STRESS AND STRAIN
IRON AND STEEL
When external force is applied to any solid
CARBON STEEL is an alloy of iron and control-
material, the material is subjected to stress. Many of
led amounts of carbon. ALLOY STEEL is a combina-
the properties of metals can best be understood in
tion of carbon steel and controlled amounts of other
terms of the manner in which they react to stress.
desirable metal elements.
Therefore, before considering the properties of metals
The percentage of carbon content determines the
and alloys, let us examine the concepts of stress and
type of carbon steel. For example, wrought iron has
0.003 percent carbon, meaning three thousandths of
one percent. Low-carbon steel contains less than 0.30
Load, which is usually measured in pounds, is the
percent carbon. Medium-carbon steel varies between
external force applied to a material. When the load is
applied, reaction forces to the load occur throughout
0.30 and 0.55 percent carbon content. High-carbon
the material. The reaction forces are stresses. Why do
steel contains approximately 0.55 to 0.80 percent
these forces occur when a load is applied to a material?
carbon, and very-high-carbon steel contains between
Newton's third law of motion states that "to every
0.80 and 1.70 percent carbon. Cast iron contains 1.8
force or action, there is an equal and opposite
to 4 percent carbon.
reaction." Stress, therefore, is the "equal and
Carbon generally combines with the iron to form
opposite" reaction to the externally applied load. It is
CEMENTITE, a very hard, brittle substance.
defined as the force per unit area resisting the load.
Cementite is also known as IRON CARBIDE. This
Unit area is important. The unit area may be stated as
action means that as the carbon content of the steel
a square inch, a square foot, or any other pre-
increases, the hardness, the strength, and the
determined amount of area that is used to figure the
brittleness of the steel also tend to increase.
amount of stress that the material will be subjected to.
When the load is applied, it is distributed equally
Various heat treatments are used to enable steel to
throughout the cross section of the material. For
retain its strength at the higher carbon contents, and
example, suppose two round metal rods with
yet not have the extreme brittleness usually associated
cross-sectional areas of 1 square inch and 2 square
with high carbon steels. Also, certain other substances,
inches are each supporting a 2000-pound weight. The
load or external force is the same on both, but since
other alloying metals, may be added to steel to
the cross-sectional areas are different and the load is
improve certain physical properties.
distributed equally over the cross-sectional areas, the
A welder must also have an understanding of the
stresses in the two rods are also different.
impurities occasionally found in metals and their ef-
You can see from the example that the stress is
fect upon the weldability of the metal. Two of the
equal to the load divided by the cross-sectional area.
detrimental impurities sometimes found in steels are
That is, equal portions of the load are distributed
phosphorus and sulphur. Their presence in steel is due
equally over the cross-sectional area. Stress is usually
to their presence in the ore, or to the method of manu-
measured in pounds (for load) per square inch (for
facture. Both of these impurities are detrimental to the
area). Conversely, the load can be determined by
welding qualities of steel. Therefore, during the manu-
multiplying the stress by the cross-sectional area.
facturing process, extreme care must always be taken
to keep the impurities at a minimum (0.05 percent or
Strain is the deformation or change in shape
less). Sulphur improves the machining qualities of
caused by the load. Some strain always occurs as a
steel, but it is detrimental to its hot forming properties.
reaction to a load. The amount of strain depends on
the magnitude and duration of the stress caused by the
During a welding operation, sulphur or
load. It also depends on the type and condition of the
phosphorus tends to form gas in the molten metal. The
material. Strain is measured in inches per inch or in
percentage. Thus, when a load is applied to a bar in
Another impurity is dirt or slag (iron oxide). The dirt
tension, the bar will elongate (be strained) some frac-
or slag is imbedded in the metal during rolling. Some
tion of an inch for each inch of bar (the strain will be
of the dirt may come from the by-products of the
the same in each inch of bar). If strain is being meas-
process of refining the metal. These impurities may
ured in percentage, the bar will be elongated a certain
also produce blow holes in the weld and reduce the
percentage; that is, the total length of the bar will be
physical properties of the metal in general.