by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
2005 by George Langford

Cast Irons, High Alloy Steels, and Superalloys
The microstructures in my photomicrographs that you will see in this, the third series of lessons, belong to more advanced classes of alloys in the M.I.T. specimen collection: multicomponent systems.  Most of these alloys were developed empirically by trial and error methods, and their structures and microconstituents are considerably harder to identify by appearance alone.  However, there are still some features common to each class which will enable you to make qualitative sense of them.
The cast irons have much lower melting points than steels as a result of their higher carbon content.  They can also be distinguished by the large amount of graphite in one form or another, or by an extremely large area fraction of cementite.
High speed steel has a substantial volume fraction of alloy carbides and is used where extreme hardness and wear resistance are necessary.
Stainless steels have more than 12% chromium and were developed for corrosion and creep resistance; they can be austenitic, ferritic, or martensitic.
Hadfield's austenitic manganese steel has an extremely low stacking fault energy and is known for its extremely high rate of work hardening and abrasion resistance.
Surface coatings are used to impart properties to metals which are not attainable in the untreated material or, if attainable, would compromise other service requirements.  Their study also helps one's understanding of the utility of phase diagrams for controlling the accompanying metallurgical reactions.
Superalloys based on nickel or cobalt were developed for long life in extremely hot and oxidizing environments, such as gas and steam turbines, and are characterized by complex microstructures as well as by adherent protective oxide scales.
I made the color photomicrographs presented here of a prepared set of specimens of known history collected by the staff of M.I.T. in the years prior to 1959.  They were made by me with Kodachrome KA135 Professional, but the color temperature of the illumination was still way too low for proper rendition of the color of the light reflected from the specimens.  I have therefore adjusted the color balance to approximate better what I remember seeing through the eyepieces of the microscope.  The microscope objective lenses used were achromatic (corrected for two colors) and, as the appropriate green filter was not used, there are sometimes color fringes and reduced resolution as a result of the use of the panchromatic color film.
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. 
Return to main Introduction.
FIRST LESSON       - Cast irons
SECOND LESSON   - High Alloy Steels
THIRD LESSON      - Surface Coatings and Heat Resisting Alloys