temperature, postheat, and welding sequence.
Even when the weld joins identical metals, or
Otherwise, a satisfactory weld is likely to crack and
metals having approximately the same coefficient of
to have reduced toughness and less strength than is
expansion, the expansion and contraction may not
required. For this reason, tool steels and certain
be uniform throughout all parts of the metal. These
differences lead to internal stresses, distortion, and
alloys like carbon-molybdenum steel are less
warping. Metal parts must be free to move or a
weldable than many other steels.
special weld sequence must be used. When heat is
applied or withdrawn, expansion and contraction set
up high stresses, which in turn may cause trouble in
phosphorus, hydrogen, nitrogen, and oxygen. If
present in large enough quantities, these impurities
the weld itself or in the adjacent base metal. In thin
may decrease weldability. For example, a steel to
materials, uneven expansion and contraction may
cause the metal to warp. In heavy material, the
which about 0.10 percent sulfur has been added to
stresses set up may exceed the ultimate strength of
improve machinability is difficult to weld because
the metal and cause cracking to occur in the weld or
the weld has a tendency to crack. An excessive
in the metal next to the weld, which is called the
amount of phosphorus decreases the ductility of the
heat affected zone.
steel and thus decreases the weldability of the metal.
The presence of hydrogen in a steel, filler material,
Even if the ultimate strength of the material is
or flux may lead to cracks in the welds.
not exceeded by the stresses developed during
Stainless steels, high-chromium steels, and other
welding, the combination of welding stresses PLUS
special steels are less weldable than the plain
the stresses developed when the material is placed
in service may be enough to cause failure of the
low-carbon steels. The elements that give these
special steels their desirable properties for some
weld. It is for this reason that many materials are
applications also have the effect of decreasing the
STRESS RELIEVED after welding.
weldability of the metals. To make these special
steels weldable, the welding procedures, the filler
Another factor that influences weldability is the
metal, the fluxes, the preheat and postheat
filler material used. The wrong electrode or
temperatures, and the welding sequence must be
incorrect welding process will make welding difficult
carefully selected. This is also true for many nickel,
or impossible, and it may lead to failure of the part
copper, and aluminum alloys.
under service conditions. It is not always essential
that the welding rod or electrode be of the same
In some metals, the heat of the welding process
important requirement is that the combination of
may cause certain elements with low-melting points
the filler metal and the base metal will make a
to vaporize, thus reducing the amounts of those
elements present in the weld zone. Nonferrous
satisfactory welded joint.
alloys containing lead, zinc, and tin are particularly
subject to such losses from vaporization. These
In some processes, the flux selected for use with
losses may seriously affect the properties of the joint
a welding rod has important effects on weldability.
by causing porosity or oxide inclusions that weaken
Also, the electrode covering influences the weld
obtained in certain steels. Molten steels have a
tendency to absorb hydrogen from the surrounding
The weldability of a metal is also affected by its
atmosphere and to expel it when they solidify.
Some types of electrode coverings send a lot of
hydrogen into the atmosphere surrounding the arc
they transfer the heat away from the weld so rapidly
and the molten puddle. This hydrogen is frequently
that the required temperature cannot be maintained
enough to cause microscopic cracks in the
at the joint.
heat-affected zone of some steels. To eliminate this
problem, low-hydrogen electrodes have been
Changes in temperature cause a metal to expand
developed to weld the newer high-tensile steels.
or contract and that also affects weldability. When
metals expand and contract at different rates, the
Joint design also influences the weldability of a
internal stresses set up by these changes can cause
metal. You need to consider several factors when
the joint to crack immediately or to crack later
you select a joint design. They include the welding