| Low Alloy Steels |
|
INTRODUCTION
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. |
| 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. |
| 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 |