EFFECTS OF DIFFERENT CRYOGENIC TREATMENTS ON DRILLING PERFORMANCE OF HSS DRILLS

Cryogenic processing is a heat treatment which has been widely used in recent years to improve the properties of tool materials. This process has been reported in the literature as it provides significant contributions to wear resistance and tool life of tool steels. This study presents the differences in tool performance between untreated and cryogenically treated M2 high speed steel (HSS) drill bits in terms of their tool wear, tool life, hardness and chip formation properties. Also, the effects of two different tempering temperatures (200 ^oC and 250 ^oC) in cryogenic treatment on tool performance are discussed. Drilling performances were studied on different workpieces of SAE 1050, lamellar cast iron and sphero cast iron, and the type of wear is characterized by a high resolution camera. Moreover, fracture tests were performed on a steel workpiece using a constant drilling speed of 1100 m/min and a feed rate of 0.5mm/rev. The microstructures of the samples were characterized using optical microscopy and SEM. The Vickers micro-hardness tests of the samples were performed using 100 g load. Microstructural studies showed that cryogenically treated samples exhibit better microstructure with finer and more homogeneous carbides which yields better tool wear and hardness properties. In paralel, cryogenically treated drills showed improved tool life than untreated drills during fracture tests. When tempering temperatures are compared, tempering at 250 °C resulted in better performance than 200 °C of tempering temperature. Also, the reduction in diameter values during drilling tests were consistent with the tool life tests.