With the rapid development of my country's industry, the performance requirements of mechanical parts are getting higher and higher. In order to improve the wear resistance of mechanical parts and prolong their service life, many structural steel parts are often quenched. However, the strength and hardness of the parts after quenching are high, and the plasticity is poor, which brings difficulties to the cutting process. The quality of tool performance is crucial to the efficiency and precision of cutting. This article mainly introduces the characteristics of hardened steel and how to process high-hardness hardened steel parts.
1. The definition and processing characteristics of hardened steel
Hardened steel refers to a type of workpiece material that obtains a martensitic structure after quenching heat treatment, has high hardness (greater than HRC50), high strength, and almost no plasticity.
The cutting characteristics of hardened steel are:
1. High hardness, high strength, plasticity close to 0: When the hardness of hardened steel is in the range of HRC50-60, its strength can be as high as σb=2100-2600Mpa. At this time, the hardness and strength of hardened steel reach 9a level, and hardened steel is the most difficult material to cut.
2. High cutting force and high cutting temperature: Due to the high hardness and high strength of hardened steel, it is difficult to cut chips from the workpiece. The unit cutting force of hardened steel can reach 4500Mpa, and the corresponding cutting temperature is very high.
3. It is not easy to produce built-up edge: Although hardened steel has high hardness, it is brittle, so it is not easy to produce built-up edge during cutting, and the surface of the processed workpiece can obtain a lower surface roughness.
4. Low thermal conductivity: The thermal conductivity of hardened steel is generally 7.12W/(m?K), which is about 1/7 of No. 45 steel.
Hardened steel is widely used in the manufacture of basic parts that require high wear resistance and high hardness, such as transmission shafts, transmission gears, steering gears, dies, etc.
2. The processing method of hardened steel
The processing methods of hardened steel include traditional grinding and modern mechanized turning.
(1) Grinding: It is a cutting method that uses high-speed rotating grinding wheels and other abrasive tools to process the surface of workpieces. The characteristics of grinding are: on the one hand, the grinding speed is as high as 30-50m/s; the grinding temperature is high, the highest is 1000°C-1500°C; Obtain high machining accuracy and small surface roughness value; Grinding can process a wide range of materials, including soft materials: such as unquenched steel, cast iron, etc. Including hard materials: hardened steel, porcelain, cemented carbide and other hard materials that cannot be processed by other tools. On the other hand, the metal removal rate of grinding is low; and the grinding process may cause personal danger.
(2) Turning: refers to the process of cutting hardened steel as the final processing or finishing process. Compared with grinding, the processing characteristics of turning are: high processing efficiency. This is because the cutting depth is large and the workpiece speed is high during cutting. The metal removal rate is usually 3 to 4 times that of grinding, and turning does not require Coolant, also saves cooling equipment, saves time and cost; enables parts to obtain good overall machining accuracy, because chips take away most of the heat produced in turning, while grinding will burn the surface of the workpiece and produce crack. So turning is currently the most suitable method for processing hardened steel.
3. Selection of tool materials for processing hardened steel
Reasonable selection of cutting tools can double the processing efficiency of hardened steel, and can obtain the best precision requirements and optimize the processing plan.
The hardness of hardened steel is higher, generally above HRC50. The requirements for processing tools are extremely strict: good wear resistance, high thermal conductivity, excellent thermal stability, and high hardness. Commonly processed hardened steel knives include carbide tools, ceramic tools and CBN tools.
However, when the carbide tool is processing hardened steel, the hardness is not high enough, it is easy to burn the knife, and the production efficiency is low, so it is suitable for fine turning and low-speed processing; ceramics have poor impact toughness, low bending strength, and low feed rate. The range is low, so it is less used in the processing of hardened steel; the cubic boron nitride tool has high hardness, good heat resistance, fast cutting speed, and its service life is several times longer than other tools. Therefore, it is more and more economical to use CBN tools to process hardened steel. Especially the non-metallic binder CBN tool has outstanding effect on processing hardened steel.
4. The solution of processing hardened steel gear with non-metallic adhesive CBN tool BN-H20 grade:
Processing parts: gear shaft
Hardness: 58HRC-63HRC
Processing procedure: finishing turning after quenching
Tool grade: BN-H20
Cutting parameters: ap=0.15mm, Fr=0.1mm/r, Vc=150m/min
5. The solution of processing hardened steel synchronizer gear sleeve shift fork groove with non-metallic adhesive CBN tool BN-H10 grade:
Non-metallic adhesive CBN tool BN-H10 grade, with good shock resistance, good toughness, suitable for poor rigidity of machine tools; CBN tool parameters: relief angle 7 degrees, secondary deflection angle 4 degrees, tool tip arc Angle R0.4; Cutting line speed: v=90-120m/min; Feed rate Fr=0.08mm/r; The number of workpieces processed at one time of tool grinding is 350 pieces.
6. The solution of processing slewing ring bearing (40CrMo) with non-metallic adhesive CBN tool BN-S200 grade:
40CrMo bearings are also called slewing ring bearings (as shown in the figure below). The reason is that the commonly used material for slewing ring bearings is 40CrMo. The hardness of 40CrMo bearings after heat treatment is HRC47-HRC55. For processing 40CrMo bearings, CBN tool BN-S200 is selected. Mainly used in finishing processes due to its superior wear resistance when machining hardened steel and heat-resistant alloys. Processing hardness: processing procedure: finish car raceway, end face, cutting parameters: ap=0.5mm, Fr=0.45mm/r, Vc=110m/min.