|Hadfield's austenitic manganese steel was one of the
earliest high alloy steels to be developed. It has 12% manganese
and 1.1% carbon; and it is used where extreme resistance to abrasion is
In the 500X photomicrograph at left, the manganese steel has been etched with Nital. It is shown in the cast and heat treated condition. Metastable austenite that is mostly free of carbides (except at the grain boundaries) has been retained by water quenching from 1040C.
The grey particles are nonmetallic inclusions, while the dark spots are shrinkage pores. This steel is extremely difficult to machine, so it is almost always cast to shape.
|Here is another specimen of Hadfield's steel, treated as
above, but with one more step. The hardness is now Rockwell C40
to C50. What has been done to it ?
The streaks are a staining effect of the Nital etch.
The photomicrograph at left is at 100X.
Before scrolling down the page, decide on a sensible answer, based on the remark about machining above.
|The specimen was heavily cold worked. In the complex
iron - manganese - carbon phase diagram, there is an hexagonal epsilon
phase field next to the face centered cubic austenite phase
field. This is a strong indication that the austenite is not a
very stable FCC crystal, and that the hexagonally packed epsilon phase
has nearly the same free energy.
This photomicrograph was made at 200X.
What does the strong work hardening imply with respect to the stacking fault energy of the austenite ?
Humor me; think about this briefly and then go on to the explanation, which assumes you already know a lot about theoretical structural plasticity ...
Since a stacking fault in FCC material amounts to a layer of hexagonal
symmetry approximately three atomic planes thick, an FCC metal of low
stability with respect to the hexagonal close packed atomic arrangement
would have a low stacking fault energy (SFE). The diagram at left
describes the effect that the low SFE has on dislocations in the FCC
structure of the material.
The dislocations in the FCC structure of any metal are split into two partial dislocations, which together have a lower strain energy than the undissociated perfect dislocation, even though the two partials have to be separated by a band of stacking fault, as shown here.
Look at it this way: The close packed planes slip over one another in two steps. In the first step, the motion is half a step to the right and forward, and in the second step, the motion is half a step to the left and forward. The atoms simply follow a zig-zag path. Dislocations are the mechanism whereby the close packed planes of metals can slide over one another, not all at once, but a little bit at a time, and still not have much metal in an in-between condition. The actual dislocation consists of the edge of an extra plane of atoms inserted in the structure. Above and below the edge of this extra atomic layer, the structure looks the same in all directions. Only at the very edge of the extra plane is there any disurbance to the orderly array of atoms in the crystal.
|The partial dislocations are very widely spread apart in this low SFE alloy because there is little penalty for a wide separation of the two partials brought about by their mutual repulsion (strain energy). The separation is so large that the first partial dislocation can get away from its companion and thereby create a macroscopically visible band of stacking fault extending clear across a grain. This stacking fault then acts as a barrier to other dislocations which try to cut through it. Consequently, Hadfield's austenitic manganese steel work hardens extremely rapidly. It is used as a hard facing material and is welded onto the wearing surfaces of railroad switches, bulldozer blades, hammer mills, and so on. Conveniently, natural cooling after welding onto a massive substrate gives sufficiently rapid cooling that it avoids carbide precipitation, obviating any need for subsequent heat treatment.|
|SUMMARY: Although this
has not been an exhaustive survey of high alloy steels, the chosen
examples serve to illustrate that the analysis and interpretation of
their microstructures and metallurgical failures are not much more
complex than for simpler alloys. It is important to realize that
many more phases (often of complex crystal structures) are involved, so
past experience and attention to published literature on the structures
are especially helpful.
Continue to the next lesson in Cast Irons, High Alloy Steels, and Superalloys. Return to the main Introduction.