Experimental Study On Physical And Mechanical Properties Of GCr15 Bearing Steel In Ultrasonic Assisted Hard Turning

With the appearance of PCBN tool, the application of hard turning in hard-to-cut materials is more and more wide. It has become a new machining method that using hard turning to instead of grinding as the finishing operation of hardened materials. While the processing technology of combining between ultrasonic and common turning is also becoming a new study method for scholars from various countries. As we know, the white layer and residual stress of workpiece can not be averted in hard turning. The cutting parameters and tool wear are the main factors effecting white layer and residual stress.In this thesis, GCr15 bearing steel is drily cut in common and ultrasonic turning with PCBN tool. The influence rules of machine speed, feed and cutting depth on residual stress and white layer are studied by orthogonal experiment. And the cutting parameters are optimized too. Through experiment study, we find that residual compressive stress is produced both in common and ultrasonic turning of GCr15 bearing steel. The influence of machine speed on tangential residual compressive stress is largest. The influence of feed is smaller than that of machine speed. The influence of cutting depth on tangential residual compressive stress is smallest. Residual compressive stress can improve the fatigue performance and life of workpiece, so the cutting parameters should be optimized to get the best residual compressive stress in turning process. The cutting parameters optimized in this experiment are: machine speed n=250r/min, feed f=0.103mm/r, cutting depth ap=0.18mm.Through common and ultrasonic turning of GCr15 bearing steel, we find that there is white layer on the surface of workpiece in common turning while there is not white layer in ultrasonic turning when the ultrasonic vibration effect is best. When the power of ultrasonic generator reduces or improves to a certain extent, there will be a thinner white layer on the surface of GCr15 bearing steel in ultrasonic turning. The cutting heat produced in common turning sufficient to achieve phase transition temperature of GCr15 bearing steel and the surface of workpiece is cooled down rapidly after phase transition, so the rapidly freezing and heating process is phase transition mechanism. In common turning, the influence of machine speed on white layer thickness is largest, and the influence of cutting depth is smaller than that of machine speed, while the influence of feed is smallest. White layer can improve the abrasion resistance, but it also can cause early brittle flaking of workpiece. So, the wokpiece machined in ultrasonic turning can avoid brittle flaking of white layer. If the abrasion resistance of white layer need be used, the workpiece can be produced by common turning. So the cutting parameters should be optimized to get the best white layer thickness. The optimized cutting parameters in this experiment are: machine speed n=250r/min, cutting depth a_p=0.9mm, feed f=0.137mm/r.