Criteria for determining machinability and techniques of improving machinability of steel and alloys

It has been established that during of steel and other work materials there is a definite cutting speed at which chip-tool-contact changes abruptly from contact with built-up-edge (BUE) formation to contact without BUE formation. This cutting speed has been termed as critical cutting speed, Vc. Intensity of wear of a tungsten carbide tool is minimum at a speed slightly lower that Vc, when a soft BUE protects the tool from intensive wear. In case of with Tungsten-Titanium carbide tool there is a second cutting speed, slightly above Vc at which a second peak of tool life is observed. These speeds have been referred to as optimum cutting speeds, Vopt1, and Vopt2 respectively. Since both these speeds have definite relationship with the critical cutting speed, Vc which has a specific value for every pair of work and tool materials and conditions of cutting, so the speed Vc may be used on an important criteria for determining machinability of work materials. Three different methods of determining cutting speed Vc has been discussed in this paper. Two different methods of improving machinability of steel has been discussed in the paper. In the first method machinability has been improved by micro-alloying of steel with calcium, which leads to a considerable shift of V towards higher cutting speed (by approximately 140%). The reasons for this improved machinability have been explained in the paper. In the second method low machinability of Titanium alloys and heat resistant steel, associated with instability of chip formation leading to micro and macro chipping of the tool is overcome by preheating of these materials to optimum temperatures. Preheating considerably raises tool life due to lower amplitude and average value of cutting force and higher chiptool contact length resulting in lower stress, acting on the tool tip