What insert is better for carburized quenched steel with a hardness of up to HRC58-62 after heat treatment? For high-hardness carburized quenched steel parts, carbide inserts, ceramic inserts, and polycrystalline cubic boron nitride (CBN) inserts can all be processed. Why is CBN insert a more suitable tool for HRC58-62 carburized quenched steel?
Carburized Quenched Steel Processing Characteristics
Carburized quenched steel hardness is generally between HRC58-62 degrees. Common steels include 20CrMnTi, 40Cr, etc., which are often used for gears, gear sleeves, tooth molds, gear shafts, and other parts. Its cutting processing characteristics are as follows.
- Carburized quenched steel has high strength and hardness, and almost no plasticity, close to 0.
- When cutting HRC58-62 carburized quenched steel, the cutting force is large and the cutting temperature is high.
- It is not easy to produce a built-up edge on the surface of carburized quenched steel parts.
- The thermal conductivity of carburized quenched steel is low.
Heat Treatment Process of Carburized Quenched Steel
Carburized quenching is a common heat treatment process for metal materials. It can make the surface of carburized workpieces obtain high hardness and improve its wear resistance. The traditional processes mainly include: low temperature tempering, pre-cooling direct quenching, primary heating quenching, carburized high temperature tempering, secondary quenching cold treatment, induction heating after carburizing and other processes.
Machining Process of Carburized Quenched Steel
Carburized quenched steel needs fine processing after heat treatment to ensure the size and surface roughness required by the drawing. However, the carburized quenched steel parts after heat treatment have high hardness and are difficult to process. Some carburized quenched workpieces are intermittent cutting, which is easy to cause tool chipping. Take carburized quenched gears as an example.
- Gear processing technology: cutting – forging – normalizing – rough turning – quenching and tempering heat treatment (quenching + high temperature tempering) – fine turning – tooth surface grinding – inspection and storage.
- From the processing technology point of view, carburized quenched steel parts need grinding to ensure the requirements of the drawings. Gears are intermittent cutting, and the turning tool cannot be processed for a time.
Tool Materials for Process of Carburized Quenched Steel
With the continuous development of cutting technology, the tool industry has successively developed coated carbide tools, ceramic tools and cubic boron nitride tools to realize the processing of carburized hardened steel by turning instead of grinding.
Coated carbide insert. It is to electroplate a layer of wear-resistant material on the carbide insert with good toughness. It has higher hardness and better wear resistance than the uncoated carbide insert, but the thickness of the coating is generally 2 to 18μm, and the wear-resistant layer is thin. Long-term processing causes the wear-resistant layer to wear. After the wear-resistant layer is worn, the processing performance is no different from that of the uncoated carbide. Moreover, when encountering intermittent working conditions, the edge of the coated carbide insert is easy to crack, resulting in increased wear, frequent tool changes, affecting processing efficiency, and increasing tool costs.
Ceramic insert. Compared with coated carbide tools, it has higher hardness and better performance, but the disadvantage is that it is brittle and ceramic inserts are easy to break during intermittent cutting.
CBN tools. The hardness of cubic boron nitride material is second only to diamond tools, and it is collectively called superhard tools together with diamond tools. In addition to high hardness and good wear resistance, it also has the disadvantage of being brittle. Through continuous efforts and research, my country’s tool industry has developed non-metallic binder cubic boron nitride tools, which solves the problem that CBN tools can only be used for finishing processes but not for intermittent processing. For carburized and quenched steel parts processing, BN-H10 can be used for continuous finishing processes, and BN-H21 can be used for strong intermittent processing processes.
Cutting Parameters of Carburized and Quenched Steel by CBN Insert
There are often two working conditions when processing carburized and quenched gear steel. Continuous cutting, intermittent cutting and rough turning. my country has developed BN-H10, BN-H21 and BN-S20 materials for the two working conditions. The processing plan is as follows.
Continuous cutting. Select BN-H10 material, which belongs to the composite CBN insert, and the cutting depth is controlled within 0.3mm. Cutting parameters: ap≤1mm, Fr=0.05-0.35mm/r, Vc=60-250m/min.
Intermittent cutting. Choose BN-H21 material, which is a composite cubic boron nitride tool with strong wear resistance and fracture resistance, and the cutting depth is controlled within 0.3mm. Cutting parameters: ap≤1mm, Fr=0.05-0.35mm/r, Vc=60-250m/min.
Rough turning. There is also a kind of intermittent cutting, which belongs to large-allowance turning. Due to the large deformation of large workpieces after heat treatment, the machining allowance is ≥2mm, and it belongs to strong intermittent cutting. In this case, choose the BN-S20 material developed in my country. This brand belongs to an integral cubic boron nitride tool with no upper limit on the cutting depth, which can fill the entire insert.
