DIRECTIONAL SOLIDIFICATION STUDIES.

Ductility of AISI 4340 alloy heat treated to 150,000 170,000 psi yield strength level is strongly affected by solidification mode. Reduction in area is doubled in heavy sections increased from 50 per cent per cent by control of solidification. Optimum ductility is found in castings with columnar structures. At the 150,000 170,000 psi yield strength level, tensile, yield, and impact strength are relatively insensitive to solidification mode. Ductility of equiaxed structures correlates well with dendrite arm spacing, decreasing with increasing dendrite arm spacing. Ductility of columnar unidirectionally solidified castings is relatively insensitive to dendrite arm spacing. Fatigue strength was significantly affected by solidification structure. The fatigue limit for columnar specimens was in excess of 60,000 psi, with an endurance ratio of 0.36, while for equiaxed specimens fatigue limit was 45,000 psi, with an endurance limit of 0.29. Choice of chill material and use of watercooling for unidirectional solidification depends on casting size and complexity. Watercooled copper chills provide rapid heat transfer but are not necessary for small castings. Author