In the process of hardened steel processing, it is very important to choose the right cutting tool. With its superior cutting ability and strong wear resistance, the end mill can provide stable cutting effect when processing hardened steel with higher hardness. To ensure processing accuracy and extend the tool life, it is recommended to use coated end mills. The coating of this kind of tool can effectively reduce friction and heat accumulation, while improving the wear resistance of the tool. In the cutting process, reasonable cutting parameters, the use of cutting fluid, and regular maintenance of the tool are all important factors to ensure processing quality and improve production efficiency.
What is Hardened Steel
In the processing of hardened steel, it is crucial to choose the right cutting tool. With its superior cutting ability and strong wear resistance, the end mill can provide stable cutting results when processing hardened steel with higher hardness. In order to ensure processing accuracy and extend tool life, it is recommended to use coated end mills. The coating of this tool can effectively reduce friction and heat accumulation, while improving the wear resistance of the tool. In the cutting process, reasonable cutting parameters, the use of cutting fluids, and regular maintenance of tools are all important factors to ensure processing quality and improve production efficiency.
- High hardness, high strength, and almost no plasticity are hardened steel’s main cutting characteristics. When the hardness of hardened steel reaches HRC50~60, its strength can reach ob=2100~2600MR3. According to the classification regulations of the processed material’s machinability, hardened steel’s hardness and strength are both 9a, which is the most difficult material to cut.
- High cutting force and high cutting temperature: to cut chips from high hardness and high strength workpieces, the unit cutting force can reach 4500MR. In order to improve the cutting conditions and increase the heat dissipation area, the tool should select a smaller main deflection angle and secondary deflection angle. This will cause vibration, requiring better process system rigidity.
- It is not easy to produce built-up edge; hardened steel has high hardness and high brittleness, and it is not easy to produce a built-up edge during cutting. The machined surface can obtain a lower surface roughness.
- The blade is easy to break and wear: Due to the high brittleness of hardened steel, the contact between the chip and the blade is short during cutting, and the cutting force and cutting heat are concentrated near the tool edge, which is easy to cause the blade to break and wear.
- Low thermal conductivity: The thermal conductivity of general hardened steel is 7.12W (m”K), which is about 1/7 of 45 steel. The machinability grade of the material is 9a, which is a very difficult material to cut. Due to the low thermal coefficient of hardened steel, the cutting heat is difficult to be carried away by the chips, and the cutting temperature is very high, which accelerates the wear of the tool.
How to Choose Tool Materials for Cutting Hardened Steel
Reasonable selection of tool materials is an important condition for cutting hardened steel. According to the cutting characteristics of hardened steel, tool materials should not only have high hardness, wear resistance, and heat resistance, but also have certain strength and thermal conductivity.
Cemented carbide: In order to improve the performance of cemented carbide, when selecting cemented carbide, cemented carbide with ultrafine particles of TaC or NbC added in appropriate amounts should be preferred. Because in WC-Co cemented carbide, after adding TaC, its original high temperature strength at 800℃ can be increased by 150~300MPa, and HV40~100 can increase the room temperature hardness. After adding NbC, the high temperature strength is increased by 150~300MPa, and the room temperature hardness is increased by HV70~150. Moreover, TaC and NbC can refine the grains and improve the ability of cemented carbide to resist crescent injection wear. TaC can also reduce the friction coefficient, reduce cutting temperature, enhance the ability of cemented carbide to resist thermal cracking and thermoplastic deformation, and also refine the grains of WC to 0.5~1μm, increase its hardness by HRA1.5~2, and increase its bending strength by 600~800MPa. Its high temperature hardness is higher than that of general cemented carbide.
Hot-pressed composite ceramics and hot-pressed silicon nitride ceramics: Adding metal elements such as TiC to AI203 and using hot pressing technology improves the density of ceramics and the performance of alumina-based ceramics, so that its hardness is increased to HRA95.5, the bending strength can reach 800~1200MPa, and the heat resistance can reach 1200℃~1300℃, which can reduce bonding and diffusion wear during use. Silicon nitride-based ceramics are made by adding metal elements such as TiC to Si3N4, with a hardness of HRA93~94 and a bending strength of 700~1100MPa. These two ceramics are suitable for turning, milling, boring, and planing hardened steel.
Cubic boron nitride composite sheet (PCRN) tool: Its hardness is HV8000~9000, composite bending strength is 900~1300MPa, thermal conductivity is relatively high, heat resistance is 1400℃~1500℃, which is the highest among tool materials. It is very suitable for semi-finishing and finishing of hardened steel.
How to Choose the Geometric Parameters of Cutting Tools for Hardened Steel
When cutting hardened steel, good cutting tool materials alone, without reasonable cutting tool geometric parameters, cannot achieve satisfactory results. Therefore, it is necessary to reasonably select tool geometric parameters according to specific tool materials, workpiece materials and cutting conditions to effectively exert the cutting performance of tool materials.
Rake angle: The size of the rake angle has a great influence on cutting hardened steel. Due to the high hardness and strength of hardened steel, the cutting force is large and concentrated near the tool edge. In order to avoid chipping and cutting, the rake angle should be selected as zero and negative values, generally γ0=-10°～0°. When the workpiece material is hard and intermittent cutting is required, a larger negative rake angle should be selected, γ0=-10°～-30°. If a positive rake angle indexable insert is used, a larger negative chamfer with a width of bγ=0.5～1mm and γ01=-5°～-15° should be ground to enhance the blade strength.
Back angle: The back angle of the tool for cutting hardened steel should be larger than that of the general tool to reduce the friction of the back tool face. Generally, α0=8°～10° is better.
Main rake angle and secondary rake angle: In order to enhance the strength of the tool tip and improve the heat dissipation conditions, the main rake angle κr=30°～60°, and the secondary rake angle κ’r=6°～15°.
Blade inclination angle: When the blade inclination angle is negative, the tool tip strength can be increased. However, when the negative value is too large, the fp force will increase, causing vibration when the rigidity of the process system is poor. Therefore, under normal circumstances, λs=-5°～0°; for intermittent cutting, λs=-10°～-20°; for hard tooth surface scraping hobs, its blade inclination angle λs=-30°.
Blade tip arc radius: Its size affects the tool tip strength and the roughness of the processed surface. Due to the influence of the rigidity of the process system, the blade tip arc radius γε=0.5～2mm is appropriate.
Cutting hardened steel tools must be carefully sharpened and honed on the basis of reasonable selection of geometric parameters to improve the sharpening quality of each tool surface and improve the tool durability.
How to Choose the Cutting Amount When Cutting Hardened Steel
The cutting amount of hardened steel is mainly selected according to the physical and mechanical properties of the cutting tool material, the workpiece material, the workpiece shape, the rigidity of the process system and the processing allowance. When selecting the three elements of cutting amount, first consider choosing a reasonable cutting speed, followed by cutting depth, and then feed rate.
Cutting speed: The heat resistance of general hardened steel is 200℃～600℃, while the heat resistance of cemented carbide is 800℃～1000℃, the heat resistance of ceramic cutting tools is 1100℃～1200℃, and the heat resistance of cubic boron nitride is 1400℃～1500℃. Except for high-speed steel, the hardness of general hardened steel begins to decrease when it reaches about 400℃, while the above tool materials still maintain their original hardness. Therefore, when cutting hardened steel, make full use of the above characteristics, and the cutting speed should not be too low or too high to maintain a certain durability of the tool. According to current experience, the cutting speed of different tool materials for cutting hardened steel is vc=30～75m/min for carbide cutting tools; vc=60～120m/min for ceramic tools; vc=100～200m/min for cubic boron nitride tools. When cutting intermittently or the hardness of the workpiece material is too high, the cutting speed should be reduced, generally about 1/2 of the minimum cutting speed mentioned above. The best cutting speed during continuous cutting is when the chips are dark red.
Cutting depth: Generally selected according to the machining allowance and the rigidity of the process system. Under normal circumstances, αp=0.1～3mm.
Feed rate: Generally 0.05～0.4mm/r. When the workpiece material is hard or cutting is intermittent, in order to reduce the unit cutting force, the feed rate should be reduced to prevent chipping and tooling.
How to Cut Hardened Steel with Ceramic End Mills
Using ceramic cutting tool materials to cut quenched steel has a significant effect compared with using carbide end mills to cut quenched steel. This is mainly reflected in the fact that the hardness and heat resistance of ceramic tools are higher than that of cemented carbide. Turning tools, milling cutters, and threading tools made with it can successfully cut hardened steel.
Taking full advantage of the fact that ceramic cutting tool materials have higher hardness and heat resistance than cemented carbide, the selected cutting speed should be higher than the cutting speed of cemented carbide cutting quenched steel, which is generally 50% higher. For example, at a cutting speed of 50m/min, the flank wear of ceramic tools is close to that of cemented carbide. When the cutting speed increases to 95m/min, its wear resistance is much higher than that of cemented carbide. For example, using a three-sided edge milling cutter made of ceramic blades to mill a quenched steel keyway with a depth of 5.2mm, a width of 16mm, and a length of 700mm at a cutting speed of 102m/min, the tool will basically not wear.
When ceramic cutting tools are subjected to impact loads during cutting, the tool should choose a small leading angle, a large tip arc or a round insert to increase the tip strength and avoid tool damage. For example, using a machine clamp end mill made of circular ceramic blades to mill quenched steel, vc=120~150m/min, vf=230~290mm/min, αp=1~2mm.
Negative rake angle and negative edge inclination angle should be used to increase the strength of the blade and tip. The roughness ra of the blade and blade surface should be less than 0.4μm.
Cutting fluid is generally not used during 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.
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 rate during cutting should be smaller, generally f=0.08~0.15mm/r.
How to Use Cubic Boron Nitride Cutting Tools to Cut Hardened Steel
Cubic boron nitride cutting tools (CBN) are not only a good material for manufacturing abrasive tools, but also easy to sharpen (can be sharpened with diamond grinding wheels). They are also good materials for manufacturing turning tools, boring cutters, milling cutters, gun drills, reamers, gear cutters, etc. CBN is mainly used to cut various hardened steels, and can also be used to cut other difficult-to-cut materials. It not only has a high metal removal rate, but also has good surface processing quality. Cutting various hardened steels can effectively replace grinding, reduce processing procedures, and improve productivity. Most of the CBN blades sold on the market are made into indexable blades or tools in the form of composite sheets with cemented carbide, the purpose of which is to improve the bending strength of CBN blades.
Due to the high hardness (hv8000～9000) and high heat resistance (1400℃～1500℃) of CBN tools, they can be used to cut hardened steel at a cutting speed several times higher than that of cemented carbide, and their durability is several to dozens of times that of hard alloy. Comparison of CBN tools and cemented carbide tools when cutting hardened steel.
Domestic manufacturers of CBN blades include Chengdu Tool Research Institute, which produces the brand name LDP-J; the Sixth Grinding Wheel Factory produces DLS-F1, DLS-F2, and DLS-F3. There are also many manufacturers that produce CBN indexable blades 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.
CBN tools are not suitable for low-speed cutting. CBN tools rely on the cutting heat generated during cutting to soften the workpiece material in a small range of the cutting area for cutting.
When cutting materials with a hardness of HRC55-65, the cutting speed of the CBN tool should be 50-120 m/min. When milling, vc=100-160 m/min, and the feed rate per minute vf=70-160 mm/min; when reaming, vc=60-130 m/min, ap=0.1-0.2 mm, and f=0.07-0.2 mm/r. CBN tools are mainly used for semi-finishing and finishing of hardened steel. The machined surface will not burn like grinding, and the efficiency is about ten times higher than grinding.
The geometric parameters of CBN tools when cutting hardened steel are γ0=-15°～-5°, α0=α’0=10°～15°, κr=30°～60°, κ’r=5°～15°, λs=0°～10°, and γε=0.3～1 mm.
When Using CBN Tools to Cut Hardened Steel, in What Cases is It Most Effective to Replace Grinding
Cutting complex surfaces and several complex surfaces on CNC machine tools instead of grinding processes can reduce 1/3 to 2/3 of the labor and ensure high position accuracy.
Inner holes or small holes with complex shapes. If grinding is used, the shape of the grinding wheel must also be complex. Sometimes grinding is impossible. At this time, turning is the most advantageous.
Several surfaces of a part (external circle, inner hole, end face, step, groove) need to be ground. At this time, turning is used, and one process can be completed, and the tooling for grinding can be reduced.
After quenching, parts are easy to deform and leave a small margin, which is easy to cause scrap. At this time, a larger margin can be left. After quenching, the excess margin can be cut off with a CBN cutting tool, and then ground to reduce scrap caused by large deformation.
When processing high-frequency parts with large load changes and used under difficult conditions, the surface structure and physical and mechanical properties of the workpiece are better than those when ground, which can extend the service life of the parts.
When selecting cutting tools such as milling cutters and end mills, the key is to properly select tool materials and geometric parameters based on the hardness of the material being processed and the shape of the workpiece. For the processing of high-hardness steel, how to optimize cutting speed, cutting depth and feed rate, as well as choosing the right tool material and coating, are important factors in ensuring processing quality and improving production efficiency. Carefully selecting cutting tools and reasonably adjusting cutting parameters can not only improve cutting results, but also extend tool life, thereby achieving economical and efficient processing goals.
