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A Brief
History of Steel
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This moduel is also
availabe in Romanain languga, thanks to Irina
Vasilescu. Here is the
link |
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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. |
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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. |
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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 (?). |
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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. |
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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. |
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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. |
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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).
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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". |
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| King Tut's daggers (Internet source "Stacey") |
Meteorite stolen from the Eskimos |
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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. |
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The Eskimos in
Greenland, matter of fact, made
their iron tools for hundred of years from a large (30 tons) meteorite.
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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: |
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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. |
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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). |
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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. |
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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). |
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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). |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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? |
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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.. |
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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). |
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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. |
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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". |
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Now you did
it: Martensite has the fivefold "strength" of wrought
iron! |
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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. |
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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! |
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But now we stop (so does the smith).
If you really want to know much more about
this, use this link. |
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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). |
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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!) |
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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. |
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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. |
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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. |
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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.
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A fascinating world unfolds behind
the catch word "damascene technique", if you like you can browse the
following links |
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Damascene
Technique in Metal Working |
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Literature
to Damascene (and Other) Techniques in the Production of Iron and Steel From
the Internet |
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A
Cross-Linked Glossary of Some Terms from the History of Metal Working |
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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. |
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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. |
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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) |
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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. |
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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. |
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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. |
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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. |
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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. |
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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). |
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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! |
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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. |
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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. |
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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. |
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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. |
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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. |
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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? |
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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. |
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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. |
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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. |
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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