by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
2005 by George Langford
INTRODUCTION - Lesson 1 - Cast Irons
The first lesson concentrates on the metallurgical microstructures of cast irons.  The photomicrographs that I made of these microstructures show the sensitivity of microstructure to the processing and environmental history of each macrosopic specimen in the M.I.T. specimen collection.  Be sure you pay particular attention to the concepts of stability and metastability as they are applied to the iron - carbon phase diagram.  Various alloying elements, the cooling rates during solidification, and the time of heat treatment all critically control whether the specimen follows the stable iron - graphite or the metastable iron - cementite phase diagram.
Iron - Carbon metastable and stable systemsThe image at left shows both systems together.  The stable iron - carbon equilibrium diagram has solid lines, whereas the metastable iron - cementite system is represented by dotted lines.  Note that the austenite <=> liquid, delta ferrite <=> liquid, and austenite <=> alpha ferrite equilibria are unaffected by the stability of the carbon-rich phase (whether graphite or cementite).  The stable temperature range of the more stable phase always extends to a higher temperature.  Thus, cementite melts at a lower temperature than graphite, and both the eutectic temperature and the eutectoid temperature of the iron - cementite system are lower than those of the iron - graphite system.

Silicon and nickel both destabilize cementite and promote graphitization, so castings made with sufficient amounts of either element are more likely to freeze to austenite plus graphite than to austenite plus cementite.
Allow plenty of time to study and to take good notes about each specimen.  About two hours per lesson would be appropriate.  You will be expected to interpret some of these specimens during the final examination.  Feel free to use the Internet to find additional information about the alloys and applications mentioned here.
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