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Writing these modules, I found it
surprisingly hard to find data or good metallographic pictures for the plain carbon steel of the preceding chapter. |
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Well, there is a simple
reason for that: There is practically no such thing as plain carbon steel - and
probably never has been. |
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Steel practically always
contains other elements besides carbon, too, which were added intentionally or
unintentionally. |
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Unintentional elements are in
particular Sulfur (S) and Phosphorous (P); but also Sn,
As and Sb. |
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All these elements tend to diffuse to
grain boundaries, where they might segregate; reducing the cohesive strength -
the steel becomes brittle. If it does not happen right away, it might happen
after some temper treatment; we have (undesired) temper brittleness. |
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By now you should be sensitive to
words like "tend" and "might", which indicate that things
are not so simple and easy. |
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Phosphorous, for example, is not always harmful. In properly treated steel, it
might be beneficial, too, as shown
below. |
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Since the "bog
iron" (German: "Raseneisenerz"), used for millennia to
make iron and steel, contained relatively large amounts of P, it
"might" have been crucial for the early smiths to keep the
Phosphorous from segregating to grain boundaries. What bog iron looks like is
shown on the right - we all have seen stones like that, but possibly not
recognized what it was. |
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However, if you were lucky, some other elements
contained in your iron "might"
have helped in this respect and you may never have noticed that you had a
problem. |
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But generally, some elements, in
particular Sulfur S (and P), are almost always bad news, and not easy to avoid. |
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But fortunately, Manganese (Mn) is also
quite ubiquitous - and it sort of "soaks up" the Sulfur (by forming
immobile sulfides). |
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We thus have a first
reason for adding something else: To neutralize bad effects of
unwanted, but hard to avoid trace
impurities. But this, while being quite important, is nevertheless only
a minor point for making
alloyed steels, sort of a fringe
benefit. |
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So small wonder that you will always find 0.5
% -1 %or so of Mn in practically any alloy steel (and "plain
carbon", too). |
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The major reasons for adding all kinds of elements to
carbon steel are: |
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1. Improved strength while
maintaining good ductility and in particular workability
("Verarbeitbarkeit"). The key words in this context are
solution
strengthening and
precipitation
hardening ("Mischkristall- und
Ausscheidungshärtung") while maintaining weldability
("Schweißbarkeit"). |
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2. Improved
hardenability. The key is to
enable martensite formation even at
relatively low cooling rates so that it can occur in the interior of massive
steel pieces , too. In German, hardenability is called "Härtungstiefe" (= "hardening
depth"), which gives a better impression of what is meant: Even regions
deep in the bulk, which by necessity cool down more slowly than surface near
region, become "hard", i.e. experience martensite formation. |
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3. Improved
corrosion resistance. The
key word is "stainless steel",
resulting from rather large additions of Chromium (Cr). |
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4. Stabilized austenite at low
temperatures. In other words, we get (nonmagnetic) austenitic steel (with an fcc lattice) at
room temperature (and somewhat below). It is almost, but not quite the same
thing as point 3. from above. |
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In addition, we should not forget
that properties like weldability, and
pedestrian concerns like money, are also
part of the alloying game |
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All the obviously desirable features
from above can be achieved to some extent by adding a suitable amount of the
right elements. |
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To make things complicated, most
elements do several things from the list above, and a combination of two
elements usually does not just produce the sum of the individual properties,
but something new. |
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In addition, improving one property
by adding a certain element might easily produce problems with some other
properties. You many have to compromise. |
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And not to forget: As we have seen by
alloying Iron with just Carbon: Many variations of properties are possible with
just one element! |
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In discussing, not to mention
making alloyed steels, a certain amount of
alchemy is in evidence, even today. And new discoveries and new steels will
certainly come forth in the future, too. |
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The link provides a
short list of some alloying
elements and what they are used for. |
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It is entirely impossible to touch
all bases here. Let's just give the four categories from above a cursory glance
and make a basic distinction at the beginning: |
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We distinguish between
- Low-alloy steels: We only add less
than about 2 weight % of the major alloying element(s) (and
usually keep the carbon concentration low)
- High-alloy steels: We add a lot more
than 2 weight % and possibly as much as 20 weight
%.
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In between is "medium-alloy", but that already goes
to far in this context. |
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Here we are generally talking
low-alloy steels, in particular with a
rather low carbon concentration. The general idea is to avoid martensite
formation, which is bad for welding and shaping, but still have good strength
properties. |
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If you want to shape a piece of material by any method (for car
bodies you just press some sheet metal in a form), you must have some
"workability"; in other words, you need some plastic deformation,
i.e. ductility. Think of pure martensite as being like glass, and you get the
idea. |
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Weldability is a particular important part of
"workability"; another one would be "hot pressability"
(Heißpreßbarkeit")or "drawability"
("Ausziehbarkeit; Tiefziehbarkeit"). Just consider how you would make
a car body, if those two properties are non-existent, and you have a good idea
of how important "workability" is for mass production! |
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We clearly need strength
(= "hardness") without
martensite formation, |
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This leaves us with all the basic
mechanisms discussed in chapter 8 for strengthening. |
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We thus add suitable elements to
obtain:
- Solution hardening. Except for
nitrogen, which dissolves as an interstitial like carbon, all other suitable
elements will always be of the substitutional solid solution type.
- Precipitation hardening. Either by
forming finely dispersed hard and small carbides of the alloying elements, or by influencing
the cementite formation to occur in fine particles, or by producing
precipitates of compounds of the alloying elements (e.g. borides, or
intermetallic phases), or by all of the above.
- Grain size reduction. You may produce
small grains (i.e. from a martensitic transformation which rips on i
"apart" in many grains), and/or keep small grains small by keepig
grain boundaries from moving (i.e. grains from growing) by precipitating
suitable elements there (without making the grain boundary brittle, of course).
This will always lead to hardening, too.
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It only remains to
check the "easy" elements of the periodic table under all kinds of
conditions. Let's do that for solution
hardening first. |
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What we find is that Carbon and Nitrogen have by
far the biggest direct effect on the yield strength (owing to their being
interstitials), and that Phosphorous in
solution is very good, too, (but, remember, very bad if segregated
in grain boundaries). |
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Then we have Silicon (Si), Manganese
(Mn) Titanium (Ti) and copper (Cu, not shown) and some
others as still pretty good solution hardener. Cu, however, has
drawbacks (including its prize), and Si causes problems here, too (also
it is much in use for other purposes). |
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This leaves Mn, Ti, and to some
extent Ni and Vanadium (V) as alloying elements (we also had
Mn to neutralize spurious S, if you remember) |
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Complex - but not difficult. We had
much the same picture before for
Copper. |
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In essence, we understand that part of steel alloying. |
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Precipitation hardening can be more
efficient than solution hardening, and indeed, very small amounts of Boron
(B; 0.005 %), or about 0.1 % of Niobium (Nb) or
Vandium (V) will produce considerable increases in strength. |
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Always provided that the
heat treatment was right, the grain size is small, and so on and so forth.
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Just on example: Niobiumcarbide
particle of about 1 nm size will increase the yield stress from about
20 MPa to 200 MPa, at a concentration of about 0.1 weight
% Nb, while "huge" particles with about 10 nm diameter
have practically no effect anymore! |
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We understand that immediately, by
looking at the mechanism of
precipitation hardening. If a 1 nm particle can stop a dislocation
completely, a 10 nm particle can do no more - but we have 1000
times fewer 10 nm particles at a fixed solute concentration. |
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We also understand why these
micro-alloyed steels are rather recent
developments: Try to optimize such a steel if you don't know what happens,
can`t see your precipitates anyway, and can't measure their size and other
properties for some quantitative data. In other words, with no knowledge about
deformation and dislocations, just optical microscopes, and without the whole
bag of microanalytical tools, you are simple blind and the best you can to is
go by trial and error following up some guesses. |
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Anyway, with some basic understanding
and giving proper care to their needs, micro-alloyed steels may have much
better strength than "mild" carbon steels, with all other properties
(exept the prize) being comparable. |
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To some extent, micro-alloyed steels
are the steel industry's answer to the Al car body challenge from
Audi, because they allow to
maintain the easy manufacture and strength of a steel car body, while
considerably reducing the weight (the sheet metal can be thinner). |
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Of course, you may now ask yourself a
simple question: |
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| Hardening mechanisms of maraging steel |
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What do I always add carbon, if I can get all kinds of hardening
mechanisms from other elements, too? |
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Good thinking. Take carbon-free Iron, add
sizeable amounts of elements like Ni and Co, and rather small
amounts of, for example, Al, Si, Mo, or Ti. This
gives you some solution hardening if nothing else happens. |
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Keep out P, S and so on, make sure
the grain size is very small and the grain boundaries not embrittled by
segregation of the wrong elements. |
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Upon cooling down this alloy, some relatively
soft martensite will form (No
carbon!). This is when you shape your piece of steel in the form it is
supposed to have when it is finished. |
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After that, you do some tempering, just right, to
now form lots of very small intermetallic
precipitates between the major elements and the minor elements. |
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This puts some precipitation
hardening on top of everything else and you end up with "maraging steel" (short for martensitic
aging), being fantastically strong while still ductile - and being rather
expensive. |
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The picture above shows the total
effect with an increase of the tensile strength to a fantastic 1500 MPa!
Even larger values have been achieved while still keeping a maximum elongation
of 6 % - 8 % before fracture! |
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A maraging steel is what you use for
landing gear of Jumbo jets, for ultra centrifuges (needed for making atomic
bombs) or for golf clubs (needed for hitting little balls). Interestingly, if
you enter "maraging steel" into Google, you will find either golf
club advertisement, or stern warnings concerning trade restriction, but very
little useful information. We have a real high-tech material here! |
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Shaping a sword, a car body, or
whatever by banging, pressing, stamping, rolling or drawing a piece of some
rather soft steel into the desired shape, and then making it hard by heating
and quenching, is actually a great way of getting strong (= hard) products with
comparably little effort. |
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So we want to keep this
old-fashioned hardening method, known for millennia for plain carbon steel, but
we also want to make the result less sensitive to the cooling rate. Remember,
with plain carbon steel, you only get hard martensite in those parts of your
work piece that cool down with cooling
rates of about 1000 K/s. |
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There is no way to achieve this kind
of cooling rate with anything thicker than a few mm! Therefore the only
option left is to alloy the right elements to our plain carbon steel, hoping
that this will lower the austenite - martensite transformation temperature.
This then might produce the good hardenability we are after - which,
remember, is not just a large hardness value,
but hardness as deep as possible into the bulk of a massive sample. |
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This brings us to an
old piece
of wisdom concerning of what is better: Being practical, or being
theoretical? If you don't have a good theory here, you do not even know if that feat is
possible at all. Even if you trust your luck, you have no idea of how much of
what you should add. Good luck and all the time in the world to you
practitioner! |
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Well, the truth is that we know a lot
about alloying and hardenability, but we really do not have a "final"
theory yet, and a lot of what is known about hardenability did come from an
empirically established data base. |
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Thanks to Walter Jominy (the Chief
Metallurgist for Chrysler Corporation sometime before the war), there is at
least a simple but accurate test to assess the hardenability of a given sample.
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Just take a standard size sample,
heat it to some high temperature, and then spritz water (at defined conditions,
of course) at one end as shown below. The cooling rate will be different from
one end to the other of the sample, and all you do after it has cooled down
completely, is to measure the hardness along its length. |
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What you might find is shown to the
right of the test set-up. |
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Plain carbon steel with sufficient
carbon (e.g. 0.8 % ) may become very hard in the region where the
cooling rate was very high, but the bulk of the sample remains "soft"
(red curve), while very mild steel with little carbon (e.g. 0.3 %) just
shows some hardening (green curve). |
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Now add some Cr, V,
Mn, Ni, or Mo (or some other suitable elements), and if
you do everything right, you may obtain the blue curves - steels with good
hardenability and adjustable hardness. |
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All you have to do now is to check
what happened to the other 10 or so properties of supreme interest
(ductility, weldabiliy, fracture toughness, corrosion resistance, ....). |
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If you are extremely lucky (and
after 10 - 20 years of work), you may find a new kind of steel
with properties just right for your purpose and better than anything else
available so far. |
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We all know it: Iron and
steel rusts! What we probably
do not know: Relatively pure iron ("wrought iron") rusts far less
then steel. |
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In Delhi is a 1600 year old
huge iron pillar (7 m tall, 6 tons in weight; see picture on the
right) that does not rust. It was forged
together from many pieces of wrought iron with low carbon content. Its
"secret" has recently been unraveled: The relatively large amounts of
P in the iron and in slag particles within the iron, catalyzed the
formation of d-FeOOH ("Misawite")
and a layer of crystalline phosphates that together form a stable protective
layer. |
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In the
"Württembergisches Landesmuseuum" (which we encounter in
"sword"
conncections, too) and in many others, iron bars in the typical
double-pyramid shape of the Celts as shown above are on display. |
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If you ever visit these
museeums (the display above is from the museeum in Heidelberg), don't miss this
part; you will experience some surprise: These wrought iron bars, about
2000 years old, look like new. There is hardly a trace of rust. |
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But these are the
exceptions to the rule: Iron and steel rusts! In the museeums mentioned above,
you can also see the evidence for this fact: Most steel objects like swords are just lumps of rust.
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In general, this is easy to
understand: Since metals have
too many
electrons by definition, and Oxygen has too few, in air metal-oxides will
form. The noble metals are just the (rather easy to understand) exception to
the rule. |
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The oxidation of a metal exposed to
air will go on as long as oxygen can meet metal, i.e. as long as either one can
diffuse through the oxide layers formed. |
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Stainless metals, obviously not decomposing
into oxides foreever, thus can only exist if the unavoidable oxide layer formed
in air will be impenetrable to oxygen as soon as a certain (small) thickness
has been reached. |
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This is not all that difficult to
achieve, after all, metals (and other reactive elements) like Al,
Si, Pb, Cu, Cr, ... are quite stable in air (at
room temperature), and, as we have seen, even some relatively pure iron does
not rust. |
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Iron, plain carbon steel, and many
alloy steels, however, do generally not
form a stable oxide - they rust! And sooner or later our car body, sword, or
cooking pot is just a piece of ugly iron(hydro)oxide. |
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And there is nothing particular
systematic that you can do. The method of choice, of course, is to paint the object, or more generally, to apply a
protective coating, e.g. paint, Zn, Cd, or Cr, or if money
is of no consequence, Au. But this will only help for some time if the
object is mechanically stressed (i.e. used)
because than the thin protective layer will sooner or later been worn off or
develop cracks - rusting just starts later, as we all know. |
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The alternative is to alloy a sufficient amount of typically Cr, so
that the surface always is covered with a
stable Cr2O3 layer. |
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The minimum amount of Cr you
must add is 13 % (a number that can actually be calculated), but up to
25 % or so are used. |
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But now you have high alloy steel; and while it may not easily
corrode, its properties may also be quite different from plain carbon
steel. |
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Staying simple, you can get stainless
steel by only alloying pure Fe (no carbon) with Cr and nothing
else. |
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But even then you will get something
new: Fe - Cr alloys stay ferritic
(i.e.. in the bcc phase) at all
temperatures - they do not form fcc austenite at all. Well, no reason
why they should, considering that this is no longer Fe with a little bit
of something. |
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The problem, however, is that now you
have no possibility of using some kind of martensitic transformation for
hardening. |
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So if you also want strength,
weldability and so on, you start a whole new game of going through the periodic
table in search of proper additional alloying elements. |
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Adding some Carbon again will help;
0.6 % is already enough to produce some martensite and thus
hardenability. Simple Fe - Cr - C stainless steels, quenched and
tempered, are indeed used for, e.g., ball bearings, kitchen knives or surgical
instruments. |
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We now have stainless steel, with a
bcc lattice at room temperature (lossely still called
"ferrite"), it is also "ferro"-magnetic (try your kitchen
ware). But we can do more than that with high alloy steel containing a lot of
Cr. |
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Besides having sufficient Cr,
add some Ni (say 10 %) and the ubiquitious Manganese (about
1%). |
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What you will obtain is a steel that
is still austenitic at room temperature
(i.e a fcc and non-magnetic)).
It is not the stable phase at room temperature, but the transformation
temperature is lowered and never takes place for normal cooling rates. |
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This is mainly a result of the
Ni addition; the transformation temperature goes rapidly down with
increasing Ni concentration (from 914 oC at 0 %
Ni to 720 oC for 8% Ni, or to 600
oC for 15 % Ni. |
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Austenitic
steels are materials quite different from regular steel. |
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Not only are they stable in corrosive
environments (thanks to stable Cr2O3 on their
surface) and non-magnetic. |
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They are relatively tough but still more ductile
than regular steel and thus are easy to work with because they can be pressed
or drawn. They also have better creep properties (we will learn what that is in
chapter 10) A certain problem is that they work harden very rapidly,
which makes them difficult to machine. |
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You will find a lot of austenitic steel around
you. Your kitchen sink will almost certainly consist of austenitic steel, but
also the inside of your electron microscope, and much piping in your nuclear
power plant. |
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Well, so much for steel in just two
short chapters. Three final remarks are in order: |
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1. I found it is impossible to do
"steel" justice without including some of its history. |
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2. It is even more impossible (excuse the
oxymoron), to do steel justice without having gone through the
"basics" first, as put down in chapters 2 to 7. |
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3. There is much more scientific stuff
around steel (and any other alloy) that was carefully avoided here in order not
produce system crash at this level. In particular there are things like
TTT-diagrams
("temperature-time-transformation"), semiquantitative complicated
diagrams that reveal , e.g., what will happen to a piece of alloy if it is
cooled down with a specific cooling rate. |
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I hope that you understood the basic
messages: |
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1. Steel is just a collective name for an
incredibly complicated set of materials with wildly different properties. |
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2. But everything can be understood
in principle by understanding the atomic
mechanisms of "strength" (and "fracture"). There is
no mystery anymore, and no magic is needed
to produce a wide variety of products reliably. |
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3. We are just in the transition period
where development of new steel alloys (of Iron and other metals) switches from
the (highly educated) "trial and error" method, to a development that
is guided by scientific principles based on the theory of the atomic structure
of the material. |
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© H. Föll
