Effect of Microstructure on Machinability of Cast Irons—I

Abstract

Abstract Test bars were prepared to study the effect of the following structures in the milling of cast iron: Graphite-pearlite of various gradations of coarseness; graphite-pearlite plus about 5 per cent of free-carbide segregation; graphite-ferrite. Quantitative results of the relative machinability of these structures are presented in terms of (1) tool life, (2) power requirements, (3) surface finish. Tool life was found to increase as much as 2:1 as the graphite-pearlite structure was varied from fine to coarse. For the same graphite-pearlite size, the presence of free carbides had no effect on tool life at a cutting speed of 200 fpm but seriously reduced tool life between 250 and 1000 fpm. The increase in tool life for the graphite-ferrite structure as compared to all other structures was outstanding, e.g., at 1150 fpm the increase was of the order of 8:1, compared to the pearlite structure, while at 300 fpm the increase was of the order of 50:1. All of the graphite-pearlite structures required the same amount of power to mill at a constant rate of metal removal for the same feed per tooth. The graphite-ferrite required approximately ½ to ⅔ the power per cubic inch for the same feed per tooth compared to graphite-pearlite structures. The sharpness of the cutter played an important role in power requirements. A “dull” cutter required as high as 3 times the power as that for a sharp cutter at 0.002 in. feed per tooth, and 1.3 times as much power at 0.020 in. feed per tooth. Surface finish improved as the cutting speed increased and as the graphite distribution became finer.