2.3 Some Definitions
2.3.1 Let's get to Work.
|Enough small talk, we need to do some work now. We have used everyday words like iron, cast-iron, wrought iron, steel, and so on, and we now need a few definitions in order to avoid confusion and to be sure that we really know what we are talking about.|
|Iron is simply a chemical element; its symbol is Fe (from Latin "ferrum" = iron).|
|Pure iron, and that means iron containing only very small amounts of impurities, melts at 1536 oC (2732 0F). We already know that this is a rather high temperature, not easily reached with "natural" means.|
|What we don't know yet are the answers to some of the more general "why" questions. Why do things melt? And why at quite different temperatures?|
|We need to look into that a bit and for that we need to discuss first what "temperature" actually means. You may now feel a bit amused; thinking that you sure know what temperature means. Well, we'll see if you do.|
|Pure iron (Fe), in a chemist's sense, is a rather recent thing. For a very long time the best approach to half-way pure iron is what we still call "wrought iron".|
|"Wrought" is just old-fashioned English for "worked", and the term "wrought iron" simply refers to the way it was made: the smith banged together the little lumps of relatively pure iron found in the "bloom" of ancient smelting furnaces after the smelting process. In German, by the way, wrought iron is called "Schmiedeeisen" ("forged iron") because it was forged together.|
|Talking about impurities in iron, we first talk about carbon, another chemical element. Wrought iron does not contain more than about 0,1% of carbon. There is no clear-cut definition, however, and a lot of times when people (or archaeologists) use the expression "wrought iron" they might refer to what is known as mild steel with up to 0.4 % carbon.|
|By the way,
concentrations given in
percent (%), if not otherwise stated, always refer to "weight percent". A concentration of
|Carbon (from Latin "carbo", meaning charcoal), is simply another chemical element (symbol C).|
|In its purest and most stable form carbon is
found as graphite but there are
many other modifications or phases of pure
|Carbon is a great element not only for carbon-based life forms like slime mold, mosquitoes or you and me, but also for illustrating a supremely important concept:|
|We need a good name for that phenomena. From now
on we will call those different modifications, structures, manifestations,
different states of being, or whatever you
like to call different forms of the same stuff by one word: different
phases of the same material.
This is something very important that you need to remember. So let me repeat:
|The concept of "phases" is supremely important for making iron, steel, and swords (and about anything else). So, what exactly is a phase? We will get into that in great detail later, for now we just ascertain the following examples:|
|Graphite is a distinct phase of pure solid
carbon; diamond is another phase of pure
Ice is a distinct solid phase of aitch-two-o (H2O). Liquid water and steam are other phases of the substance or chemical compound H2O, consisting of one oxygen (O) atom and two hydrogen (H) atoms. Whatever phase you look atit's still aitch-two-o. I must call it aitch-two-o or H2O in chemical shorthand becausesurprise! there is no common name for this substance. There are only names for some of its phases.
|If one and the same material exists in different phases, logic dictates that we must be able to induce phase changes. That will be a decisive thing for sword making.|
|So let's look at phase changes in iron right away. Iron (Fe) crystallized in one way and then called "ferrite" is a different phase from iron crystallized in a different way called austenite. In contrast, iron carbide (called "cementite" (Fe3C)) and rust (a mixture of iron oxides and hydroxides) are not different phases of iron. They are different chemical compounds and thus different materials|
|Phase changes actually are rather
common. When the ice in your whisky melts, the material
|When iron melts, or if anything
else melts, there is also a phase transformation from solid to liquid, for that
matter. Phase changes from solid to liquid are a bit trivial, you might think,
so let's look at phase changes from one solid phase to another one.
The phase change from carbon in its diamond phase to carbon in its graphite phase, for example, is easy to do: heat up a diamond without admitting air (otherwise it just burns) and you get graphite. It is easy to do because carbon prefers to be in the graphite phase at room temperature, and the diamond phase just needs a little incentive to change to graphite. You can try an experiment in your kitchen when your wife is not around. Women tend to oppose experiments that turn their diamonds into coal. That's probably while few experiments along that line have been performed. Reversing the process, making diamonds from graphite, is not quite that easy, by the way.
Different phases of the same material may have very different properties. Tell your betrothed-to-be that the lump of graphite in that engagement ring you gave her is practically the same as a diamond, and she will tell you far more about the relative merits of different phases of the same element than you ever wanted to know.
|Pure solid iron (Fe), as it happens, comes in different phases, too, just like carbon. Phase changes occur whenever you heat it up beyond a certain temperature and when it cools down again.|
|Some of the different phases of pure iron have
names like some of the different phases of
|Some really big "why" questions come up now. The answers will come up later:|
|Steel and Cast-Iron|
|If we add a bit of carbon to
(otherwise pure) iron, we obtain a range of materials that we call:
|There are a lot of iron alloys (mixtures of iron
and other elements) around since there are a lot of other elements you can mix
iron with. Some of those alloys we call steel and some others we call cast-iron. These names are thus just generic or
collective names for a whole bunch of quite different iron alloys with
The only common denominator is that all steel and all cast-iron varieties contain iron.
|There are qualifiers for special steels and a plethora (= a hell of a lot) of names for it.|
|Steel names might be historical
("Wootz" steel, Sheffield steel, Tamahagane), scientific
(hypoeutectoid vs. hypereutectoid steel), relating to people (Bainite,
Martensite, Ledeburite) or describing appearances, processes or properties
(pearlite, maraging steel, spring steel, mild steel, stainless steel, TRIP
I'm just showing off here. We need to go into some of that later, however.
|What we note right here is the essential "why" question concerning (carbon) steel|
|A carbon concentration somewhere between 0 % and
4 % determines if we have:
|Why? And how does the carbon work this miracle?|
|Melting Point of Alloys|
|What we should note right here is a
simple truth: The melting point of (almost) all alloys, e.g. the alloy of iron and 4 % carbon
that we count among
| The same is true if we don't have a technical
"alloy", implying that we
mixed the constituents for the alloy intentionally, but just "dirty" or
impure iron, implying that whatever
is in there is not of our doing, and likely not what we want.
This makes clear why copper (Cu) alloys like bronze and brass were easier to deal with than the pure metals at the advent of metal technology.
Now why does the melting point go down if you add a bit of this or that? That's a really tough question that we nonetheless need to answer.
© H. Föll (Iron, Steel and Swords script)