9.1.2 Problems with Alloying
|Setting the Stage|
|In everyday life we use almost
exclusively metal alloys and not pure metals because alloys are better. All
alloys with iron are called steel a long
as iron is the base metal with more than 50 wt% of the total. With copper (Cu)
as base metal, we use more names and call its alloys
example. There are no generic names for
alloys, even so they are very important.
So alloying is good. Well, yes - but there are also problems. I shall start reminiscing a bit about the history of alloying and go from there to point out some problems with alloying. Finally I will delve deeply into the joys of alloying
|In writing this article I found it surprisingly hard to find data or good metallographic pictures for the ideal plain carbon steels, i.e. steel with only carbon as alloying element. That's the kind of steel I have discussed in the preceding chapters. There is a simple reason for that:|
|There was and is no such thing as ideal plain carbon steel since nowadays we always add some other elements (typically manganese and silicon) , and in the old times you always had some stuff (often phosphorus) in the iron unintentionally.|
|Steel is iron with some
alloying elements. That is a relatively recent insight. Looking back three
millennia, we might distinguish several steps on the way to understanding the
system we are dealing with here: iron plus a bit
of the rest of the periodic table. The times given below are approximate. They
may vary between here and there, and there is always considerable overlap.
|Whatever is in your steel might have been put in intentionally or unintentionally. In a strict sense, putting specific elements intentionally into your steel did not start before, roughly 1850. Steel makers through the millennia, however, might have used certain ingredients (like leaves of some special plants in crucible steel making) for the same purpose. What you had in your steel depended on what you used for making it, and trial and error lead to recipes that "worked" - sometimes by alloying the right stuff by good luck.|
|Your forebears put (mostly) the right
things into their personal systems, too. They did not eat certain mushrooms,
fruits and plants even so they had no idea about the biochemistry of toxins and
had never heard of cholesterol or calories. But they did not consume the really
good stuff either (chocolate ice-cream, coffee, cognac, my wife's gooseberry
cake, .. ) because they lacked the ingredients and the know-how for making it.
Try to make ice-cream without a freezer. It can be done but I bet you don't
Most of the time your ancestors ate mediocre food that was easy to get. But they had very good food on occasion, using special and expensive ingredients and followed time-tried complex recipes.
In analogy, very good steel was produced sometimes and then used for swords or other high-value items. Most of the time, however, the stuff was mediocre or bad.
|Major Problem: Embrittlement|
|If we now look at whatever was in there intentionally or unintentionally, it either could be good or bad (and anything in between). Sulfur (S) is practically always bad, manganese (Mn) is practically always good, and phosphorous (P) is ambivalent. Of course, as with all medicine or spices, the dosage is important. The headline of this sub-chapter was "Low Alloy Steels", meaning that we look at low concentrations below at most a few percent.|
|If a given steel gets worse upon
alloying it with something depends on how you define "worse". That
depends on your intended uses. Does a whisky get worse when you alloy it with
A decreasing hardness, for example, might be bad for sword users but not for others. When you need to make a chastity belt in a hurry, it's easier with not-so-hard steel. Since your wife doesn't appreciate a hard one any more that a soft one, why bother and use hard steel?
|Everybody agrees on one issue, however:|
|If your steel
undergoes a ductile-to-brittle
transition (or transformation), you have a problem! It is such a well
known and big problem that it has its own abbreviation:
DBT! If alloying induces a DBT
transition, your steel is now perfectly brittle and behaves essentially like
glass. Not good. Neither for swords nor for chastity belts.
Phosphorous and sulfur are notorious for causing DBT behavior. They are also particular hard to keep out of (ancient) steel, so we now have one of the major problems with (typically unintentional) alloying in ancient and recent times:
|The terms mean
nowadays called brittleness,
that occurs at low or high temperatures, respectively. I've already
introduced this in chapter 3.2, now we will go to the
core of these really big problems in steel technology that haunted steel makers
and users up to about 1950.
To be perfectly clear: phosphorous (P) causes a DBT when the temperature drops below some critical temperature TDBT, and sulfur causes a DBT if it gets above TDBT. Other alloy elements might also cause these effects.
|Cold and hot shortness were two of the banes of steel throughout the millennia. Even today DBT causes major worries for users and makers.|
| Cold shortness is a major problem for you, the
user. Just imagine that your steel turns
brittle in the winter, when the temperature is around freezing. Your sword will
shatter into thousand parts on impact, your steel steamer will just break apart
see below), and your chastity-belted wife only needs to sit in the snow for
some time and then look for that nice smith apprentice with his hammer. One
little bang will do it (for starters).
In other words, cold shortness is bad for you if it occurs in the range of temperatures where regular humans live, love, work, and do business. If cold shortness occurs at temperatures too low for regular humans, you can live with it. However, if you, like me, belong to the irregular humans who, for example, keep liquid nitrogen (N) or helium (He) in steel containers at temperatures around -200 oC (-328 oF) or -270 oC (-454 oF), resp., you better watch out for what kind of steel is used!
|Hot shortness doesn't affect you, the user, all that much - except if you plan to go to hell - because sword fights rarely take place around 500 oC (932 oF) or so. The smith has the problem now since his steel turns brittle when its hot. If red shortness occurs, there will be no forged steel objects because the material shatters when the smith is trying to forge it at high temperatures. So all embrittlements are simply bad.|
|I hear you. You now want me to answer your why questions. Why are phosphorus and sulfur causing DBT transitions at some special and inconvenient temperatures? And how? For starters I give you a first clear answer:|
|Well, let's take that with a grain of salt. I don't really know the deep-down details - and that's also true for my colleagues. In the words of Jianming Huang, who wrote a PhD thesis entitled: "Ductile-to-Brittle Transition in Body Centered Cubic Metals..." in 2004: "The (..) mechanism of this (ductile to brittle) transition still remains unclear despite of large efforts made in experimental and theoretical investigation".|
|By the way, there are many more things in Materials Science that present-day Material Scientists don't know about for sure. What a relief! This means that there is still enough work for Materials Scientists and Engineers in the future. My kids can go for decent careers and do not need to become lawyers or bankers. (The brats actually went for medical doctor (2) and literature science (1)).|
|OK - so I don't really know what exactly is going on at DBT transitions. But I do now quite a bit of general stuff about DBT's, however, and I will share that with you.|
|I do know, for instance, that hot shortness, in very general terms, occurs if some impurity forms precipitates with a low melting point that like to sit in grain boundaries. When it gets too hot they melt. The ferrite / pearlite grains of your steel then are held together in parts only by a liquid. They will still cling together - even if the grain boundaries are completely liquid - just like wet hair. But hit it with your hammer and the steel falls apart. Sulfur (S) does just that, so hot shortness is now explained (haha).|
|What happens at cold shortness is far more trickier. Quote: "Large quantities of phosphorus (in
excess of 0.12% ) reduce the ductility thereby increasing the tendency of the
steel to crack when cold worked. This .... is called cold-shortness". You will find a statement like
this in most steel books.
Notice that the quote above doesn't say that phosphorus causes cold shortness, only that it increases the tendency for it. But why is phosphorous increasing the tendency for cold-shortness, or as we call it now, a ductile-to-brittle transitions (DTB) at low temperatures? And how does it do that? I have promised (or threatened?) to tackle these hard questions in this Hyperscript, so I will give it a shot.
|In what follows here I give you a few simple hard facts about cold shortness. Any attempts to delve deeper into that subject will open a humongous can of major and very wriggly worms. That's why I will deal with the deeper aspects of cold shortness mostly in the science module.|
|When I started to look into the topic in more detail, I had no idea about the size of the can of worms I was about to open. I spare you the gruesome details, you will probably find more than you are looking for if you use the link above. The explanation for cold shortness, in very simplistic terms, follows a line of arguments like this:|
|So the long and short of cold shortness in (ferritic) steel is not if it occurs (it will), but at what critical temperature TDBT you will encounter it. With respect to alloying we therefore want to know how the alloy element affects the critical temperature TDBT and possibly the magnitude of the effect. The figure below makes clear what I mean:|
test provided the fracture energy data on the left scale.
The specimen for the red line, shows essentially pure iron. Its detailed composition was: 0.005 % C; 0.001 %N; 0.0045 %O; <0.005 %P; 0.004 %Si; 0.01 %Mn; 0.004 %Si; 0.002 %Ni; 0.002 % Cr.
"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.
|Those are rather dramatic curves that
tell us a lot about the DBT transition phenomena. Let me enumerate the major
points for you:
|Now you see why "the (..) mechanism of this (ductile to brittle) transition still remains unclear...". We can account more or less for the points 1 and 2 right above, but point 4 is a tough nut to crack with theory. There are other open questions concerning DBT transitions; look up the science module if you need to know.|
|Alloying iron with anything, or alloying iron already containing a mix of alloying elements (then called steel) with one more element, will always change the DBT transition behavior. The changes might be small but you must be aware of it.|
|Generalizing a bit,
alloying will always change all the
properties of your steel.
Playing around with one specific element might leave some property almost
unchanged, make it better, or make worse. The "playing around" part,
meaning the concentration and detailed processing you use, is important.
If you only look at the information about phosphorus (P) as alloying element that is contained in the figure above, your conclusion would be to avoid phosphorus at all costs. To some extent that is what we do. But before you throw out phosphorous (P) forever (assuming you can do that), you should first look at what it can do for you.
If you cannot throw out phosphorous because you, the ancient smith, do not know how to do that, you better figure out how to live with it.
We have few modern steels that contain phosphorous (P) intentionally because whatever good it can do for you is mostly far easier done with other elements. But we do have plenty of ancient steels that contained phosphorous (P) unintentionally. Despite all the problems it causes, it still could be used to makes steels that were better than wrought iron or bronze. In fact, phosphorous (P) steel was so important in old times that it has its own sub-chapter, coming up soon.
|The basic problem with alloying should be quite clear now:|
|If you intend to "invent" a
better metal alloy today, you are well advised to assemble a highly
knowledgable group of 5 - 10 people, supply a infrastructure with the necessary
hardware like furnaces and electron microscopes, and give them at least 10
years and a sizeable budget.
The odds are actually rather low that they will come up with a new alloy that will be successful in the market place. Nevertheless - it's the only way to go.
© H. Föll (Iron, Steel and Swords script)