Solidification structures of the AISI M2 type high speed tool steel were studied under various solidification conditions by metallographic and thermal analysis. Small laboratory melts were unidirectionally solidified to simplify the freezing process. Also cooling curves were obtained on 300kg ingots to investigate the differences in the solidification structure and freezing process between the unidirectionally frozen ingots and practical ingots.
The results obtained were as follows:
1) The secondary dendrite arm spacing, SII (μ), in columnar crystals of unidirectionally frozen ingots, was influenced only by the average cooling rate, R (°C/min), during solidification, exhibiting relationship, SII=100R-0.28, for the M2 steel.
2) The progression of liquidus isotherm in 300kg ingot varied linearly with square root of time, but that of solidus isotherm did not.
3) In 300kg ingots, grain diameter of equiaxed crystals, ι(μ), varied with average cooling rate, R, as did the dendrite arm spacing, showing relationship, ι=220 R-0.29. These results indicate that the mechanism of the growth of grobular grain is similar to that of the secondary dendrite arm.
4) The morphology of the dendrites changed from equiaxed to cellular type with decreasing liquidus isotherm drift rate, V (cm/min), and with increasing temperature gradient, G (°C/cm), across solidified layer.