The primary ingredient in steel is iron, one of the most common elements found in the earth.  It is usually found  in the form of iron oxide (iron ore) and then smelted down into cast iron (pig iron).  The cast iron is then further refined and other elements are added to bring about the desired qualities in the finished alloy.
The most common element that has an effect on hardness in an alloy is carbon, which not only increases the hardness but can also make the steel quite brittle.  Thus, using a steel with a high carbon content will result in a very hard (and sometimes brittle) blade, while using a steel with a low carbon content will
result in a tough  blade that will not hold an edge.  (Of course there are ways around most problems, but that's another story)
The element content of an alloy is expressed as a POINT , with each point
signifying 0.01 percent of the alloy.  As an example, a 60-point carbon alloy
will contain approximately 0.60% carbon.
CARBON (C):  Has by far the greatest influence of any of the elements.  Steel
could not exist without carbon.  Martensite, along with banite give steel a
microstructure of hard, tough carbide.  None of the other elements so dramatically
alter the strength and hardness as do small changes in carbon content.  Carbon
iron crystalline structures have the widest number and variety known to exist in
metallurgy.  They also combine with other elements to furnish steel with an
assortment of iron alloy carbide systems.

MANGANESE (Mn):  Is normally present in all steel and functions as a
deoxidizer.  It also imparts strength and  responsiveness to heat treatment.  It is
usually present in quantities of 0.5 to 2.0 percent.


NICKEL (Ni):   Increases strength and toughness but is ineffective in
increasing hardness.  It is generally added in amounts ranging from 1 percent to 4
percent.  In some stainless steels it is sometimes as high as 20 percent.


SILICON (Si):  Has a beneficial effect upon tensile strength and improves
hardenability of an alloy.  It has a toughening effect when used in combination
with certain other elements.  Silicon (Si) is usually added to improve electrical
conductivity of an alloy.  Its average concentration is between 1.5 and 2.5
percent.


CHROMIUM (Cr):  Increases the depth penetration of hardening and also the
responsiveness to heat treatment.  It is usually added with nickel (Ni) for use in
stainless steels.  Most of the chromium (Cr) bearing alloys contain 0.50 to 1.50
percent chromium;  some stainless steels contain as much as 20 percent or
more.  It can affect forging, causing a tendency in the steel to crack.


VANADIUM (V):  Retards grain growth within steel even after long exposures
at high temperatures, and helps to control grain structures while heat treating.  It
is usually present in small quantities of 0.15 to 0.20 percent.  Most tool steels
which contain this element seem to absorb shock better that those that do not
contain vanadium (V).


MOLYBDENUM (Mo):  Adds greatly to the penetration of hardness and
increases toughness of an alloy.  It causes steel to resist softening at high
temperatures, which defeats the purpose of forging.  If the alloy has below 0.020
percent molybdenum (Mo), you should be able to forge this alloy with little
difficulty.


TUNGSTEN (W): Also known as wolfram, is used as an alloying element in
tool steels, as it tends to impart a tight, small, and dense grain pattern and keen
cutting edges when used in relatively small amounts.  It will also cause steel to
retain its hardness at higher temperatures and hence will have a detrimental
effect upon the steel's forgeability (otherwise known as "red hard")


SULFUR (S):  Is usually regarded as an impurity in most alloys and its addition
to steel is held to a minimum as it is damaging to the hot forming characteristics
of steel.  It is, however added to increase machinability.  A word of caution, some
alloys are offered in different forms, an example is E52100.  This particular steel
can be had in either a "Bearing Quality" or "Machining Quality"  the latter having
sulfur (S) added to increase machinability.


LEAD (Pb):  Increase the machinability of steel and has no affect upon the
other properties of the metal.  It is usually added to an alloy only upon request
and then in quantities of 0.15 to 0.30 percent.


PHOSPHORUS (P):  Is present in all steel.  It increases yield strength and
reduces ductility at low temperatures.  It is also believed to increase resistance to
atmospheric corrosion.  Phosphorus (P) is usually treated as an impurity in most
alloys.
Some common blade steels....and their GENERAL element content.
I strongly DISCOURAGE the use of scrap or recycled materials for the Bladesmith/Knifemaker.
(more about scrape and recycled steel here)
52100:  Carbon  1.00%    Manganese  0.35%    Silicon  0.25%    Chromium  1.50%

5160: Carbon .60%  Manganese .85%  Chromium .80%  Phosphorus .035% max  Sulphur .040% max

1084: Carbon .80%/.94%  Manganese .60%/.90%

1095: Carbon .90%/1.04%  Manganese .60%/.90%

15N20: Carbon .75%  Manganese .75%  Silicon .25%  Nickel 1.5%

O-1: Carbon 1.00%  Manganese .60%  Silicon .30%  Chromium .50%  Vanadium .30%  Molybdenum 1.10

L-6: Carbon .75%  Manganese .70%  Silicon .25%  Chromium .80%  Nickel 1.5%  Molybdenum .30%

A-2: Carbon  1.00%    Manganese  0.85%    Silicon  0.35%    Chromium  5.25%    Molybdenum   1.10%    Vanadium  0.25%

D-2: Carbon  1.55%    Manganese  0.35%    Silicon  0.45%    Chromium  11.50%    Molybdenum  0.90%    Vanadium  0.80%
Knowing the alloying elements and how they effect steels alone, as well as in combinations, will allow the perceptive bladesmith to create a higher quality product, and give the end consumer a better value.
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Copyright 2017:  "The Montana Bladesmith"




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