Doctoral Dissertations

Date of Award

8-1954

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

E. E. Stansbury

Committee Members

John H. Fryer Jr. Lawrence K. Jetler, Walter S. Snyder, Anton Besaumas, H. K. Garber

Abstract

Plan of the Investigation: The data needed to determine the thermodynamics of the transformation of a steel are: (1) the specific heat of austenite from above the critical temperature to the transformation temperature, (2) the specific heat of the transformed product from the reaction temperature to room temperature, (3) the enthalpy change at the transformation temperature, and (4) the variation of these with temperature of reaction.

Since the calorimeter could not be used to determine these data directly, that is, during the transformation of austenite to pearlite at temperatures well below the critical temperature, the problem was approached from the reverse transformation. The specific heat of the pearlite and austenite are known throughout the temperature range over which the transformations are to be studied, and the enthalpy of transformation of pearlite to austenite is known at one temperature, then the thermodynamics of the reaction of austenite to pearlite may be calculated as a function of temperature.

In order to determine the effect of the variation of the reaction temperature on the transformation of austenite to pearlite, specimens of known alloy content were completely transformed at various subcritical temperatures, for times just long enough to complete the transformation. These were run in the calorimeter to determine the specific heat of the pearlite from room temperature to the reaction temperature, pearlite to austenite, the enthalpy of transformation of pearlite to austenite, and the specific heat of austenite from the reaction temperature to approximately 900°C. Any differences in the specific heat of the pearlite to the reaction temperature would be due to differences in inerlamellar spacing, to any partitioning of the alloying element between ferrite and cementite in the pearlite, and to inherent differences in the pearlite itself since pearlite of the same physical structure reacted at a higher temperature. Any differences in the enthalpy of transformation of pearlite to austenite would be due to the differnces in the partitioning of the alloying element, in the interlamellar spacing, and possibly due to energy state of the pearlite. By comparison of the specific heats and the enthalpies of transformation of the specimens that had been treated to different conditions, the effect of each of the possible variables and conditions could then be determined.

Since the kinetic data for a high purity 0.75% C-1% Mn steel (15) had been determined and the specimens of the proper size and shape had been transformed to 100% pearlite at several subcritical temperatures for this investigation, the primary object of this investigation was a determination of the thermodynamics of this steel. In order to help evaluate the effect of the addition of the manganese as an alloying element, specimens of high purity iron, carburized to eutectoid carbon content were to be isothermally transformed at the same subcritical temperatures and the thermodynamics of this alloy determined. Also, a specimen of high purity electrolytic iron was to be run to determine the specific heat for comparison with the alloy steels. The enthalpy of transformation of iron was to be determined to provide a check for the self consistency of the data for the steels and to compare with the values for the specific heat and enthalpy of transformation reported in the literature since a fairly wide scatter exists in the reported values.

From the above specimens it would be possible also, by knowing the interlamellar spacing of the pearlite for each specimen, to determine the interfacial energy of the interface between ferrite and cementite and the effect of the alloying element on the interfacial energy.

By running hyper- and hypo- eutectoid steels in the calorimeter, and comparing to the eutectoid alloys, the heat of solution of ferrite and cementite in austenite could be determined.

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