Thermionic Emission Microscopy of Metals. II. Transformations in Plain Carbon Steels

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Abstract
The direct observation of transformations in plain carbon steels at temperatures above about 625°C is demonstrated for both the A3 and A1 transformations. It is concluded from emission studies of the decomposition of austenite below the A3 line that the transformation is of a diffusionless type rather than carbon diffusion controlled. It is shown that carbon is associated with ``memory'' in the austenite grain structure, the memory improving with increasing carbon content. The emission images also indicate that the growth of a ferrite grain in austenite induces a recrystallization of the surrounding austenite. A model to account for the memory is advanced based on thin sheets of retained austenite in newly transformed ferrite. Studies of a 0.77 percent carbon steel below the A1 line illustrate the formation of pearlite. The images suggest that these decompositions take place in two distinct steps. The first is the formation of a metastable crystal which subsequently decomposes by carbon diffusion to the final ferrite‐carbide products. Austenite recrystallization about a growing pearlite nodule is demonstrated. A model for the transformation based on a diffusionless type reaction followed by a diffusion decomposition is suggested to explain the emission images.