||The logical way in categorizing
major steels would be to follow some system. Of course there are steel-categorizing systems
and as it turns out, there are at least two basic ways
to do this: |
In either case you want to come up with a short string of symbols, e.g. S355J2+N or St
52-3 N (same thing, by the way) that, if you are familiar with the code, gives you the major
parameters of the steel in question. The American SAE standard goes mostly for the first option, the European Norm for the second. There is a lot of compromising though, because rigidly sticking
to one basic system just will not get you there.
- By composition (and
- By properties.
| ||It comes as a a great relief that you, like me, do not care for strict and highly
formalized codes. I'm sure about this because sticklers to formalized details would never
have made it that deeply into this Hyperscript. Or else they would still be pondering this link. Thus I will not categorize steel in some formal way but go through some major points my way. Since I'm not
a steel expert, I must rely on what's around in the literature. I will also emphasize on steels
that illustrate some point I find interesting, and not so much on actually making and using
| || |
| || || |
|Classification according to time |
|Name ||Description / Properties |
||The trial-and-error-type. Nobody had the faintest
halfway correct idea of what makes a steel in terms of composition, microstructure, and so
on. This includes all steels from the very first ones around 1500 BC to - roughly - 1775 AD. |
I picked 1775 because it was around then that it finally became clear that carbon was an element and that a small amount of
the stuff mixed with iron produced steel. We can take that as the beginning of:
steel ||Steel making still involved following working recipes but reasoning
about cause and effect was there now, triggering experiments based on some (often wrong) forethought.
This culminated in the various methods (Bessemer, Siemens-Martin, ...) for mass-producing
steel. This period lasted until about 1900, when we move into: |
||As a starting date we can take the year 1897 when Roberts-Austen
published the first iron-carbon
phase diagram in preliminary form. The next decisive period in time centers around 1930
(metals = crystal; plastic deformation
by dislocations) and 1960 (Electron
microscope and advanced analytics). A lot of steel related things were understood in principle
but steel making still relied on science-based general rules, experience plus trial and error
steel ||This period started around 1980, when computing power became large enough
for calculating some properties of newly conceived alloys
and using the results for optimizing a steel before first
samples were made and tested. The impact of this period is becoming noticeable since about
let's look at the three
major alloy groups of the backbone again. The emphasize now is on the concentration of
major alloying elements,
not counting the carbon and the ubiquitous manganese, silicon, aluminum etc., needed
for the usual reasons (taking care of sulfur, "killing" the steel, ..). This allows a general and often used classification:
| || || |
|Classification according to alloying
element concentration |
|Name ||Description / Properties |
"Micro" Alloy steels |
|Always with a very
low carbon content. That ensures formability and weldability. Strength comes from
small concentrations (usually less than 0.10 %) of carbide-forming
alloying elements like niobium (Nb). titanium (Ti) or vanadium (V) with a total alloy element
concentration that is less than 0.15 %. |
I put the "micro" in quotation
marks because it doesn't mean 106 but just "little".
steels of this group go under the generic name: High-strength low-alloy steel (HSLA).
|Low Alloy Steels ||The "normal" everyday
steels, covered to some extent in the backbone.
We only add less than about 3.5 % of the major alloying element(s) and keep the
carbon concentration mostly (but not always) in the hypoeutectoid range.
High Alloy Steels ||We add a lot more than 3.5 %, possibly as much as 20
weight %, of alloying elements like nickel (Ni) and chromium (Cr). The (low) carbon
concentration then may not be important. |
|| || |
||If we only look at ("low alloy") carbon steels
, the sub-groups to distinguish are: |
| || || |
|| || |
according to carbon concentration |
||Description / Properties |
Mild and low carbon steel || 0.05 % - 0.25% carbon content |
Simple, easy-to-work with everyday
steels. "Mild steel" at the upper end of the concentration
range is the most common form of steel. It is rather cheap and good enough for many applications.
carbon steel || 0.25 % - 0.6% carbon content. |
The typical "tempered steels"
of the 19th century. The best steels up to beginning of the 20th century are found
in this group.
Good hardenability; extensively used for machinery parts that need to be
mass produced and "strong", as well as for 19th century swords.
|High carbon steel
|| 0.6 % 1 % carbon content. |
Hard and brittle steels, difficult to work with,
decreased machinability, poor formability and poor weldability. Good for springs or when abrasion
is of concern, e.g. plough shares, scythes, wrenches, hammers, mauls, pliers, screw drivers
and cutting tools. They also make high-strength wires for your piano or wire saw.
|Ultra-high carbon steel
||1 % - 2 % carbon content. |
These steels can be tempered to great hardness. They
are used for special purposes like axles or punches.
Wootz steel also falls into this category.
| || ||
||Most of the Hyperscript up to chapter 8 was about plain carbon
steel, including various hardening mechanisms. Knowing
about things like phase diagrams, ferrite, austenite, pearlite, martensite, driving forces,
transformation temperatures and so on, allowed to produce amazingly good "plain carbon"
steel in modern times. |
Not knowing about all of that nevertheless allowed some artists
to produce amazingly good and beautiful steel swords in ancient times. So why would we want
to alloy steel with other elements if carbon already does
| || ||Because
hardness is not everything. And
because we want to be in control. In more ancient times you, the black smith, didn't like
it all that much that you weren't in control but rather some typically irresponsible and finicky
God. You had to make expensive sacrifices to keep your iron / steel in good order. You might
have hated doing that (the oxen, lambs, goats, chicken and so on also hated this) because
it often didn't work, but it was your only option. |
Your boss wasn't in control either.
He was dependent on you, his master smith, for forging that special sword. He also needed
you to pass on your craft to apprentices in good time since he himself didn't have the faintest
idea about how it was done, and it wasn't written down somewhere either. His only degree of
freedom concerning sacrifices to proper Gods was that he could sacrifice you if he wasn't
too happy with your work.
Being in control means knowing why and how - that's what we call science.
It works completely without Gods and sacrifices, and it works much better, too. Making it
better or cheaper comes after that and is called engineering.
Since we are human, there is a lot of overlap and confusion between the two. That makes life
richer and more fun.
| || ||
Let's look at some reasons for making "alloy steel",
as we call all steels with intentionally added elements not being carbon (and here I include
manganese, silicon aluminum, ...). More reasons in this
It goes without saying that
alloying does not just mean to pitch some alloy elements in your heat of steel but to follow
precise and complex processing routines; cf. this
module for an example.
- Neutralize unwanted but
unavoidable impurity elements that are impossible or too expensive to keep out.
Manganese (Mn), counteracting the detrimental effects of sulfur (S), is the typical example.
If you like, you can also file the "killing" of modern steel (i.e. the removal of the oxygen used to purify the
pig iron) with silicon (Si), aluminum (Al) or calcium (Ca) under this heading.
- Improved strength not based on carbon. Cementite,
pearlite, and martensite, the stuff you get with carbon steels, do increase strength or hardness
but are not so good for "workability", i.e. properties like ductility or weldability,
not to mention corrosion. The key words in this context are: solution hardening , precipitation hardening, and grain size hardening. The first two mechanisms are clear in principle, the third
one relies on alloy elements to keep grain boundaries from moving, disallowing grain growth.
Two kinds of (always small) precipitates are used: hard metal carbides or nitrides like niobium
carbide, and hard intermetallic compounds like nickel-titanium Ni3Ti.
- Improving hardenability by quenching. The key is
to enable martensite formation
even at relatively low cooling rates. It then can occur in the interior of massive steel pieces
and is not limited to a thin surface-near layer..
- Improved corrosion resistance. The key word is "stainless steel", resulting from rather large additions of Chromium
(Cr). But between "no corrosion at all" and "rusting like crazy"
is much room for improvement.
- Stabilizing austenite
at low temperatures. Reduce the transition temperature and you may get (nonmagnetic) austenitic steel (with an fcc lattice) at room temperature and below. That's
a rather different material from bcc steel.
- Lower costs. It's rewarding if you you can make some new
cheap steel by alloying cheap stuff that has about the same properties as some presently used
expensive steel containing expensive alloying elements like nickel. Boron (B) is he champion in this field.
| ||The following links give
plenty of information about alloying elements, what you use them for, and the science behind
it. But beware! |
| || ||Some
of the desirable features from above can be achieved by adding a suitable amount of the right elements. But 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. In addition, improving
one property by adding a certain element might easily produce
problems with some other properties. You will have to compromise.
Never forget: Very different properties can result from alloying just one
element in a given concentration by different kinds of processing. We have seen that by
alloying iron with only carbon.
In other words: processing
matters very muchfor plain carbon steels and for alloy steels!
| || |
the linked modules I will give very short essentials of some interesting steels. It will not be an exhaustive overview . If you want that, you must consider
buying a good-sized library. |
| || ||Some
steels have already been treated elsewhere in the Hyperscript. Here are the links: |
| || || |
© H. Föll (Iron, Steel and Swords script)