| | A Brief History of Steel |
 | This moduel is also
availabe in Romanain languga, thanks to Irina Vasilescu. Here is the link |
| | |
| This is a much
embellished translation of an earlier version written in German (it can be found in the Hyperscript "Matwiss I") and with some footnotes added
later.
If you really want to know about the history of iron and steel use this link. |
| |
| In order to make steel
not accidentally, but conscientiously, you obviously first need to make iron.
In contrast to the noble metals like gold, silver or platinum (and the occasional find of pure copper), iron is never (?) found as an element but practically always as an oxide. |
|  | However, in contrast to other metals
found as oxides (especially Cu and Sn oxides needed to make bronze), the temperature of a
"normal" fire is not sufficient to reduce iron oxide and to make the
elemental iron liquid - the melting point of iron is Tm(Fe) = 1535 0C; far above
the (1000 - 1100) 0C that the ancients could produce (?). |
| | For Copper (Cu), e.g., it is different - its melting point is Tm(Cu) =
1083 0C. Throw some copper minerals in a nice hot fire made with plenty of charcoal (producing CO
which is great for reducing oxides), and liquid copper will result almost automatically. |
| | This happened and was noticed probably a good 6000 years ago, when early potters tried to adore
their pottery with nice green malachite - a copper mineral known in antiquity and used
as a gem stone. What a surprise, when one day in
a particularly hot fire, instead of decorated pots they found an ingot of pure - and then extremely precious - copper
in their oven. Copper was otherwise only found in small quantities (much less frequent then the (then) ubiquitous
gold) in mountain ranges and river beds. |
| This was a decisive
discovery for mankind: Precious and shiny metals could be made from dull stones. Things could be changed from one
seemingly immutable form into a completely different one - alchemy has its roots right here, and the yearning for
"transmogrification" has never stopped since. |
| |
| | Early metal industry and the short-lived "copper age" began to be replaced rather soon by the bronze age (Cu + (5 - 10)% Sn and often some
As); and the bronze age lasted more than 2000 years (it was not abruptly replaced by the iron age, but
coexisted for about 1000 years). | |
 | From the
"Kieler Nachrichten", front page, one day after after I wrote this paragraph. It says:
On the Track of
Charcoalers
Up to the 16th century, Schleswig-Holstein was woodland. Then the trees were felled to
produce charcoal (among other things). How that is done will be demonstrated by Stefan Brocke in the Loher woods. |
|
| | Here we first encounter the importance of
impurities: A little bit of As as an impurity atom makes bronze "harder", it doesn't deform so easily
any more. Of course, nobody knew this. All that was probably known was that some sources of copper and tin ore,
together with all kinds of tricks (including some magic or prayers, of course) produced superior bronze. |
| | It is quite natural that tin and other metals were
discovered shortly after the momentous discovery of copper smelting. Once you saw that
precious copper could be made from some kind of rock, everybody not completely stupid would of course try what you
could get with other rocks. |
| | We also have the
beginnings of an environmental disaster, because for metal smelting you need tremendous quantities of charcoal. First in order to obtain high temperatures but, just as important, for reducing
the metal oxide according to |
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| | About 100 kg charcoal are needed to smelt 5 kg of copper. |
| | Besides shipbuilding, charcoal production
is responsible for the disappearance of large parts of European forests (the disappearance of yew trees (which were ubiquitous in
antiquity) from present day forests, by the way, is due to the middle age bow-and-arrow industry - nothing beats a yew
bow!). Charcoal production was a major industry and the source of the many charcoaler ("Köhler") stories in fairy tales and
folklore. |
| | Beside Cu and
Sn, Pb, Hg, Ag, and of course Au, were known and produced on an industrial scale - especially by
the Romans. But the Romans (and the Chinese, and the Indians, and the ...) had also Fe - but still no fire hot
enough to melt it. |
| Early experience with the smelting and melting of other metals did not help in
producing iron - it first came into use about 1000 years later than bronze. This must have been a kind of
puzzle, because the ancients did know that iron existed. It was
extremely rare and precious - because it fell from the sky in exceedingly small quantities. |
| | |
| | King Tut, matter of fact, had a little iron dagger made from meteorite iron right on his
breast - obviously his most precious object. In old Sumeria, iron was called "sky
metal" and the pharaohs in old Egypt knew it as "black copper from the
sky". | |
 |  |
| King Tut's daggers (Internet source "Stacey") | Meteorite
stolen from the Eskimos |
|
| Of course, only pictures of his less precious and
useless but more showy gold dagger are easy to find. The picture on the right shows both. |
| The Eskimos in Greenland, matter of fact, made their iron tools for hundred of years from a large (30 tons)
meteorite. |
| Some American explorer (Admiral R. Peary) finally stole it (he wouldn't have expressed it
that way, though) in the 1890s and had a hard time to transport it to the Natural History Museum in New York.
Here it is: |
| |
| We may safely assume
that the old materials scientists tried everything to smelt iron from suitable stones. They did have tricks to raise
the temperature of a fire - in a 4500 old mastaba in Egypt, I took a picture of a relief showing six gold smiths (probably rather their Ph.D.
students) blowing into the fire with hollow reeds. But just blowing with lung power will not do the trick for iron -
maybe you get 1200 oC, but that's it. |
| | So in a typical fire with temperatures well beolw 1500 oC you do not get
liquid iron - but you do get solid iron because
reduction does take place - in a solid state reaction. What you get is an iron bloom ("Eisenblüte" in German), a mixture of fine iron particles, unreacted iron
oxide, slag and charcoal residue. Here is an actual picture of some ancient bloom (from around 600 AD; I
actually "found" this myself (in some museum). |
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| | The iron in the
bloom was rather pure (and thus comparatively soft) because a solid state reaction produces only iron - carbon or other impurities have to diffuse in from the outside (if the iron
would be liquid, it would just dissolve the dirt up to the solubility limit). |
| The early iron smiths (probably being Hethites of some form)
could "wring" the iron from this bloom by separating the iron from the rest mechanically and repeatedly
hammering together what was left at high temperatures (about 800 oC; some of the slag then is liquid
and gets squeezed out) with, no doubt, proper prayers to the respective gods and many (magical) tricks. |
| | What they finally obtained was "wrought iron" ("Schmiedeeisen"), i.e. a lump of rather pure iron
consisting of small pieces welded together, with plenty of small inclusions (small, because of the hammering that
breaks up large pieces of slag). |
| | Extreme
care was necessary - from the selection of the iron ore, the reduction process and the hammering business. If you were
careless, the iron oxidized again (it really "burns" at temperatures in excess of about 800
oC), and if you kept your reduction process going too long, carbon diffuses in and you may end up with
cast iron (C content about 3% - 4%; melting point as low as 1130
0C). Then you actually got it liquid - "casting" was possible - but cast iron is brittle and
useless (for weapons, that is). |
| | Somewhat later, with larger furnaces and increased experience, the bloom obtained may have contained some
high-carbon melted parts on its top layer. It then consisted of a whole range of iron-carbon alloys - from rather pure
wrought iron to cast iron with good steel - say 0,5 % - 1,5% carbon - in
between. The art of the smith than included to pick the right pieces. This was a highly developed skill, we know about
it especially from Japan; but that does not
mean that the Kelts or others did not do it just as well. |
| But beware. The
art of making iron and steel, developed over 2000 years in many civilizations, cannot be contained in a few
lines, not to mention that very little is known about that story - iron, after all, rusts (see the link showing an old sword), and not much has been found that gives detailed knowledge
about how the old romans, Indian, Chinese, etc. made their steel and iron products. |
| | Nevertheless - the early smiths, starting
with the Greek god Hephaistos (the roman Volcanos) and
containing many fabulous figures like the Nordic "Wieland the smith" or "Mime" in Wagners "Ring
des Nibelungen", could produce articles, especially swords, from the iron bloom that were much better than the
customary bronze stuff (and than of course "Magical" swords). In other words, they sometimes
succeeded in making good steel. |
| What was their secret? It is rather
simple - looking at it retrospectively: You need the proper concentration of C in the Fe bcc lattice at
room temperature (some other impurities are helpful, too; while others - especially S and P - were
harmful). Raising the about 0,1% C in wrought iron to an optimal 0,7 -0,9%, raised the hardness (or better the yield point) threefold! But if you got
too much - say 2% - you were on the road to brittle cast iron not useful for swords. |
| | Not being able too melt iron (and thus
not being able to throw some magical stuff into the brew) the only way to get carbon (or on occasion N which
also "works") into the Fe lattice was diffusion via the surface. What you needed to do was to
"roast" you iron (possibly the whole sword) for the right time at the right temperature in a charcoal fire.
Magic and praying helped - it did indeed: How do you keep track of the time without a watch? You utter a long prayer
that you learned from your master - the right ones "worked"! The rest of the magical ritual was helpful in
providing reproducible conditions. |
| | Of course the old practitioners had no idea of what the really were doing; if they thought about it, they
felt that were purifying the iron in the (more or less holy) fire. This erroneous believe (like so many others) goes
back to the (from a materials science point of view somewhat questionable) philosopher Aristoteles who certainly asked the right questions about life the universe and so on,
and is righteously famous for that. His answers, however, were invariably wrong - even in the few instances where he
could have known better. |
| | Well,
we have made but the first step to steel. We now must make a few more steps for good homogeneous steel - or we delve
into a fascinating world of its own, the various damascene
techniques, one of which is blending different kinds of steel into a compound material. More to that in the
link. |
| Here we look first a bit on what
happens in heating up and cooling down your material. We know, after all, that
going up in temperature, iron changes at 910 0C from the bcc ferrite phase to the fcc austenite phase. |
| | Carbon feels much more at home in
austenite - its solubility is higher than in ferrite. If
the smith kept his iron in a good fire very long, he now might have had a rather carbon rich austenite in the outer
layers of his sword. So what happens upon cooling down? |
| | Well, it depends. If the iron cools down s l o w l y, the carbon rich austenite will change to
carbon rich ferrite. If there is more carbon in the austenite than the ferrite can dissolve, carbon will precipitate,
forming a new Fe - C phase called cementite (with a quite complicated lattice).
We now have cementite particles in fcc ferrite; usually in a very typical structure - both phases appear like a
stack of plates. This kind of structure is called perlite because, looking at it under a
microscope, it has a luster like pearls.. |
| | Perlite, the mixture of ferrite and cementite, however, is not much better than bronze as far as its
mechanical properties are concerned. So you must prevent the phase change from austenite to perlite if you want to
keep your sword "magic"! In other word, you must not allow enough time for the carbon atoms to diffuse
around during cooling as would be necessary for forming precipitates. In other words: You must
cool down rapidly (hopefully you did the proper exercise for calculating how fast you must cool down). |
| Here we have the next big trick - after making bloom,
extracting wrought iron, and carburization: Quenching - often the big secret of master
smiths (there is a whole Japanese mythology to this subject). The hot sword is stuck in a liquid for some time and
thus quenched - and only very unimaginative smiths would have taken common water at room temperature for that. |
| | If the cooling time was too short to
allow Fe-C precipitate formation, we now have a supersaturation of C in the ferrite phase which then will
have a strongly disturbed lattice structure. A kind of mixture between fcc and bcc phases will prevail
which has its own name: "Martensite". |
| | Now you did
it: Martensite has the fivefold "strength" of wrought iron! |
| | Unfortunately - if you got martensite at
all, it tends to be brittle! Now the next bag of tricks is needed: Heat up your
sword again - but keep the temperature moderate. |
| | Some of the defects that make martensite brittle anneal out and its
ductility goes up. Bang it (i.e. deform it plastically), and you produce dislocations (hey, that's were we started from some time back!). Now you are manipulating a
second kind of defect for optimizing mechanical properties! |
| | But now we stop (so does the smith). If you really want
to know much more about this, use this
link. |
| | Anyway, if everything worked,
you now have a very good (and of course magical) sword which was far superior to the bronze stuff of your opponents.
In particular, you could make it longer without having to worry that it might break
in battle (which was about the worst health hazard imaginable then). |
| And
don't think that an increase in strength by a factor of 4 - 5 is not all that much. The old Gauls, Asterix and Obelix notwithstanding, were conquered by the Romans not least because their swords bent
and needed straightening (over your knee) after a forceful blow - something the Roman swords did not need. (Haha - don't you believe all this Roman propaganda!) |
| | |
| Well, making a good steel sword was lots of work, lots of knowledge, and lots of
luck. Considering what could go wrong, it is quite remarkable that the old smiths actually did produce superior steel
swords now and then. Of course, probably more often then not, only the outer layer was steel, while the inside was
still soft wrought iron - the sword was made from compound materials, in fact. |
| | This gives us (and possibly also the old smithies)
the idea of doing that from the start: Weld together soft and hard layers, carefully picked from the bloom or made by
carburization, and hope that the result will combine the positive properties of both materials. We are talking damascene techniques here. |
| | However, the word "damascene techniques" is a
collective identifier of several very different technologies. Most people associate it with a kind of compound
technology where two different kinds of steel were put together in layers and then forged into a sword or whatever.
While this is something that was done - especially by the Kelts and other North
Europeans - it was not what the guys in Damascus did, the purported source of the
famous damascene blades. |
| | As far as we know
today, the "true" damascene technique actually worked with a famous kind of steel, so called "wootz" which was produced in India for maybe
a 1000 years in a kind of closely guarded monopoly. Wootz was rich in carbon (about 2%; there was a
secret carburization technique) and the trick was to precipitate the surplus carbon in a pattern of fine
FeC3 precipitates. |
| A fascinating world unfolds
behind the catch word "damascene technique", if you like you can browse the following links |
| | Damascene
Technique in Metal Working |
| | Literature to Damascene (and Other) Techniques in the Production of Iron and Steel From the
Internet |
| | A Cross-Linked Glossary of Some Terms from the History of Metal Working |
| Steel technology was not confined to the Mediterranean and the European North West. India may
well have been at the apex of steel technology and China had its own technology
centered around cast iron, used not so much for warfare but for civil objects like pots
and pans. |
| | And lets not forget the Haya, a people who lived in what is now Tanzania.
They had a highly developed Fe technology and used it for beautiful sculptures, too. Their myths and fairy tales
contain many stories relating to the making of iron, using a vocabulary that was heartily enriched with expressions
relating to the making of humans. |
| | There is even some evidence - collected recently (and, of course,
being discussed controversially), that the
old Africans had the highest temperatures of all, even reaching the melting point of iron some 2000 years ago
(long before everybody else did) |
| Whatever happened whenever and
wherever, during the millennia, and despite the many difficulties, iron and steel became common materials. At some
time in the middle ages or Renaissance, the melting temperature could be reached, but the mass production of good
steel still had to wait for the 19th century. Before, only "thin" objects - the paradigmatic
"sword" or katana, scimitar, saif, shamshir, tachi, tulwar, yatagan,.. - could be made by in-diffusion of
carbon. |
| Charcoal was replaced in the 17th century with coal, but not without
unpleasant surprises. Iron that was smelted with coal instead of charcoal was very brittle and completely useless. We
now know, of course, that minute amounts of sulfur in the Fe lattice - it segregates in grain boundaries - are
sufficient to make Fe brittle, and S, like other harmful impurities, is contained in regular coal in
rather large concentrations. |
| | The solution to
this problem, surprisingly, did not come from the military related strata of society, but from the second most
important enterprise dear to the hearts of men: beer brewing. Brewers had tried to use coal
instead of charcoal for roasting the barley - and produced a stinking abominable brew. Thusly coke was invented: Roast
coal in an environment deprived of oxygen - the stinky stuff will evaporate and what remains is clean carbon - called
coke - which could not only be used to brew beer, but was also usable for the iron smelting
industry. |
| The beginning of the industrial
revolution was severely hampered by the lack of a large-scale process for the production of good steel. (Just imagine
how the Si revolution would have fared without large dislocation free and rather perfect Si crystals).
The (at least in German and French) paradigmatic Eisenbahn
(chemin de fer in French), the rail road, needs rails; with regular wrought iron or cast iron the rails had to be
renewed every 6 month because they deformed under the load (or cracked). Accidents were frequent and often
catastrophic. |
| | The production of large
amounts of iron was common by then - the essential part was blowing large amounts of air into the fire with the aid of
mechanical bellows powered by steam engines. The leading British production accounted for 2,5 million tons of
iron in 1850, but the production of steel was still a cumbersome and expensive business, accounting for a few
percent of the total production. |
| | It was also known
for sure since 1786 that steel had something to do with carbon; the first person suspecting this was one Tobern Bergmann in 1774 (other sources, however, refer to Vandemonte, Berthollet and Monge from
France). |
| | Still, all efforts to produce iron with
the proper carbon content (and the right structure) "from scratch", were in vain. Sometimes things worked,
sometimes they didn't - there was no large-scale, reliable, and reproducible process. And thus no big bridges, sky
scrapers, safe railroads, big ships, efficient engines, and so on - one rarely reflects how much cheap steel changed
the world! |
| This time, however, progress came from
the military industrial complex. It became simply too embarrassing that the big canons (made from cast iron) had a
tendency to explode. Something had to happen. |
| | It was Henry Bessemer who was especially interested in good steel for big canons, because he had just
invented a new kind of projectile that received some spin even from smooth bore guns (and thus was harder to
destabilize during flight). Unfortunately, the canons couldn't take the additional pressure building up while the
projectile was building up spin as well as speed- they exploded more than ever. So Bessemer was looking for large
amounts of cheap steel. |
| | He was then the
first person (so it was believed for a while) who had the genius idea of making steel by getting carbon out of cheap, carbon rich cast iron, instead of using
the cumbersome way of getting carbon into low-carbon wrought iron. The way to
"drive out" the surplus carbon was to blast large amount of oxygen
through the cast iron melt (which, by the way, definitely needed the steam engine; quite hard to do this through a reed). CO
will form in the melt which not only burns off to CO2 upon hitting the air, but by doing this
supplies the heat to increase the temperature of the melt because the melting point will go up with decreasing carbon
content. If you stop at the right time (looking at the color of the flame), you will be able to adjust the carbon
content of a large amount of iron to just the right value and thus produce large amounts of good steel. |
| Mr. Bessemer, who was not exactly unknown before (he already had some fame as the
inventor of the "lead" pencil (which in reality contains graphite), after publishing his finding on Aug.
12th, 1856 became very famous - and very rich - quickly; everybody wanted his process. The London Times went as
far as printing the whole paper two days later. |
| | But point defects were fighting back. The industrial realization of
the Bessemer process with large quantities of ore and coke yielded a big and very
unpleasant surprise: Bessemer steel from large size production, in contrast to the
Bessemer steel from "laboratory" experiments, was brittle and not fit for anything. Bessemer felt like
"being hit by a flash of lightning from the blue sky"; the descend from the Olympic heights of top inventors
to desperation was quick and brutal. |
| | But Bessemer was a good materials scientist and engineer; if it worked once, it must work again. There
must be reasons for what happened, and with diligence, one can find out what is going wrong. What had happened? |
|
Well, Bessemers work, and the work of many others, supplied the (here much
simplified) answer. Bessemer used Swedish iron ore for his experiments (you always
use the best in lab experiments), while his industrial country fellows used English
ore - and this stuff contained some phosphorous. The Bessemer process (possibly in contrast to the
old-fashioned steel making process) did not remove the phosphorous, and small amounts of P are sufficient to
render steel brittle. As we know now, P segregates in the grain boundaries and changes the local properties in a
detrimental way. |
| | Phosphorous had to be removed
(if you lived in merry old England, out on a conquest to assemble an empire, you did not want to have your steel
production depend on the supply of Swedish iron ore). Two cousins, Sydney Gilchrist Thomas and Percy Carlyle Gilchrist, found the way in 1875: Take (among other things) chalk stone for the
lining of the Bessemer converter and even add some to the melt. The phosphorus
would react with the CaO of the burnt chalk and end up in the slag which could be skinned from the liquid steel,
or stuck to the lining. |
| | There were plenty
of other problems - on occasion, e.g., some oxygen remained in the steel and rendered it useless. Mr. Mushet, another Englishman coming to the aid of his country,
found the solution: Add some "Spiegeleisen" (an iron - manganese alloy found somewhere in Germany) and your
problems are gone. The Mn reacts with the surplus O and forms slag. It also neutrlizes any sulfur in the
mix, which would otherwise create real trouble. |
| So besides
Bessemer, many people were involved in bringing large scale steel production to fruition. And, as it practically
always will turn out with great inventions, somebody else did it before. In this
case it was one Mr. Kelly from the USA, who had the
"Bessemer" idea 10 years before Bessemer himself. While he made a mint over patent hassles, the name
Bessemer remains attached to steel, and Kelly is quite forgotten as a materials scientist. |
© H. Föll (Defects - Script)
Preface to "Defects in Crystals" 
With frame