on the surface of the part. Drilling holes in such hard materials or surfaces is very difficult, and sometimes only a small hole is drilled on the surface. The drill bit whistles and wears heavily. In the absence of a cemented carbide drill, high-speed steel drills with varying geometrical parameters can be used and the hard materials can be successfully drilled. Drilling hard material drilling parameters shown in Figure 2-2.

In order to increase the blade strength and improve the heat dissipation conditions, a method of reducing the rake angle, increasing the radius of the arc groove, and the tool tip angle of the outer edge is adopted. The geometry of the drill bit is shown in Figure 2-2. Shorter drills should be selected for drilling to increase the rigidity of the drill. When the workpiece rigidity is insufficient, auxiliary support is added. The amount of drilling is generally selected νc = 2 ~ 5m/min, feed rate f = 0.03 ~ 0.05mm / r or manual feed. Cutting fluid should use cutting oil or extreme pressure cutting oil.

11. What are the advantages of grinding high-speed steel with cubic boron nitride grinding wheels?

High-speed steel is a special purpose steel in hardened steel. CBN grinding wheels are particularly suitable for grinding high speed steels. The use of corundum grinding wheels for high speed steels, especially for grinding high vanadium high speed steels, results in high grinding forces, high grinding temperatures, prone to burns, rapid abrasive wear, easy clogging, easy threshing, and difficult grinding. As with the CBN grinding wheel W18Cr4V, W2Mo9Cr4VCo8, when five kinds of bars W6Mo5Cr4V5SiNbA1, W6Mo5Cr4V5Co3SiNbA1, W10Mo4Cr4V3A1 like high speed steel, abraded accumulated 17mm, the grinding wheel is still sharp. Another example is a cubic boron nitride abrasive grains of the copper wheel, high-speed steel grinding W6Mo5Cr4V5SiNbA1 Broach, grind 150 teeth, and corundum grinding wheel with a grinding cutter can not move, add the microcrystalline corundum crystal corundum Grinding wheels can only grind out 7 to 8 teeth.

Cubic boron nitride grinding wheels have a high degree of durability when grinding HSS. Its wear is 1/64 of the single crystal corundum, 1/104 of the green silicon carbide, and 1/99 of the white corundum.

CBN grinding wheels have a high grinding ratio for grinding HSS. The grinding rate of high-vanadium high-speed steel (W12Cr4V5Co) is 140, while corundum-like grinding wheel is 10, and diamond grinding wheel is 16. The grinding ratio of superhard high-speed steel (W2Mo9Cr4VCo8) is 660 for it, while the corundum grinding wheel is 10 and the diamond grinding wheel is 18. The grinding ratio of tungsten carbide molybdenum high-speed steel (W6Mo5Cr4V2) was 980 for grinding, 10 for corundum type grinding wheels, and 20 for diamond grinding wheels.

CBN grinding wheels can avoid burns when grinding HSS. When high-speed steel is ground with a corundum grinding wheel, it wears out quickly. If it is not repaired in time, there will be burns and the grinding quality will be affected. The cubic boron nitride grinding wheel does not need to be trimmed until it is worn, and no burns will occur.

The cubic boron nitride grinding wheel has a high cutting capacity, and the unit grinding power is 2/3 to 5/6 smaller than the corundum grinding wheel.

12. What are the examples of cutting hardened steel?

Hardened steel cutting is the most representative of hard-to-cut materials. In the past, people believed that it had a high hardness and could only be ground. Nowadays, due to advances in cutting technology, turning grinding, milling, planing, and drilling instead of grinding is a common practice.

(1) Car cold rolling bearing roller mold top rod, made of alloy tool steel Crl2MoV, hardness after quenching is HRC62 ~ 66. As the workpiece is slender, afraid of quenching, large deformation, leaving a 2mm margin, difficult to grind, instead of turning. With the LT55 ceramic turning tool, the tool geometry is: γ. =-8o, α. = 8o, κr = 75o, κ'r = 15o, γε = 0.5mm. The cutting amount is: νc = 35m/min, aP = 0.3 ~ 0.5mm, f = 0.1 ~ 0.2mm/r. The tool has a durability of 25 minutes and a surface roughness of Ra=3.2 to 1.6 μm.

(2) High-speed steel after quenching, material is W18Cr4V, hardness is HRC64, workpiece size is 120mm×40mm. The tool material is YM052 hard alloy. The tool geometry parameter is: γ. = -10o, α. = 6o to 8o, κr = 45o, κ'r = 15o, γs = -10o, γε = 1mm. The cutting amount is: νc = 8m/min, aP = 0.5 - 0.8mm, f = 0.1 - 0.15mm/stroke, the tool life is 21min, and the surface roughness is Ra = 3.2μm.

(3) The hardness of the bearing steel (GCrl5) after quenching with a cubic boron nitride tool is HRC62. The workpiece is a bearing outer ring. The tool geometry parameter is: γ. =-6o,α. =8o, κr=45o, κ'r=45o, γε=1.2mm, λs =0o. The cutting amount is: νc = 115m/min, aP = 0.3mm, f = 0.1 - 0.2mm/r. The tool has a durability of 55 min and a surface roughness of Ra=1.6 μm.

(4) Drill the hinged hardened steel hole, the workpiece material is GCrl5, and the hardness after quenching is HRC58-60. The borehole diameter is 4 (+0.03,0) mm and 10 mm deep. This hole was drilled after the product was improved. The drill and reamer are made of YG6X cemented carbide. The drill is in the form of a non-standard twist drill, and the reamer is a double-edge reamer. The drilling and reaming are two processes. The machine tool used is a bench drill. The cutting speed is νc=15.5m/min, manual feed. Grinding a drill can drill more than 100 holes, reaming more than 200, the quality fully meets the process requirements.

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