What is commonly known as carbon steel is a metal composed of iron
containing varying amounts of carbon. Steels containing much carbon are
called tool steels to distinguish them from the low carbon steels. Tool
steel, when heated red hot and plunged into cold water, will harden,
while low carbon steel treated in the same way will not do so. This is
an excellent way of testing two bars of steel for carbon when one is
not able to distinguish grades of steel accurately. It is carbon that
gives the hardening quality. When steel is heated it becomes red at
1000° F. At 1300° F. it passes a point at which it absorbs considerable
heat without any increase in the temperature, showing that some change
in the structure of the metal must be taking place. If the steel is
heated above this point and allowed to cool slowly, a brightening of
the colour may be noticed as it passes this point, known as the point
of recalescence. The brightening is due to a liberation of the heat
previously absorbed.
_Method of heating steel in forge fires:_ All steel work, including
tool dressing, hardening, and tempering, was formerly done in an
ordinary forge fire. Now we have special furnaces for that purpose.
However, in using a forge fire, care must be taken to insure good work.
The fire must be very deep--that is, a large body of coke must be put
between the tuyere, and the tool, so as to prevent the blast reducing
the carbon on the surface of the steel. Sulphur will injure the quality
of any steel tool. Hence, a fuel low in sulphur should be used.
Charcoal is the best for this purpose, but the cost and the difficulty
in maintaining the heat prevent its general use in blacksmith shops. If
the coal does contain sulphur a great deal of it can be extracted or
reduced by one making his own coke. This is done by burning the green
coal to a coke and in this way driving off much of the sulphur. Gas
furnaces or oil furnaces are used. This is much better than coal, for
a uniform heat can be kept and an oxidation of the steel prevented.
Whether natural or artificial gas is used, all that is necessary is
to adjust the supply of gas in such a way that there will be a very
slight excess of gas present beyond the proper amount for combustion.
The presence of this gas excludes all air from the steel and therefore
prevents its oxidizing the surface of the metal.
_Heating in lead:_ In order to prevent oxidation molten lead makes
a most satisfactory bath. The lead is melted in a cast-iron pot and
heated in the forge. The steel must be left in the bath until it has
all been heated to the required temperature. As the steel will float in
the molten lead it must be weighted down to keep it submerged.
_Hardening solutions:_ In many cases clear cold water is used in
hardening steel. Some use soft or rain water. The temperature of the
water for general work should not fall below the temperature of the
shop; otherwise it would extract the heat too quickly from the steel
and cause cracks or breaks in the work.
_Salt solution:_ Salt is often added to water which is to be used as a
hardening solution. (1) It increases the rate at which the bath will
extract the heat from the steel; (2) it prevents the formation of steam
on the surface of the water. Put as much salt in rain water as it will
dissolve. This is considered one of the best and easiest made solutions
for hardening steel.
_Oil solution:_ Linseed oil, lard, cotton seed oil, whale oil, and
melted tallow make good hardening solutions. They are used mostly
for fine work. The oil prevents the sudden chilling of the steel and
lessens the chances of cracking and breaking. Springs are mostly
tempered in oil.
_Metallic hardening baths:_ For very delicate tools mercury is
sometimes used. It has a greater heat conductivity than any solution
mentioned. However, the fumes given off are poisonous and for this
reason it is not extensively used.