TRUCK SERVICE MANUAL
STEEL AND ITS HEAT TREATMENT
The heat treatment of steel consists of annealing,
hardening and tempering.
Annealing consists of heating above the "critical range",
then cooling slowly, for the purpose of refining the grain,
softening the steel to machinability and relieving the internal
strains set up in the steel by forging and hammering, these
strains sometimes amounting to several thousand pounds per
Hardening consists of heating above the critical range
and cooling quickly, as by quenching in oil or water, the
degree of hardness depending upon the carbon content of the
steel and the severity of the quench.
Water quenching is more severe than oil quenching
and is frequently followed by tempering or drawing to reduce
the brittleness imparted by the severity of the quench.
Casehardening or Carburizing
Carburizing, carbonizing or casehardening are names
applied to the process wherein a piece of low-carbon steel is
packed in a carbonaceous material such as bone or leather,
or a commercial carburizing material and heated for a number
of hours, just above the "critical range" of the steel, or above
its point of decalescence, thereby causing the low-carbon
steel to absorb carbon on the outer surface for a depth directly
dependent upon the number of hours it is heated. Under such
conditions, a carbonized case is produced which is capable of
responding to ordinary hardening or tempering operations.
The Brinell test is commonly made with a hydraulic
testing machine in which a steel ball of ten millimeter
diameter is pressed into the test piece by a load of three
thousand kilograms. The diameter of the impression the ball
produces in the test piece is then measured and checked
against a standard. Thus an impression four millimeters in
diameter indicates softer steel than a diameter of three and
one half millimeters.
The Brinell test is definitely related to the ultimate
strength of the material.
The Shore Scleroscope test is made with a small
approximately ten inches through a small glass tube upon a
smooth surface of the steel to be tested, and the height of the
rebound of the hammer measured against a scale at the back
of the glass tube. Hard steel is taken as being 100 hard on
the Scleroscope and soft steel approximately 30 to 35 hard.
A test bar of the standard S.A.E. form is machined
from the material to be tested, and is held in threaded grips in
a vertical position in the testing machine. The machine is set
in motion and the test bar is slowly stretched until it is broken.
The point at which the elongation ceases to be proportional to
the load is designated as the elastic limit. This is the highest
point at which, if the load were removed, the bar would
resume its original length. This is also the point at which, if
exceeded, failure of the part commences. The weight of the
load at this point is read on the weighing beam of the testing
machine and converted into pounds per square inch, to be
checked against S.A. E. specifications for that particular steel
from which the test bar was made.
CTS-2128-L Page 6
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