Formation mechanism of surface defect during high-speed ball-end milling process of hardened die steel Cr12MoV

Cr12MoV die steel is frequently used for cold stamping, cold extrusion, and other cold work applications. However, surface defects such as micro-pits and micro-cracks are easily produced during the cutting process, which impacts the fatigue resistance of die. The formation mechanisms and influencing factors of various surface defects of hardened die steel are investigated by a combination of high-speed ball-end milling experiment and simulation. Results show that there are mainly five kinds of defects on the machined surface: micro-pits, micro-cracks, grooves, surface texture, and surface material residues. Micro-pit defects are formed when carbide particles are integrally pulled out, or broken and then spalled. When the carbide particles are large in size or have sharp edges, local microscopic stress concentrations will be generated between the sharp edges and the material matrix, causing micro-cracks within and around the spalled pits. The form of carbide particles spalling is changed from brittle spalling to plastic plowing under the effect of thermal softening, which causes the formation of groove defects. However, with the aggravation of thermal softening effect, machined surface is highly susceptible to extrusion and scraping from tool flank, which results in an increase in the number of defects and increasingly pronounced textural features. The research has important implications for improving the fatigue life of die and optimizing the cutting process parameters.