| Cast Irons, High Alloy Steels, and Superalloys |
|
INTRODUCTION
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. |
| FIRST LESSON
- Cast irons SECOND LESSON - High Alloy Steels THIRD LESSON - Surface Coatings and Heat Resisting Alloys |