Processing characteristics of hardened steel
Hardened steel refers to steel that has undergone a quenching process (usually followed by tempering) to a high hardness. Its hardness is generally above HRC45, and some, such as mold steel, bearing steel, and tool steel, can even reach HRC60-70. Hardened steel offers high strength, high wear resistance, and excellent dimensional stability, making it widely used in machinery manufacturing, mold processing, the automotive industry, and other fields. However, it also presents significant processing challenges, making it a typical difficult-to-machine material. The processing characteristics of hardened steel are primarily reflected in high cutting forces, high cutting temperatures, rapid tool wear, and difficulty in ensuring surface quality. Understanding these characteristics and implementing appropriate processing measures are crucial for improving the efficiency and quality of hardened steel processing.
The high hardness of hardened steel is the primary reason for its machining difficulties. During the cutting process, the contact stress between the tool and the workpiece is extremely high, generating cutting forces significantly greater than those encountered when machining ordinary steel. Typically, cutting forces in hardened steel are 1.5 to 2 times greater than those encountered when machining 45 steel. The increase in radial cutting forces is particularly pronounced, which not only predisposes the tool to bending and vibration but also increases the load on the machine tool, impacting machining accuracy. To reduce cutting forces, appropriate tool materials and geometry are crucial. Tool materials should exhibit high hardness and wear resistance, such as cubic boron nitride (CBN) and ceramic tools. The rake angle should be appropriately reduced (even negative) to enhance cutting edge strength, and the lead angle should be large to minimize radial cutting forces. Furthermore, cutting parameters should be appropriately reduced, with smaller feed rates and depths of cut to reduce specific cutting forces and prevent tool chipping.
High cutting temperatures are another significant characteristic of hardened steel machining. Due to the poor thermal conductivity of hardened steel (approximately 1/3 to 1/2 that of ordinary steel), the heat generated during cutting is difficult to dissipate through the workpiece and chips . Instead, a significant amount of heat is concentrated in the cutting zone, causing a sharp rise in cutting temperatures, typically reaching 800-1200°C. High temperatures not only increase tool wear but also cause thermal deformation in the workpiece, compromising machining accuracy and surface quality, and even leading to surface defects such as burns and cracks. To reduce cutting temperatures, enhanced cooling and lubrication measures are necessary. High-performance cutting fluids, such as extreme pressure emulsions or cutting oils, should be used. These fluids should be delivered directly to the cutting zone via high-pressure jets to dissipate heat. Choosing the right cutting speed is also crucial. Excessively high cutting speeds can rapidly increase cutting temperatures. Therefore, a moderate cutting speed (50-150 m/min) is typically used when machining hardened steel to ensure a certain level of machining efficiency while avoiding excessive temperatures.
Rapid tool wear is a prominent problem in hardened steel machining. Due to the high hardness and cutting temperatures of hardened steel, cutting tools are subjected to intense friction, impact, and high temperatures during the cutting process, resulting in accelerated tool wear and shortened tool life. Tool wear primarily occurs in abrasive, adhesive, diffusion, and chemical wear forms. Abrasive wear is caused by the scratching of the tool surface by hard particles (such as carbides) in the workpiece material and is particularly pronounced when machining high-hardness hardened steel. Adhesive and diffusion wear at high temperatures further exacerbate tool damage. To improve tool wear resistance and tool life, tool materials suitable for machining hardened steel should be selected. Cubic boron nitride (CBN) tools, with their extremely high hardness (HV 3000-5000) and wear resistance, maintain excellent cutting performance at high temperatures, making them the preferred tool material for machining hardened steel. Ceramic tools (such as alumina-based and silicon nitride-based ceramics) also offer high hardness and wear resistance and are suitable for machining medium-hard hardened steel. In addition, the quality of tool sharpening must also be guaranteed. The cutting edge should be sharp and have a certain strength to avoid microcracks.
The surface quality of hardened steel is difficult to guarantee, primarily due to high surface roughness, residual stress, and work hardening. Due to the low plasticity of hardened steel, the cutting process easily produces chipping chips, which scratch the machined surface and increase surface roughness. Furthermore, the drastic fluctuations in cutting forces and temperatures can generate high residual stresses on the workpiece surface. Excessive residual stress can cause deformation or cracking during use. Work hardening is also a serious phenomenon. The cutting forces cause plastic deformation of the workpiece surface, increasing the surface hardness. The hardened layer can reach a depth of 0.05-0.3mm, which not only affects the efficiency of subsequent machining (such as grinding) but also reduces the fatigue strength of the workpiece. To improve surface quality, sharp cutting tools and reduced feed rates are required to reduce cutting forces and surface plastic deformation. Furthermore, proper cooling and lubrication can reduce friction and thermal damage, thereby reducing surface roughness. For hardened steel parts with high precision requirements, grinding is often performed after turning or milling to further improve surface quality and dimensional accuracy.
Hardened steel has poor machinability and places high demands on machining equipment and processes. Due to the high cutting forces and torques involved, machine tools for machining hardened steel require sufficient rigidity and power to avoid vibration and deformation during machining. Key components, such as the spindle and guideways, must also exhibit high precision and stability. Traditional machining methods (such as conventional turning and milling) are insufficient for hardened steel machining, leading to the use of hard turning (turning instead of grinding) or grinding. Hard turning involves the direct machining of hardened steel using ultra-hard tools. This method offers advantages such as high efficiency, low cost, and excellent surface quality, but places high demands on both the tool and the machine. Grinding, while capable of achieving high dimensional accuracy and surface quality, is associated with lower efficiency and higher costs. In actual production, the machining process should be appropriately selected based on factors such as part precision requirements, batch size, and production costs. If necessary, combined machining methods (such as hard turning plus fine grinding) may be employed to balance efficiency and quality.
