Properties of Steel
Steel which has only a low percentage of carbon has the same properties
as wrought iron. While it melts only at a very high temperature,
it is soft enough to be welded, and is ductile and malleable. As
the carbon is increased these qualities are all lessened, and high-carbon
steel would be very brittle if it were not tempered. This process
does not make it less hard, but renders it extremely tough and strong.
Annealing is the process of softening metals and increasing their
flexibility and ductility by heating them very hot and cooling them
Hardening steel is accomplished by heating it to a red heat and
cooling it suddenly. This process not only makes it harder but also
more brittle and less elastic.
Tempering is a process applied only to hardened steel by which
a part of its brittleness is drawn out and it is rendered tougher.
This is done by reheating the steel and cooling it gradually but
not so slowly as for annealing.
Steel is hardened by bringing it to a red heat and plunging it
into water or brine. It may then be tempered by putting it into
a sand bath or in oil. The sand is heated by a fire beneath it and
the steel is placed on top of the sand until it reaches the desired
temperature, which is indicated by its color. The higher the temperature
used, the softer will be the steel which results. The following
table shows the varying degrees of temperature, color, condition,
Tempering in oil can be gauged by the thermometer, as the steel
cannot become hotter than the oil. It is therefore a more accurate
method. To harden the edge or point of a tool its cutting edge may
be plunged at a bright red heat into water, and tempered by allowing
the heat which remains at the other end to reheat it up to the desired
temperature and then quenching it again.