Machinability analysis of microstructures in pre-hardening plastic mold steel 718

Owing to strict dimension accuracy demands, pre-hardening treatment has been widely used in the mold for production of large plastic parts. However, the large volume of mold leads to the existence of tempered martensite and bainite structure on the cross section by pre-hardened heat treatment, and the uneven structure makes great influences on the cutting performance of the pre-hardening plastic mold steel. For service materials, machinability is affected by strength, work temperature, cutting conditions, plastic deformation, phase. Pioneering researchers tended to focus on the influences of temperature, cutting conditions and little is known about the effect of different microstructures in same materials. In this work, 718 steels with tempered martensite, lower bainite and grain bainite structures were prepared by heat treatment. The microstructures and mechanical properties were characterized by optical microscopy, scanning electron microscopy, X-ray diffractometer and universal tensile testing machine. Meanwhile, the effects of mechanical properties and structure on processing properties were studied by high-speed milling experiments and optical profilometer. The results show that when the cutting speed was lower than 145 m·min⁻¹, the bainite was easier to cut than tempered martensite, and the life of tool cutting for bainite was 30%‒40% higher than life of tool cutting for tempered martensite. When the cutting speed was higher than 165 m·min⁻¹, tempered martensite microstructure worked softening and the life of tool cutting for it increased, moreover, its workability advanced. The ridges were observed on the milling surface of grain bainite because of severe tool adhesion and tempered martensite structure has the best milling surface roughness. Under consideration, the comprehensive machinability of the three kinds of microstructure are ranked from high to low: lower bainite structure, martensite structure and granular bainite structure. The adoption of 300 ℃ austempering process can effectively improve the synthesis cutting performance of 718 plastic die steel.