Application of pulse-echo ultrasonics to locate the solid/liquid interface during solidification and melting of steel and other metals

The velocity of sound and the density have values that are sufficiently different for liquid as compared to solid phases of metals and alloys to permit the use of pulse-echo ultrasonic techniques to locate the solid/liquid interface during solidification and melting. Experimental results are presented for pulse-echo observation of the melting and freezing of pure iron, 304 stainless steel, and tin, using Bridgman-type furnaces with unidirectional heat flow, at frequencies from 1 to 5 MHz. For both iron and steel, rapid grain growth in the solid phase at high temperatures can strongly attenuate the sound waves and can also produce backscattered waves which obscure the identification of the solid/liquid echo. Additionally, in alloys the presence of a ‘‘mushy zone’’ rather than a sharp interface further reduces the reflected signal. These signal/noise problems were successfully overcome by the use of a transducer spatial scanning technique with computer signal averaging that permits the interface to be located even in concentrated alloys.