by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2015 by George Langford
Low Alloy Steels - Lesson 1 - Thought Experiment
| Metastable Iron-Carbon Phase Diagram |
Discussion and Preparatory Explanation |
 |
In
the early 1970's I was doing an experiment at Monsanto's Research Park
Development Center with the casting of steel wire directly from the
melt using their technology, which was based on the addition of small
amounts of silicon to the liquid steel and on the selective reaction of
that silicon with carbon monoxide to form a silica envelope around the
liquid stream to stabilize it against breakup into droplets by
surface-tension effects. I was trying to speed up the solidification process in an effort to avoid shrinkage porosity, which was the bugaboo of Monsanto's wire-casting efforts. I added forced cooling with helium using a helical pattern of gas jets directed crosswise to the liquid stream. The stream could be seen to follow a wavy pattern, but it would tolerate pretty high velocities of the helium without getting disrupted. As the helium flow rate was increased, the recalescence point in the stream rose higher and higher until it reached a critical point, then suddenly dropped down significantly, and became both extremely steady (not diffuse as before that point) and quite pronounced, from below red heat to white hot. The resulting wire was mostly light tan in color, but had occasional lengths of a blue color. |
| The tan color was due to
oxidation of the still-hot wire by exposure to air below the cooling
chamber, but the blue color was a surprise. We later found out that the
blue portion of the wire was martensitic and was blue because of the extra heat of transformation from austenite to martensite.
There had been no austenite in the tan wire, which we found to be a
mixture of ferrite and iron carbide by metallographic examination. Our addition of extra cooling had caused the liquid iron - ca. 0.4 percent carbon liquid to freeze directly to body-centered cubic iron, skipping the face-centered cubic austenite region of the iron-carbon phase diagram. The liquid steel was supercooled so much that it went almost instantly to solid ferrite, as inferred from the sharpness of the transition from barely red hot liquid to white hot solid ferrite, the liquid/solid interface traveling up the stream as fast as the liquid was flowing downstream. The blue sections of the wire were the result of occasional formation of grain boundaries in the ferrite at which austenite grains could then grow and later transform to martensite. Armed with this history, make a new iron carbon phase diagram (in the region below one precent carbon) without the austenite phase. Bear in mind that delta ferrite and alpha iron both have the same body-centered cubic crystal structure. They are isomorphous. It is helpful to make use of schematic free-energy composition curves in your thought process. Find the Answer Here |