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The (huge)
Hyperscript "Iron,
Steel and Swords" contains a lot about steel. Check it if you
like |
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The "Iron
Age" lasted a long time. It actually should
be called "Steel age", because
pure iron is not only difficult to make,
but has only limited limited use as a
structural
material. |
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Steel and cast iron, both
Fe - C alloys or compounds, and not just iron made the difference to the bronze tools and
weapons in use before the iron age. Note, however, that bronze products were
used for a long time parallel to steel products. |
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The beginning of the iron age in Northern Europe dates to
about 800 BC; one could debate if it ever really ended. The industrial
revolution in the 19th century has one of its deeper roots in the
discovery of how steel could be mass produced; and the car industry, for
example, is still perfectly impossible without steel but quite possible without
Silicon. The computer, of course, is perfectly impossible without Si -
but does this mean that we are now in the Silicon Age? |
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Iron technology was invented in the Mediterranean
about 1500 BC; present day knowledge ascribes its discovery to the
Hethites from what is now
Turkey. |
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India and China also mastered iron technology in ancient
times. It appears, however, that the Hethites were earlier by several hundred
if not 1000 years. The Japanese, of course, had and still have a heavy
cult around their steel swords for a longe time, and there was some early iron
techology in Africa, too. |
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In the "Hyperscripts of AMAT", a growing
number of modules deals with the history of iron and steel; in particular with
the ancient paradigm of this material: the (magical) sword. These modules will give you
an idea of what iron technology meant to our ancestors, and why your conception
about it is probably totally wrong. Available are: |
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A short history of Iron and steel
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Myths
around making a sword (In German; not for the faint of heart) |
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The
Ring of the Nibelung (In German; Wagner's opera in the context of forging
Siegfrieds sword). |
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Damascene Technology (In
English; contains many links to other sources). |
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An early (magical) sword
(In English; shows the original and its reconstruction). |
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Magical swords (In German; What
makes a sword magical - how is a Japanese sword made?). |
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In this module, however, we will look at steel
from a scientific point of view. |
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This is not easy: Steel is an extremely complicated material
with an amazingly large number of variants; and new discoveries are still being
made. |
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Here we will only look at some basics; this involves cutting corners and being at
bit imprecise at times. To simplify things, we will treat carbon steels, alloy
steels, and cast iron separately; even so this does not make much sense for
many real steels. |
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The Iron - Carbon Phase Diagram |
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The Iron - Carbon Phase Diagram is one of the most
important diagrams of mankind - but not part of public education. |
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Here is the important part: |
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What we see is: |
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The color a smith sees at the
temperature given (sort of). Notice the "bright cherry red" at the
996 K boundary |
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Absolutely pure iron changes from the a-phase (which has a bcc lattice) to the fcc
g- phase just below 1200 K (1180
K, to be precise). Around 1700 K it changes to the bcc
d-phase; at 1800 K and some it
melts. |
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There is no b Phase ??? Well - b -ferrite is simply non-magnetic
a - ferrite; here we can just forget about
it. |
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The situation is quite different with
a little bit of carbon - somewhat more than 0.1 weight %, say.
Around the melting point tricky stuff is going on, which we will not consider
any more. |
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Below 996 K (= 723 oC) the iron
cannot dissolve all the C and we have some mixture of a-iron and Fe3C. |
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Above 996 K, however, the g-phase can keep quite a bit of C in solid
solution up to a maximum of 2% at 1403 K (= 1130
oC). |
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At about 0.8 % C, we have an
eutectoid
composition at a temperature of 966 K, and around 4.5 % we
have a true eutectic composition at 1403 K. |
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The diagram extends only to about
6.7 % C; at higher C concentration nothing of interest will be
found. This means we are actually considering the Fe - Fe3C
phase diagram. |
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If you wonder why so much happens
with so little carbon, don't forget: At 6.7 weight %, every
fourth atom in the soup is a carbon atom; we have 25 atom %! |
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All important true
phases have (old) names; these are
- Cementite (German: Zementit). The stoichiometric Fe3C
phase. It is a compound with a complicated lattice; it is rather hard and
brittle.
- Ferrite (German: "Ferrit). The a-phase
with the bcc lattice. If you want to be precise, you call it a - ferrite.
- Austenite (German "Austenit). The g-phase with the fcc lattice.
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Just to get you used to the facts of
life concerning iron and steel, there are some more old-fashioned names still
very much in use, and absolutely de rigeur for everybody who calls herself a
materials scientist:
- Pearlite (German: Perlit), the two-phase mixture obtained right below
the eutectoid point at 0.8 % C
concentration - we will encounter it quite soon and excessively.
- Ledeburite (German: Ledeburit); the two phase mixture obtained right
below the eutectic point at 4.5 % C
concentration; we will not have much dealing with that because it should not
exist in equilibrium at room temperature.
- Martensite (German: Martensit); a kind of metastable version of
austenite + carbon; but with a tetragonal
lattice and different mechanical properties; this will exercise us a great
deal.
- Bainite (German: Bainit); a mixture of a
- ferrite supersaturated with carbon and cementite, but in a (non-equilibrium) structure
quite different from pearlite.
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If you wonder why there are so many
strange names, consult the
link. |
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Considering cast iron, the important part is the eutectic at
around 4.4 % C. |
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At that concentration, casting at
about 1400 K is easy; and the temperature is so low that it was easily
achieved in ancient times. |
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But
cast iron contains a lot of carbon (mostly
in the form of graphite; which is not
directly evident from the phase diagram), is brittle, and while employed in
huge quantities, not what we are after. What we are after is steel. |
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That is different from an eutectic reaction, where both components need to
solidify. Here nothing needs to solidify, we have some grain structure with
a and b
grains, and the a grains could remain
unchanged. |
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We won't go into more
details here, but you can look at an
illustration of the
solidification and phase change process in an illustration module. |
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What the structures (=
Gefüge) you
get look like is shown in the pictures below. |
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Ferrite and blackish Pearlite grains
at 0.1 % C |
Ferrite and more blackish Pearlite grains
at 0. 4% C |
Pearlite and (white) cementite
at 1.3% C. |
Interestingly, cementite seems to be black for hypoeutectice
steel and white for hypereutectic steel. Six sources ignore
the obvious problem with pictures like these. The resolution of the apparent
paradox (probably) is as follows: Both, ferrite
and cementite are "white". The black part comes from the boundaries between ferrit and cementite (a simple
optical effect
at high magnifications). |
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Now there is a first
complication: If we start well above the eutectoid temperature of 996 K,
the composition of the a + g, or the g +
Fe3C phase mix has to change according to the lever rule as the
temperature decreases. |
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It only can do so by diffusion in the solid state. But this is a
relatively slow process, so we might expect that what we will get, will depend
on the cooling rate. Only for very slow
cooling will we obtain the "equilibrium" structure shown in the phase
diagram. |
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What kind of mechanical properties do
we expect? We have, after all, some kind of composite made from a relatively
soft and ductile material (ferrite) and a hard and brittle material
(cementite). |
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As far as Youngs modulus is concerned
we could use the rules derived for compound
materials. |
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But we are more interested in
properties like hardness, yield stress, ultimate tensile strength, and maximum
strain. The latter is a kind of direct measure for ductility. Since all these
properties are "defect sensitive" (as we learned in chapter 8!),
simple rules cannot exist. We only can make educated guesses. |
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We certainly would expect that an
increasing cementite concentration would lead to an increase in hardness but to
a concomitant loss of ductility (it becomes hard and brittle). |
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Let's see what we get. Some
essential mechanical properties are shown on the right. Since not only the
carbon concentration, but also other structural details determine what you get,
the graphs give a whole ranges of properties for structures somewhere between
"annealed" and "normalized". |
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"Annealed" and "normalized"
refer to different standardized heat treatments designed to give comparable
structures with respect to grain size and shape, and dislocation density. |
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"Annealed" is as close to equilibrium
as possible. We have large grains and small dislocation densities.
"Normalized" means that some heat treatment was used so that the
history of the material (it might have been heavily deformed, for example), is
essentially wiped out, but grain sizes are small and we are far from
equilibrium. |
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The first graph shows the
yield
stress RP (German: Fließgrenze) and the
ultimate tensile
strength RM (German: maximale Zugfestigkeit). |
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We see that both increase with increasing carbon
content, but the more important parameter RP sort of
tapers off and remains constant around 0.5 % C |
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The second graph gives the maximum
elongation that can be achieved in a tensile test and the impact energy or
fracture
energy (German: Zähigkeit). |
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The maximum elongation is a fairly direct measure
of ductility; we see that the ductile behavior gets worse in a rather linear
fashion with increasing carbon content. |
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It is pretty close to zero as soon as there is no
longer a contingent matrix of ductile a-
ferrite. |
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The impact energy is a fairly direct measure of
"brittleness". Low energy means easy fracture - the material is
brittle. |
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It decreases steeply, tapers off around around
0.5%, and reaches a relatively constant low value for hypereutectic
steels. Essentially, the fracture toughness than is determined by the fracture
properties of the cementite, which now forms a continuous skeleton with
embedded pearlite grains 1). |
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So much for the simple part. Iron-carbon compounds or
plain carbon steels now become
difficult for two major and related reasons: |
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1. The phase
diagram from above does not show the real
equilibrium structure - FeC3 called cementite is not
the phase with the absolute minimum of the free energy; that is actually
carbon. |
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However, FeC3 is
metastable; it simply forms before pure C (= graphite) can develop; and
it may take a long time before all FeC3 is decomposed into
C. |
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The real, i.e. true equilibrium phase diagram of
Fe and C, however, looks a lot like the C -
FeC3 from above. Just take out
the vertical line for stoichiometric FeC3, substitute
"C" for "FeC3" everywhere, and
shift the horizontal lines downwards a few K; leaving everything else
the same (except for the "L + graphite" liquidus line, which
goes up much steeper). If you can't imagine this,
look it up in the
link. |
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This has some consequences for
cast iron (it is essentially the reason why we
find pure graphite and not just cementite in cast iron as already
mentioned above) |
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2. The solubility of carbon in
austenite is much larger than in ferrite. At temperatures somewhat higher then
the "magical" 996 K, austenite can easily accommodate any
carbon concentration around the eutectoid concentration of 0.8%. |
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Passing the eutectoid temperature during cooling
now requires a radical change. Practically all the homogeneously dissolved
carbon now has to go to the inhomogeneously distributed cementite - by
diffusion, there is no other way. |
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This simply takes time, and if
that time is not available, because the
austenite is quenched, i.e.
rapidly cooled (really rapidly
at this point, with at least 1000 K/s for hypoeutectics), something
new happens. |
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The carbon stays in place - more
or less - and this necessarily prevents pearlite and ferrite formation.
Instead, a new lattice type is found, called "martensite". It is a body-centered
tetragonal
lattice; essentially a bcc lattice elongated somewhat in one
direction. |
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The transformation from the
fcc austenite lattice to the
tetragonal martensite lattice does not need
long range diffusion (as, e.g., the transformation from the austenite lattice
to pearlite). |
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It takes place by a shear process (involving special dislocations); and
all transformations of that kind are called
martensitic
transformations. There are
many martensitic transformations in materials science. |
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It looks like martensitic
transformations are easy - if you don't have to move atoms around, it could
just happen! |
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Not so: Changing from one lattice type to another
one will (almost) always involve a volume change. If there is no moving around
of atoms, if nothing can "give", such a transformation will then
automatically produce a lot of stress and strain, and thus requires plenty of
energy. Martensitic transformations therefore are difficult; they only happen if there is a large
driving force. |
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As a corollary, martensitic structures are rarely
equilibrium structures; they are metastable at best. But that does not mean
that they can last a long time at normal temperatures. |
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The pictures below illustrate what
happens and what it looks like. |
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The martensite shear transformation |
Martensite "needles" or lathes in
austenite (Magnification ×1000) |
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Looking at the schematic representation of the
shear transformation; it is clear that the process is not easy, stores a lot of
elastic energy, and tends to make thin needles or plates (called "lathes"). |
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Pure martensite is
soft and ductile. However, we don't have pure martensite; we have martensite
with interstitially dissolved carbon - and this is an extremely hard and
brittle substance. |
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Hard martensite is of
not much use by itself - but it is the key to things like "magical swords"
or high strength steels |
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Look at the Vickers
hardness diagram
below to get an idea of how much better the edge of a Japanese swords - pure
martensite - will be, compared to a regular decent steel. |
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If you want to know
what "hardness" means in some
detail - look up the link above. For steel, the Vickers hardness
HV is pretty much the same thing as the yield stress
RP; we have in a good approximation |
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So in all the diagrams here or in
books, you can always substitute hardness for yield stress and vice verse at
least qualitatively. For numbers you have to watch out what kind of hardness
(usually Vickers or Brinell) is given |
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Vickers hardness of annealed and
quenched plain carbon steel. |
Yield stress as function of martensite
induced dislocation density |
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Why is martensitic steel so hard? A better question is: Why is the yield stress so high, because this addresses basic
mechanisms we learned about in chapter 8? |
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Above we said because carbon
containing martensite is hard - but that is a bit of a tautology. |
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Do we need new mechanisms for raising
the yield stress of a given material that we did not address in chapter
8? |
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As you would guess: Not really!
Martensite formation simply generates a high density of dislocations since a
lot of (local) plastic deformation is needed to accommodate the martensite
"lathes" |
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Indeed, if we plot the yield stress
(or hardness) versus the root of the dislocation density introduced by various
martensitic structures, we obtain exactly the straight line we would expect
from the discussion in chapter
8.3.4 |
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While this kind of knowledge is not
(yet) very helpful if you actually make
steel; it simply proves that we understand what is basically going on - and
therefore also what can go wrong. |
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Steel Making Now and Then |
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So, if you were an ancient smith and
you would quench your red-hot sword blade in cold water, you might on occasion
find that the edge of the sword is now extremely hard, while the interior is
still tough, but far softer. |
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What has happened then
is easy to see - for us: Only the surface near parts will cool down fast enough
for martensite formation. The inner parts simply will stay hot longer and just
produce pearlite with ferrite or cementite. |
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But this will only work on occasion. Why it does not always work, you don't
know. Your best guess would be that some magic is involved, or that some goods
need to be in a good mood on quenching day, if things are to work out. |
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Because what you ancient smith -
living sometime between 1500 BC to 1850 AD - can't know, is that
what you get also depends very much on the exact carbon content (which,
whatever it was when you started, you changed a lot simply by forging the blade
in your fire). |
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Even worse: What you get also depends
a lot on all the other elements that your raw steel will contain in some small
amounts. And on the temperature you quench from. And on the temperature you
quench to. And on the liquid you use, not to mention if you agitate it it or
don't. And on the clay, or whatever you use to coat parts of the blade. |
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And no god, priest,
nobleman, politician, philosopher, general, or feuilleton writer ever told you
anything helpful for about 3000 years of steel technology;
notwithstanding all the sacrifices, spells, offerings, prayers, tithes, etc.
that you made or were forced to make, and their ever-present air of general
superiority. |
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For all you know, and for all the
bullshit they did tell you (try
Aristotle): Some of
your products might be good, some might be bad. And nobody really knew why.
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Thank god scientists
and engineers, we don't have to slit the
throat of some living being anymore so that some macho or tussy up in the
heavens or down in the netherworld feels obligated to help us to produce good
steel on occasion. We know almost
everything there is to know about martensite formation in plain carbon steels.
Not yet everything there is to know, but
enough to turn martensite into full scale use for special steels. In particular
we know three major tricks that help us to
produce a wide variety of steels reproducibly and reliably: |
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1. We can produce martensite
with rather low cooling rates, too. All we
have to do is to add some suitable elements (which will bring us to the
next subchapter dealing with alloy
steel). This will allow us to produce homogeneous
martensite even in bulky steel. That's nice, but still not of much
use. |
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2. We can
anneal the material in a
well-defined way; i.e. heat it up again. Since martensite is only metastable,
we can expect that at enhanced temperatures we will get some change to the
stable cementite + ferrite mixture. If we do it right, we will retain some of
the hardness of the martensite while gaining some of ductility of low-carbon
steel. |
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3. During cooling, we can
keep the steel at some medium to high temperature for a while (this process is
called tempering (from German)) - and produce
yet another structure. |
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For the second point, all that needs
to be done is to get the carbon in the austenite mobile again, so it can form
cementite and ferrite. |
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Obviously, you want to stay below the
eutectoid temperature for this; 300 oC - 600 oC is
what you use. |
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You won't get pearlite + ferrite,
however. You rather end up with small FeC3 (= cementite)
particles in a-ferrite. Your grain size is
also smaller, because you retain the small grain structure of the
martensite. |
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What you are doing now is optimizing
precipitation hardening. The fine
FeC3 particles make dislocation movement difficult (which
gives a high yield stress and hardness), but do not completely prevent it
(which keeps the material ductile). |
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But don't "over-temper"! If
the cementite particles get too coarse, you loose hardness without gaining much
ductility anymore. |
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If you do it just right, you end up with tempered steel, a synonym for the ultimate
combination in strength, hardness, toughness - you name it - for a good part of
the 19th and 20th century. |
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By now you got the idea: The exact
structure of the cementite - a-ferrite
mixture is of prime importance. We have all the strengthening mechanisms
discussed in chapter 8 in combination; in addition we keep microcracks
from happening or spreading. What you get will depend sensitively on the carbon
concentration, and in particular at the heat treatment (cooling and annealing /
tempering). |
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Very slow cooling gives an
equilibrium structure with largish grains 1) of pearlite and ferrite. For low carbon
concentrations (say 0.1% - 02 %) we get mediocre strength properties of
this "mild" steel. However, the material is easy to work with and it
can be welded! That's why your car body and much else is made from this basic
kind of steel (however, with a few more alloying elements thrown in). |
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Rapid cooling plus tempering gives
"tempered steel". The ultimate in strength for plain carbon steel,
but not easy to work with; it is also not weldable. |
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How about rapid cooling to some
intermediate temperature; and slow cooling after that? Followed by some
tempering or not? |
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You get the idea once more. There are
innumerable possibilities for plain carbon steel; and then we have the whole
periodic table for alloying - but essentially we understand what is going
on. |
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The "rapid cooling to some intermediate temperature and slow cooling
after that" suggestion is actually a good one. It produces yet
another characteristic mix of FeC3 - a-ferrite, called "Bainite". |
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The picture below shows some
transmission electron microscope pictures of the structures discussed: |
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Pearlite |
Bainite |
Tempered martensite |
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Pretty much the same mixture of
Fe and C, but different structures and very different properties |
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As promised, we only did some basics
- there is much, much more to plain carbon steel! |
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Now we look at all that stuff from a
"distance" and realize that what we do when we make plain carbon
steel is just what some ancient smiths did: We used "damascene
technology": We always produced an intimate mix of "soft" and
"hard" iron. But there are differences: |
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We do it on a much finer scale and in
far more tricky ways. We also understand what we do. We don't need magic of any
kind or help from above (or below). |
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We also know how to define and
measure the properties of the steel we make. We do not have to cut through live
people to assess the quality (as the Japanese did). |
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But for many kinds of the steel we
can make, we have essentially the same problem as our elders: We cannot
cast everything we want. |
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While we can easily achieve the
needed temperatures, just pouring some liquid Fe - C mix in a form will
not give the structure we need. Maybe some additional heat treatment helps, but
if not - then you bang your material into shape like all smiths before you |
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If you can't cast it, you also will
have a hell of a time to weld it. After all, welding means to liquefy portions
of your material and then solidify again. If the structure at the seam is not
what you need, you have a problem. |
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This is the main reason while car
bodies, to give one example, are not made from very strong steel. |
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If instead of pressing sheets of the
stuff into the right shape and then weld everything together you would have to
bang it out of some big lump of steel with a hammer, you and I and most
everybody else would not be driving a care. |
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So be glad we have
mild steel and understand its properties
perfectly. Otherwise, the parts of society
mentioned above would do what they always do and did: They drive a car, and you
and me do the banging - as slaves. |
© H. Föll (MaWi 1 Skript)