|Producing Ghosts the Simple Way.|
|In iron alloyed with about 0.2 % to 1.2 % phosphorous (P) most variants of defect etching come up with a peculiar structure termed a ghost structure. If you have only an ideal iron - phosphorous alloy (without anything else) you will expect at room temperature to see ferrite grains in an etched sample, separated by well-delineated grain boundaries. You will see that - but there is more. As soon as you defocus a little, a second kind of structure appears: dark or bright "ghosts" that are superimposed on the now slightly defocussed ferrite grain image. They look like grains - but there are no grain boundaries!|
|Here is the picture from the backbone once more. Now let's see how it came into being in somewhat more detail what has been already given in the backbone.|
|What you see are the "ghosts" of dead austenite grains. They existed at high temperature and then disappeared completely, leaving only their ghosts back - in the form of phosphorous poor areas in the ferrite grains. In many etchants these areas dissolve a little faster than those parts of the grain containing phosphorous. They are then standing up like little mesas and become visible under the right conditions. They are best seen if you defocus your microscope a little. The picture then is not as crisp as it could be - but you see the ghosts!|
|Why do we get such a structure? A picture says more than many words. The one below is different from the one in the backbone; here we start at a higher temperature. Contemplate it and see if you can figure out what it shows.|
|We start with the red state point at high temperatures. We will have a
large-grained bcc ferrite crystal (a) with a
bit of dissolved phosphorous - 0.72 % to be precise.
As the temperature goes down, we will eventually hit the a + g region. That necessitates that fcc austenite (g) grains develop. Nucleation for the austenite grains most likely starts at the grain boundaries between the ferrite grains. According to the phase diagram the g grains must be phosphorous-poor and the phosphorus needs to get out of the forming austenite. At high temperatures the phosphorous has no big problem to diffuse into the a grains, it will become enriched there.
As soon as the temperature falls below the value where the a + g region has its maximum extension, the g grains have reached their maximum size. If the temperature keeps decreasing, they must now shrink. That's what the will do until they eventually disappear completely after the state point left the a + g region and we need to have pure ferrite once more.
The phosphorous concentration is supposed to equilibrate again, and that's what it would do, if there would be sufficient time. However, during the shrinking part and after the austenite has disappeared, it's getting colder and atomic movement by diffusion slows down rapidly. Hardly ever will there be sufficient time for the phosphorous to "reclaim" the former austenite grain regions and the phosphorous concentration in the regions of the former a grains is still larger then in the rest of the steel.
If some etchant responds to that, you are now going to see ghosts. The ferrite grains need not to have changed very much throughout the whole ordeal; they just gained some phosphorous relative to the austenite and never gave all of that it back.
|Here is what the structure of the picture in the backbone must have looked like at about 1100 oC (2012 oF) the temperature where the austenite grains are largest:|
|In the example that structure defines the "ghosts"
seen at room temperature. Of course, the austenite grains at high temperatures
have different crystal orientations, like the ferrite grains where this is
symbolized by different colors. They are shown in just one color to make them
When the austenite grain shrink, the ferrite forming just keeps the orientation of the original a grain, and the final grain structure is more or less identical to the starting one - except that the phosphorous is now distributed in an inhomogeneous way.
|So, whenever you see a ghost structure, you know
that you have phosphorous steel with phosphorous concentrations somewhere
between 0.5 % and 1,2 %?
Maybe. But it is not certain. Why not? Read on!
|Not All Ghosts are Equal|
|The nice ghost structure above is not
the last word on phosphorous steel structure for a variety of reasons. Here is
|Here is a picture that shows ghosts in P,As - iron:|
|The original picture caption says: "Longitudinal section of the spear point 1,573b etched with
nital and treated with Stead's reagent to show composition differences retained
from the a + g
phase field. Upon heating to 925 °C the g-phase that formed in the a-phase grain boundaries rejected phosphorus
(and arsenic) into the a-phase. The resulting composition gradients have been
retained. Dark areas have low phosphorus / arsenic
I added the arsenic but the paper made clear that the authors were aware of having some arsenic in their steel. It can be seen quite nicely how the austenite grains nucleated at the ferrite grain boundaries - some "ghosts" line a grain boundary like leaves on a branch. This is what one would expect if one starts with pure ferrite, i.e. at a temperature above the a + g region.
What we learn is that some arsenic does not seem to change the general "Ghosting" behavior, as one would expect.
|Summing up points 1 and 2, the present knowledge is that all kind of mixes can produce ghosts. The ghost picture by itself, however, cannot tell you what, exactly, you had in your steel. However, some mixtures may not produce ghosts. That is in part also due to the observation that it matters also how you cool down.|
|That is not a surprise. If you cool down quickly, the phosphorous has not much time for diffusing and cannot get very far. It may make it out of the austenite but may not have enough time to spread out in the ferrite. Then you must expect that there is some phosphorous enrichment at the interface ferrite - austenite. If you get ghosts at all, the look more like blurry lines|
|If there is also carbon in your steel, you will form some pearlite upon cooling. What you see later at room temperature might look like this:|
|You figure out exactly what has happened! Being a
nice guy, I give you a hint:
"Pearlitic ghosting was a new structure discovered during this study. This ghosting structure may be a clue into the relationship between phosphorus and carbon in archaeological iron. This was a halo of ghosting seen around pearlite. The testing of this ghosting showed a depletion of phosphorus immediately adjacent to grain boundary pearlite (figure 78). This ghosting structure could have been the result of the combination of phosphorus segregation to the austenite grain boundaries and the formation of pearlite from the austenite within the low phosphorus areas upon cooling."
|If we have ancient phosphoric bloomery iron, we have slag inclusions. These inclusions might change what you see. Here is a pictures that makes that very clear:|
|Most of the austenite grains nucleated at slag inclusions, grew into the ferrite and disappeared again, leaving only their ghosts behind. The ferrite grain boundaries are not visible; the straight lines are due to scratches during (insufficient) polishing|
|Professional ghost busters came up
with 5 different ghosts - but I won't go any deeper into this. Suffice it to
say that at present we go by the following two assumptions:
|To say it with Samantha R. Rubinson, who has written a whole Ph.D. thesis (see below) and papers related to this topic: The role and significance of phosphorus in iron is poorly understood.|
|1)||Samantha Rebecca Rubinson: "AN ARCHAEOMETALLURGICAL STUDY OF EARLY MEDIEVAL IRON TECHNOLOGY; An examination of the quality and use of iron alloys in iron artefacts from Early Medieval Britain; Ph.D. thesis, Division of Archaeological, Geographical and Environmental Sciences, University of Bradford, 2010|
Science of Alloying
9.4.2 Phosphorous Chaos - a Chaotic Succession
Odds and Ends about Researching the History of Iron Technology
11.3.4 Metallography of Pattern Welded Swords
© H. Föll (Iron, Steel and Swords script)