RESTORATION OF STEEL
The result of the high heat to which the steel has been subjected is that
it is weakened and of a different character than before welding. The
operator may avoid this as much as possible by first playing the outer
flame of the torch all over the surfaces of the work just completed until
these faces are all of uniform color, after which the metal should be
well covered with asbestos and allowed to cool without being disturbed.
If a temporary heating oven has been employed, the work and oven should
be allowed to cool together while protected with the sheet asbestos. If
the outside air strikes the freshly welded work, even for a moment, the
result will be breakage.
A weld in steel will always leave the metal with a coarse grain and with
all the characteristics of rather low grade cast steel. As previously
mentioned in another chapter, the larger the grain size in steel the
weaker the metal will be, and it is the purpose of the good workman to
avoid, as far as possible, this weakening.
The structure of the metal in one piece of steel will differ according to
the heat that it has under gone. The parts of the work that have been at
the melting point will, therefore, have the largest grain size and the
least strength. Those parts that have not suffered any great rise in
temperature will be practically unaffected, and all the parts between
these two extremes will be weaker or stronger according to their distance
from the weld itself. To restore the steel so that it will have the best
grain size, the operator may resort to either of two methods: (1) The
grain may be improved by forging. That means that the metal added to the
weld and the surfaces that have been at the welding heat are hammered
much as a blacksmith would hammer his finished work to give it greater
strength.
The hammering should continue from the time the metal first
starts to cool until it has reached the temperature at which the grain
size is best for strength. This temperature will vary somewhat with the
composition of the metal being handled, but in a general way, it may be
stated that the hammering should continue without intermission from the
time the flame is removed from the weld until the steel just begins to
show attraction for a magnet presented to it. This temperature of
magnetic attraction will always be low enough and the hammering should be
immediately discontinued at this point. (2) A method that is more
satisfactory, although harder to apply, is that of reheating the steel to
a certain temperature throughout its whole mass where the heat has had
any effect, and then allowing slow and even cooling from this temperature.
The grain size is affected by the temperature at which the reheating is
stopped, and not by the cooling, yet the cooling should be slow enough to
avoid strains caused by uneven contraction.
After the weld has been completed the steel must be allowed to cool until
below 1200° Fahrenheit. The next step is to heat the work slowly until
all those parts to be restored have reached a temperature at which the
magnet just ceases to be attracted. While the very best temperature will
vary according to the nature and hardness of the steel being handled, it
will be safe to carry the heating to the point indicated by the magnet in
the absence of suitable means of measuring accurately these high
temperatures. In using a magnet for testing, it will be most satisfactory
if it is an electromagnet and not of the permanent type. The electric
current may be secured from any small battery and will be the means of
making sure of the test. The permanent magnet will quickly lose its power
of attraction under the combined action of the heat and the jarring to
which it will be subjected.
In reheating the work it is necessary to make sure that no part reaches a
temperature above that desired for best grain size and also to see that
all parts are brought to this temperature. Here enters the greatest
difficulty in restoring the metal. The heating may be done so slowly that
no part of the work on the outside reaches too high a temperature and
then keeps the outside at this heat until the entire mass is at the same
temperature. A less desirable way is to heat the outside higher than this
temperature and allow the conductivity of the metal to distribute the
excess to the inside.
MECHANICAL PROPERTIES
Strength, hardness, toughness, elasticity, plasticity,
brittleness, and ductility and malleability are
mechanical properties used as measurements of how
metals behave under a load. These properties are
described in terms of the types of force or stress that
the metal must withstand and how these are resisted.
Common types of stress are compression, tension,
shear, torsion, impact, 1-2 or a combination of these
stresses, such as fatigue. (See fig. 1-1.)
Compression stresses develop within a material
when forces compress or crush the material. A column
that supports an overhead beam is in compression, and
the internal stresses that develop within the column are
compression.
Tension (or tensile) stresses develop when a
material is subject to a pulling load; for example, when
using a wire rope to lift a load or when using it as a
guy to anchor an antenna. "Tensile strength" is defined
as resistance to longitudinal stress or pull and can be
measured in pounds per square inch of cross section.
Shearing stresses occur within a material when
external forces are applied along parallel lines in
opposite directions. Shearing forces can separate
material by sliding part of it in one direction and the
rest in the opposite direction.
Some materials are equally strong in compression,
tension, and shear. However, many materials show
marked differences; for example, cured concrete has a
maximum strength of 2,000 psi in compression, but
only 400 psi in tension. Carbon steel has a maximum
strength of 56,000 psi in tension and compression but
a maximum shear strength of only 42,000 psi;
therefore, when dealing with maximum strength, you
should always state the type of loading.
A material that is stressed repeatedly usually fails
at a point considerably below its maximum strength in
tension, compression, or shear. For example, a thin
steel rod can be broken by hand by bending it back and
forth several times in the same place; however, if the
same force is applied in a steady motion (not bent back
and forth), the rod cannot be broken. The tendency of
a material to fail after repeated bending at the same
point is known as fatigue.
Strength Rockwell "C" number. On nonferrous metals, that are
Strength is the property that enables a metal to resist
deformation under load. The ultimate strength is the
maximum strain a material can withstand. Tensile
strength is a measurement of the resistance to being
pulled apart when placed in a tension load.
Fatigue strength is the ability of material to resist
various kinds of rapidly changing stresses and is expressed
by the magnitude of alternating stress for a
specified number of cycles.
Impact strength is the ability of a metal to resist
suddenly applied loads and is measured in foot-pounds
of force.
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Warmly,
Pat Mitchell
http://www.weldingsecrets.net/main.html
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