Influence of Hardness on Machinability of Quenched and Tempered Bearing Steel

Machinability of a bearing steel of which hardness was changed in a wide range by quenching and tempering was investigated. Sawing, drilling and turning were carried out for work materials of the hardness of HV180-770 and cutting time, thrust force and resultant cutting force were measured respectively. 1) In all machining methods the minimum cutting resistance (cutting time) was observed at the hardness of about HV350. Multiple regression analysis was applied between the above measuring values and the mechanical properties of work materials. The regression equation indicates that the cutting resistance increases with increase of the tensile strength, hardness, the elongation and reduction of area. 2) It was suggested that the minimum in the cutting resistance at the hardness of_??_HV 350 was brought about by the formation of the strong built-up edge. The cutting resistance increased in the hardness range of HV180-250 and 430-550. The former is due to an enlargement of the chip shear region and the latter to an increase of the deformation resistance per unit area of the chip shear region. 3) Sawtooth like chips were produced, when work materials of the hardness of HV550-700 were machined at high speed. The mechanism is considered that the chip shear region is subjected to the shear deformation by softening arising from the concentrated generation of heat.