Ductile to Brittle Transition
|Cold shortness was a big problem for
a long time in sword making. It means that things get brittle when it's cold.
The word "short" is old
English, meaning "having a tendency to break or crumble" as in
shortcake or shortening. It doesn't mean that things get smaller when it's
Nowadays we call the more general effect that properties change from ductile to brittle with decreasing temperature "ductile to brittle transition" or DBT.
|Contrariwise, if we look at increasing temperatures, brittle materials may
become ductile. DBT transitions are not restricted to iron and its alloys,
however, they are rather common. They occur in more or less all bcc metals, to some extent in hexagonal (hcp)
metals, in many non-metallic crystals and in polymers. There is, however, no
cold shortness for fcc metals. They stay more or less ductile down to very low
The decisive question is what value we will find for the critical temperature TDBT where the transition takes place. For pure iron and some steels TDBT is well below "normal" temperatures but for some other steels it might be close to room temperatures. If we take silicon (Si), for example, we find it to be quite brittle at room temperature but ductile above a critical temperature of about 700 oC (1290 oF).
|Here is an example of what cold shortness looks like when you measure the fracture behavior in a Charpy impact test.|
|A small impact energy means that it is easy to
fracture the material. In other words: it is brittle. A high impact energy means the material is ductile. In
the picture above the critical temperature TDBT for
mild steel is -50 oC (-58 oF).
The impact energy is given in "a.u." = arbitrary units, meaning that numbers might be different for the different curves but that is not important here. Upper shelf" and "lower shelf" is steel engineering slang for the high and low fracture energy associated with ductile or brittle behavior, respectively. Their difference is a measure for the magnitude of the effect.
shortness or red shortness, also a
bane in early sword making, is completely different. Here we look at a ductile to brittle transition when the temperature
goes up. Things that are ductile at low
temperatures get brittle at high temperatures. That happens, for example,
whenever some small amount of sulfur (S) is in the steel and precipitates at
I could not find any pictures for red shortness in steel that is tied to sulfur. It doesn't seem to exist anymore because sulfur is under control. In contrast to cold shortness, the mechanism is rather clear and I won't go into it here.
|Ductile to brittle transitions do not only happen as an effect of temperature. They may also be caused at a temperature that is initially well above the critical temperature TDBT by, e.g., radiation (a major problem for steel in nuclear power plants), or exposure to hydrogen produced by some corrosion ("stress corrosion cracking"). Things like "creep" and "fatigue" might also be considered to fall under this heading.|
|If a ductile-to-brittle transition occurs in your material, you have a serious problem. If your material is under some mechanical load, it might simply break, often suddenly and without any warning. Major disasters we know about were caused by this. Most certainly there were also a lot of minor disaster we don't know about, for example when some warrior's sword suddenly broke because it became too cold for its TDBT.|
|For iron and steel, as I have stated rather early, red shortness is supposed to be caused by sulfur (and phosphorous, and ...), and cold shortness is supposed to be caused by phosphorous. That's all true but perfectly clean iron shows cold shortness, too; just at lower critical temperatures than iron with some phosphorus in it. So phosphorous only raises the critical temperature TDBT of the DBT transition to noticeable levels. It doesn't really cause it. Below is a figure that illustrates this.|
|Care was taken to have an "ideal" iron - phosphorus system, with nothing else in there. Charpy impact tests provided the fracture energy (given in Joule [J]). Some (not all) experimental points are shown to give you an idea about experimental scatter.|
|Several things become clear:
| Looking at the two figures above, a
few big "why" questions suggest
themselves. I will state right away that they will not be easy to answer:
|The answer, in the words of Jianming Huang, who wrote a PhD thesis entitled: "Ductile-to-Brittle Transition in Body Centered Cubic Metals..." in 2004 1), is:|
|I hate to admit it but I must agree with Jianming Huango. I can't give you a quick and easy explanation. I, or better Jianming Huango and his ilk, do know a few things, however, about brittle-to-ductile transitions. I can give you a general idea of what is at the root of "cold shortening" in iron even so some details are not yet clear. Before I do this, let's look at a few facts.|
|Here are two figures that shows basic cold shortness in iron and plain carbon steel:|
|While the curves look rather similar on a first glance, they give distinctly different TDBT values for pure iron: -75 oC or -50oC (as before), respectively. On the absolute temperature scale (the only relevant one) that would be about -200 K or -225 K; a 12% difference. That's not too bad but tells us that things like grain size, possible unrecognized traces of other impurities, the way the Charpy impact test was done, and God knows what else, also influences TDBT. In this case it might have been the difference between a "notched" and an "un-notched" specimen in the Charpy impact test..|
|Here are a few rules for the
dependence of the DBT transition temperature on test procedure and composition.
The transition temperature at which brittle fracture occurs is lowered by:
|The second point is rather
interesting. It tells us that the always very rapid
deformation your sword experiences when hit by the cutting edge of your
opponent, leads to a higher transition temperatures. In other words: In a sword
fight your sword blade may behave like a brittle material even so it is
perfectly ductile under normal conditions.
The last point, for example, tells you that "it is desirable, therefore, to use steel deoxidized or "killed" with aluminium and normalized to give fine pearlitic structure and to avoid the presence of bainite even if tempered subsequently and increasing the manganese content". Aha. For normal people that means: steel is a complex material, indeed!
|Here are two more measured DBT transition curves dealing with manganese (Mn) and sulfur (S).|
|Obviously, manganese (Mn) is good for decreasing the critical temperature TDBT to values that can be tolerated for most intended uses. But how come that TDBT » 120 oC (248 oF) for manganese-free mild carbon steel? That does not agree with the curves shown above at all. Nevertheless, we have rather sharp transitions once more.|
|What we see is that sulfur is really bad for youeven in very low concentrations. It does not raise the DBT transition temperature very much like phosphorous, but tends to make the steel more brittle at all temperatures.|
|One last curve just to make clear that the large changes in fracture impact strength seen around the BTD transition temperature have nothing to do with tensile strength or hardness. You can be brittle and hard or brittle and soft, the same goes for ductility. What we see in the picture below is the yield strength (or hardness if you change the scale properly) of various phosphorous (P) steels as a function of temperature.|
|Shown is the yield strength of the
same phosphorus steels as in the figure above
as a function of the temperature. It is quite clear that phosphorous hardens
iron at higher temperatures since the yield
stress with 0.6 % phosphorus is 3 to 4 times large than that of pure iron. We
already but what we didn't know is that the hardening effect more or less
disappears at lower temperatures because the yield stress of the pure iron
increases substantially with decreasing temperatures. It also becomes brittle
at the temperature indicated.
Note that the DBT transition temperature is not recognizable in the yield strength behavior.
|The Explanation (Sort of)|
|By now you must be thoroughly
confused. That's fine, the experts in this area are also somewhat confused
(they just don't admit it). What became quite clear, however, is that DBT
transitions are always there and that exactly what will happen depends on many
parameters, including some that the experimenter might not be aware of.
As I have stated above, the exact mechanism of DBT transitions are still unclear. It is quite clear, however, that DBT transition behavior in bcc metals results form a competition between nanocrack growth and dislocation generation and movement.
| Imagine a piece of steel under some
tensile stress with a nanocrack at the surface or wherever. The specimen
"wants" to get longer and has two options to do this as schematically
shown in the figure below:
|Left: A piece of material
with a nanocrack under tensile stress.
Right top: The response is growth of the nanocrack. The material is brittle.
Right bottom: The response is generation and movement of dislocations. The material is ductile.
|Both mechanisms compete against each
other in a "winner takes all" kind of contest. Either the crack
growth process wins, then almost no dislocations are generated and move, or the
dislocation process wins, then the crack will not grow at all.
From the experiments it is clear that the temperature and impurity / alloy elements influence the fitness of the contestants in different and tricky ways.
|If, for starters, we look at an
hypothetical ideal crystal, the major
parameters are only
|In many articles to DBT it is stated that dislocation movement in bcc crystals is more difficult in comparison to fcc crystals because the latter have more glide systems or different ways to move dislocations around. That is not really true. It's not how many ways you have to move dislocations around but how it depends on temperature. That dislocation movement in bcc crystals needs more thermal activation than in fcc crystals is, of course, tied to the same lattice geometry that defines the glide systems, but let's not confuse the issue here.|
|DBT behavior is thus simply a result
of which process needs less energy for its functioning. Energy is supplied by
the applied stress and by the temperature. The dislocation process uses both
contributions, the crack growth only the stress part. When the temperature goes
down, the thermal energy available for the dislocation process goes down too,
which means you need more stress to move them.
At the critical temperature TDBT the stress is sufficiently large to promote crack growth and at smaller temperatures than TDBT the crack growth mechanism takes over completely.
|Alloy elements, as we know by now,
can do a lot of different things with respect to the microstructure. Let's
enumerate the options to know what we are talking about next:
|With respect to DBT transitions, we
must consider how the impurities in one of those structures will influence
crack growth and dislocation generation / movement.
As far as the dislocation mechanism is concerned, it is very likely that impurities make dislocation movement more difficult. That is the essence of hardening, after all. There might be just a small probability that in some special cases the generation of dislocations is made a bit easier with some suitable nuclei around, but the movement will still be more difficult. That would tend to raise TDBT and that is what we see in most of the diagrams above.
|As far as crack growth is concerned,
it might go easier or might be more difficult with impurities around. The first
option will win if phases / precipitates formed by impurities are easier to
crack than the regular crystal, meaning that their surface energy is smaller
than that of iron. Since sulfur or phosphorous is not a particular strong
material it stands to reason that a layer of sulfur or phosphorous in a grain
boundary will give a crack a great opportunity to run along the grain boundary
with little effort. That certainyl happens; freshy fractured phosphorous iron
does fracture along the grain boundaries, which look silvery-white due to the
phosphorous collected there.
On the other hand, a tough-to-crack particle might stop a crack that hits it; at least it will slow it down.
|So about everything imaginable might happen, and as you have seen in the small collection of figures above, taken from the large world of DBT transitions in steels, a lot does happen. The DBT transition temperature might go up or down with some impurity concentration, the transition might become less sharp, and so on.|
|You might be inclined to believe now that we do, after all, have a fairly good idea of what is going on. What is it we don't know?|
|Here is one of the
major points, hotly disputed and not yet clear: Is it the generation of dislocations at the crack tip or their
movement away from the crack tip that gets
too sluggish low temperatures? Only fairly involved calculations with powerful
computers, that are just now coming into the game, will answer that question in
the near future.
So just wait a while longer and DBT transitions will be fully understood.
Meanwhile - here is a puzzle for you. Shown is very different DTB behavior for nominally identical chromium-molybdenum steel:
|The compositions of the two steels, if looked at closely, were:|
|While there is a noticeable
difference in the concentration of typically "good" alloy elements
like manganese (Mn) or
(Si), it is not likely that those elements are responsible for the huge
different in the DTB transition temperature observed for this particularly
steel. It can only be the little bit of carbon and nitrogen still present
in the "dirty" variant.
One might speculate that we have a "Cotrell-Bilby cloud" at work once more (look it up!) but I don't know that for sure. What I do know is that here is one of the reasons why "clean steel" and "interstitial-free steel" gain more and more attention.
|1)||Huang, Jianming: "Microstructural Effect on the Ductile-to-Brittle Transition in Body Centered Cubic Metals Investigation by Three Dimensional Dislocation Dynamics Simulations", Thesis for Doctor of Philosophy in Mechanical and Aerospace Engineering; University of California, Los Angeles, 2004.|
|2)||W. A. Spitzig: "The Effects of Phosphorus on the Mechanical Properties of Low-Carbon Iron", Metallurgical Trans., Volume 3, May 1972, p. 1183 - 11 88.|
General Remarks to Literature and Sources
Books and Other Major Sources
Overview of Major Steels: Scientific Steels
3.1.2 Stiff or Hard?
3.2.2 The Charpy Impact Test
Fracture Mechanics I
Science of Welding Steel
Overview of Major Steels
Alloying Elements in Detail
11.3.3 Evolution of Pattern Welding
Lee Sauder and Skip Williams Smelt Iron
9.4.2 Phosphorous Chaos - a Chaotic Succession
9.1.2 Problems with Alloying
10.4. Crucible Steel 10.4.1 The Making of Crucible Steel in Antiquity
Units and Constants
Overview of Major Steels
Charpy Impact Test: Example
The Liberty Ships
© H. Föll (Iron, Steel and Swords script)