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

Low Alloy Steels
Steel is the most important commercial metal alloy.  It has also had the most metallurgical development and as a result may have the widest available range of physical properties of any metal.  Of course, "steel" is not a single alloy, but instead is a bewildering array of compositions whose common component is iron.  The microstructure of steel is the key to its behavior, because the crystal structure, size, carbon content and arrangement of the microconstituents (BCC ferrite, BCT martensite, FCC austenite, orthorhombic cementite, etc.) determine its properties.  The alloying elements mainly make it easier to obtain the phase transformations necessary for successful heat treatment.  A few alloying elements are added for solid-solution strengthening, and a couple are added for control of undesirable and unavoidable impurities.  Of course, high-alloy steels are a different matter ... they are the subject of a later instructional set. 
The color photomicrographs presented here were prepared from a set of specimens of known history collected by the staff of M.I.T. in the years prior to 1959.  They were made 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 ferrite is generally white to slightly gray in appearance, and the cementite is a light lemon yellow when it sufficiently coarse to be resolvable.  Pearlite often looks blue and other colors of the spectrum because of diffraction from the lamellae, whose spacing is close to the wavelength of the incident light.  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.
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Return to main Introduction.
FIRST LESSON       - Fundamental Carbon Steel Microstructures
SECOND LESSON  - Heat Treated Steels
THIRD LESSON      - Failures in Heat Treated and Mechanically Stressed Steels
FOURTH LESSON  - Surface Treated Steels