WELDING THEORY - OD1650 - LESSON 1/TASK 1
When the percentage of carbon is less than 0.25 percent, its effect in producing hardness is
slight. But when the carbon content exceeds 0.25 percent, or when such elements as manganese,
vanadium, chromium, molybdenum, or titanium are present, together with a carbon percentage of
less than 0.25 percent, the weldability of the steel is decreased. Special steps should be taken to
control preheat, interpass temperature, postheat, and welding sequence. Otherwise a satisfactory
weld is likely to crack and to have reduced toughness and less strength than is required. For this
reason, tool steels and certain alloys like carbon-molybdenum steel are less weldable than many
other steels.
Steels contain certain impurities such as sulfur, phosphorus, hydrogen, nitrogen, and oxygen. If
present in large enough quantities, these impurities may decrease weldability. For example, a
steel to which about 0.10 percent sulfur has been added to improve machineability is difficult to
weld because the weld has a tendency to crack. An excessive amount of phosphorus decreases
the ductility of the steel and thus decreases the weldability of the metal. The presence of
Stainless steels, high-chromium steels, and other special steels are less weldable than plain low-
carbon steels. The elements that give these special steels their desirable properties for specific
applications also have the effect of decreasing the weldability of the metals. To make these
special steels weldable, the welding procedures, the filler metal, the fluxes, the preheat and
postheat temperatures, and the welding sequence must be carefully selected. This is also true for
many nickel, copper, and aluminum alloys.
In some metals, the heat of the welding process may cause certain elements with low-melting
points to vaporize, thus reducing the amounts of those elements present in the weld zone.
Nonferrous alloys containing lead, zinc, and tin are particularly subject to such looses from
vaporization. These losses may seriously affect the properties of the joint by causing porosity or
oxide inclusions that weaken the weld.
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