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Ferrous Metals
     Metals containing primarily iron are classified as "ferrous metals".
They range from pure iron through exotic high alloy steels. Many
stock parts are made from cast iron, defined as metallic iron in which
more than 2 percent carbon is dissolved. One preferred variation-
ductile or nodular iron- has all its carbon contained in the form of
tiny spherical graphite nodules uniformly dispersed throughout the
metal's matrix. This makes the material more ductile ( deformable
rather then brittle) and eases casting and machining.   

   Even the best iron has only limited tensile strength. Increasing
ductility, hardness, malleability and fatigue resistance requires
removing most carbon and at the end, alloying iron with other
elements, creating "steel", an iron with less than 2 percent carbon.

  The most basic form is carbon steel, which contains up to 1.7
percent carbon and minimal additional alloying elements. Carbon
steels are designated by a four digit number. The first two digits
indicate the basic type and the last two digits indicate the
approximate midpoint of the carbon content. Stock forged rods and
cranks are usually made from 1045 or 1053 steel. The "10" ID's these
alloys as nonresulfurized carbon steel with some manganese
(popularly called medium carbon or mild steel); "45" or "53" means
the steel contains about 0.45 or .053 percent carbon, respectively.    

Alloy steels are where things really get crazy. Over time, as
manufacturing techniques improve and chemical knowledge grew,
metallurgists developed whole families of alloy steel custom tailored
to make metals stronger, lighter, more durable, more ductile, springer
and /or harder. Alloy steels are also identified by a four digit number,
but now the first two digits indicate the major alloying element or
elements, with the last two digits again indicating the approximate
midpoint of the carbon range. The alloy steel series and the major
alloying elements most often used to make forged or billet auto
engine parts are:                                                                               
41xx
Chromium 0.50, 0.80 or 0.95 percent; plus
molybdenum 0.12, 0.20 or 0.30 percent
43xx
Nickel 1.83 percent, plus chromium 0.50 or 0.80
percent, plus molybdenum 0.53 percent
Chromium 0.80, 0.88, 0.95 or 1.00 percent
51xx
Within these major categories, these are the most common alloy
steels used for auto parts:
The best known chrome moly steel, it's a great
high strength / high stress alloy in thin sections
(sheet or tube). But 4130 possesses poor deep
heat treating characteristics and doesn't like
varying cross sections, which make it a bad
choice for machined or forged parts.
4130
A deep hardening chrome moly steel, it forges
well and has good impact resistance, fatigue
strength and general all around toughness.
4140
A nickel chrome moly deep hardening steel, this
alloy is used to make premium rods and cranks.
4340 has fantastic tensile strength, toughness
and fatigue resistance. Modified 4340 alloys
with vanadium and more silicon cant his already
great alloy even tougher and more fatigue
resistant.
4340
This chromium alloy increases tensile strength,
hardness, toughness and wear resistance over
carbon steel but is more affordable than the
4000 series alloys.
5140