Machining of ultra-high strength steel bolts
Ultra-high-strength steel bolts have been widely used in important fields such as aerospace, automobile manufacturing, and bridge engineering due to their extremely high strength, good toughness, and fatigue resistance. They are key connectors to ensure the safe operation of mechanical equipment and structures. However, the high strength, high hardness, and low plasticity of ultra-high-strength steel also make it a difficult material to machine. The turning process faces many challenges, such as high cutting forces, high cutting temperatures, rapid tool wear, and difficulty in ensuring the quality of the machined surface. Therefore, studying and mastering the turning process of ultra-high-strength steel bolts is of great practical significance for improving product quality and reducing production costs. When turning ultra-high-strength steel bolts, it is necessary to optimize multiple aspects, such as tool selection, process parameter setting, cooling and lubrication, to achieve efficient and high-quality processing.
The proper selection of tool material is crucial for successful turning of ultra-high-strength steel bolts. Because ultra-high-strength steel typically has a hardness between 30-50 HRC and a strong tendency to work harden, ordinary tool materials are unable to withstand its intense wear. Therefore, tool materials with high hardness, excellent wear resistance, and strong heat resistance are preferred, such as ultra-fine-grain carbide, ceramic tools, and cubic boron nitride (CBN) tools. Ultra-fine-grain carbide tools offer high hardness and toughness, capable of withstanding high cutting forces. They are suitable for roughing and semi-finishing stages, exhibiting excellent wear resistance at cutting speeds of 50-100 m/min. Ceramic tools offer even greater hardness and heat resistance, enabling them to operate at higher cutting speeds (100-200 m/min), making them suitable for semi-finishing and finishing. However, they are more brittle and have poorer impact resistance, so sudden changes in cutting forces must be avoided during use. CBN tools are one of the hardest tool materials currently available. They can withstand extremely high cutting temperatures and forces and are suitable for finishing ultra-high-strength steel bolts. They can achieve high dimensional accuracy and surface quality, but they are relatively expensive and are generally used in demanding applications.
Optimizing turning process parameters is crucial for machining quality and efficiency of ultra-high-strength steel bolts. The cutting speed should be determined based on the tool material and the specific grade of ultra-high-strength steel. Excessively high cutting speeds can lead to a sharp increase in cutting temperature, exacerbating tool wear; excessively low cutting speeds can increase cutting forces and cause severe work hardening. For ultra-fine-grain carbide tools, the cutting speed is typically controlled between 60-90 m/min; ceramic tools can be increased to 120-180 m/min; and CBN tools can reach 200-300 m/min. The feed rate should be kept low, typically 0.05-0.15 mm/r, to reduce cutting forces and work hardening while ensuring surface quality. The depth of cut should be determined according to the machining stage. For roughing, a depth of 1-3 mm is recommended to quickly remove excess material; for semi-finishing and finishing, it should be reduced to 0.1-0.5 mm to reduce surface roughness. In actual machining, process parameters should be adjusted through trial cutting to find the optimal cutting combination.
The rational use of the cooling and lubrication system can effectively reduce cutting temperatures, reduce tool wear, and improve the quality of machined surfaces. The large amount of cutting heat generated during the turning process of ultra-high-strength steel will cause rapid wear of the tool, and at the same time cause thermal deformation and burns on the workpiece surface, so effective cooling and lubrication measures must be adopted. Extreme pressure emulsions or extreme pressure cutting oils are usually selected as cooling lubricants. They have good lubricity, cooling properties, and extreme pressure properties. They can form a lubricating film between the tool and the workpiece, and between the tool and the chips, reducing friction and heat generation. The cooling method should adopt high-pressure jet cooling, and the cooling lubricant should be sprayed directly into the cutting area to ensure sufficient cooling. At the same time, it is necessary to ensure an adequate supply of cooling lubricant and good filtration to prevent impurities from entering the cutting area and scratching the workpiece surface or aggravating tool wear. For some difficult-to-process ultra-high-strength steel grades, oil mist lubrication or low-temperature cooling technology can also be used to further improve the cooling and lubrication effect.
During the turning process of ultra-high-strength steel bolts, attention must also be paid to aspects such as workpiece clamping, design of tool geometry parameters, and post-processing quality inspection. The workpiece clamping should be firm and reliable to avoid vibration that affects the processing accuracy and surface quality. For slender bolts, a tool rest or center rest should be used for auxiliary support to prevent the workpiece from bending and deformation. The tool geometry parameters should be optimized according to the characteristics of ultra-high-strength steel. The rake angle is generally -5°-5° to enhance the strength of the tool; the back angle is 5°-10° to reduce friction on the back tool face; and the main deflection angle is 90°-100° to reduce radial cutting forces. The processed bolts are subject to strict quality inspection, including testing of dimensional accuracy, surface roughness, thread accuracy, and mechanical properties to ensure that they meet the design requirements. Only by comprehensively optimizing the turning process can the efficient and high-quality processing of ultra-high-strength steel bolts be successfully achieved.
