**Abstract**: Machining hardened steel presents significant challenges due to the high hardness of the workpiece. Traditional hard alloy tool grades often struggle with efficiency, accuracy, and shape tolerance when processing such materials. In recent years, advancements in tool materials have introduced new coated alloys and ceramic inserts, which are now widely used in various machining applications. However, these alternatives still face limitations. For instance, coated inserts tend to burn easily when machining HRC45 hardened steel, while ceramic inserts are not suitable for materials harder than HRC52 and lack cost-effectiveness compared to superhard tools. Additionally, the brittleness of ceramic and cubic boron nitride (CBN) inserts limits their use to finishing operations, as they are prone to chipping during heavy cuts or intermittent machining.
With the rapid development of the mechanical manufacturing industry, the concept of machining has evolved from handling high-hardness materials to focusing on high-efficiency processing of difficult-to-machine materials like hardened steel. As competition increases and demands for precision grow, the focus of tool usage has shifted from merely extending tool life to improving machining efficiency and reducing costs.
This evolution has led to the development of advanced PCBN (Polycrystalline Cubic Boron Nitride) tools. The new grade PCBN inserts, such as BN-H10, BN-H20, and BN-S20, developed by China Hualing Superhard Tool Co., Ltd., are specifically designed for machining hardened steels and superalloys with hardness ranging from HRC45 to HRC79. These tools are widely used in turning, milling, boring, and grooving operations for quenched steel parts, including flanges, shafts, and disc components. They are suitable for a variety of materials such as 45# steel, 40Cr, 9SiCr, T8, T10, H13 tool steel, carburized and quenched steels like 20CrMnTi and 20Cr, as well as GCr15 bearing steel and other high-hardness materials.
The BN-H10 grade is a fine-grained CBN insert (0.5–5μm), ideal for high-speed and high-precision machining of HRC50+ quenched materials, commonly used in continuous cutting operations. It is also suitable for non-standard tools like boring and grooving blades. The BN-H20 grade features a mixed crystal structure (3–15μm) that offers better impact resistance, making it suitable for interrupted cuts, such as turning slender shafts or perforated flanges. The BN-S20 grade is a monolithic sintered CBN insert made with a non-metallic binder, allowing it to handle large roughing cuts without compromising tool life. It is especially effective for high-speed machining of materials with hardness above HSD85, such as cold-rolled steel rolls and GCr15 bearing steel.
In terms of material properties, cemented carbide inserts typically have a hardness of 89–94 HRA (equivalent to 71–76 HRC), which makes them unsuitable for HRC40+ hardened steel due to wear and burning issues. Ceramic inserts are too brittle and prone to chipping. In contrast, CBN tools have a hardness of HV3000–5000, equivalent to HRC95–100, making them the most economical choice for high-speed machining of high-hardness quenching materials. CBN tools generally last several times longer than carbide or ceramic inserts and are increasingly adapted for complex high-hardness materials.
Hualing Superhard’s BN-K10 grade can even process tungsten carbide with hardness above HRC70, a first in China. While CBN tools are more brittle than traditional hard alloy blades, Hualing has developed specialized grades like BN-S20 that offer excellent impact resistance and wear resistance, making them suitable for both roughing and high-speed finishing.
From 2003 to 2013, Valin Superhard introduced several new CBN tool grades, such as BN-H10, BN-H20, BN-S20, BN-K10, BN-K20, BN-K1, BN-S30, and BN-S50, each tailored for different applications, including quenched iron castings, powder metallurgy, and more.
These tools are applicable across a range of machining operations, including turning, milling, and boring, on CNC lathes, milling machines, and machining centers. They come in various structures, such as indexable blades and forming tools, and are available in standard and non-standard configurations.
To evaluate the quality of CBN tools, two key factors are impact resistance and wear resistance. High-quality CBN tools should not chip or break during roughing and should maintain efficiency while replacing carbide or ceramic tools at a lower cost. From a manufacturing perspective, the purity of raw materials, precision grinding, and strict inspection processes are critical to ensuring long tool life and consistent performance.
Hardness conversion between HRC, HSD, and HV is essential for accurate material evaluation. For example, a hardness of 64–69 HRC corresponds to 85–100 HSD, and further conversion to Vickers hardness helps in selecting the right tool for specific applications.
Common CBN blade models include BN-S20 SNMN120408, BN-H20 CCMW09T304, and others, with cutting parameters varying based on the machining allowance. When the allowance is less than 0.5mm, BN-H20 is preferred, while BN-S20 is used for larger allowances to avoid multiple passes and improve efficiency.
In conclusion, the new PCBN inserts BN-H10, BN-H20, and BN-S20 address the inefficiencies and reliability issues of traditional tools when machining hardened steel. They provide improved accuracy, reduced tool wear, and greater cost-effectiveness, marking a significant advancement in the field of high-hardness machining.
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Xuzhou Surun wear-resistant material Co., LTD , https://www.suruntools.com