by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
2005 by George Langford
This is the first lesson in a set of images that I made that demonstrate metallurgical microstructures of low alloy steels that are in the M.I.T. specimen collection.  These microstructures are shown to be sensitive to the processing and environmental history of each macrosopic specimen.  Steel usually consists simply of ferrite and cementite.  The former is ductile and soft, the latter is hard and brittle.  The amount of cementite varies from nearly zero up to as much as 15% by volume.  The cementite can be either the continuous phase or it can be present as isolated islands.  Cementite can vary in size from a couple of hundred nanometers (10^-9m) to hundreds of micrometers (10^-6m) ... a range of three orders of magnitude.  The strength (and inversely, the ductility) can vary by about half of that ... a factor of fifty or so.  Even the arrangement of the cementite can be crucial.  Local or macroscopic variations in carbon and alloy contents cause further purturbations in properties. 
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.
Whenever the narrator asks a question, be sure to commit yourself to an answer before going to the next page.  You can backtrack with your browser's BACK button at any time, of course.  Just be sure to click the FORWARD button to return to the proper page before proceeding so that you don't get lost.