THE EFFECT OF COOLING CONDITIONS ON THE SOLIDIFICATION MODE OF STAINLESS STEELS

Abstract

Effect of different coolil).g conditions on solidification mode of two types of stainless steels, martensitic and austenitic have been studied through experimental investigations using simulation. model. The cooling conditions are varied through both indirect and direct cooling times. The effects of ferritic former elements such as Cr and austenitic stabilizing elements such as Ni on the formation of obtained phases, are expressed as a ratio of Creq I Nieq. This ratio could be used as a guide in order to indicate the solidification mode and final microstructure. Such ratio has been applied successfully, in this work, under the present cooling conditions for austenitic stainless steel. The present results suggest that the solidification sequence of martensitic stainless steel could be as follows: L . L + 8 . L + 8 + y . 8 + y . 8 + y + Carbides

The solidification of martensitic stainless steel starts with ferrite followed by austenite phase in two paths: direct from the rest of the melt and partially transforms from solid ferrite. The carbides precipitate in martensitic stainless steel at grain boundaries between ferrite and austenite. It becomes thicker and precipitate as a D-aggregate by decreasing cooling rate i.e. decreasing directionality. The present results suggest that the solidification sequence of austenitic stainless steel could be as follows: L . L+ y . y . y +Carbides ---.y + Carbides + a

The final microstructure consists of austenite grains dendritic and Widmanstatten carbides at grain boundaries and a layer of ferrite between carbides and austenite.