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Welding
Beginning welders and even those that are more experienced
commonly struggle with the problem of weld distortion, (warping
of the base plate caused by heat from the welding arc). Distortion
is troublesome for a number of reasons, but one of the most
critical is the potential creation of a weld that is not structurally
sound. This article will help to dene what weld distortion is and
then provide a practical understanding of the causes of distortion,
eects of shrin!age in various types of welded assemblies and
how to control it, and nally loo! at methods for distortion control.What is Weld Distortion?
Distortion in a weld results from the expansion and contraction of
the weld metal and ad"acent base metal during the heating and
cooling cycle of the welding process. Doing all welding on one
side of a part will cause much more distortion than if the welds
are alternated from one side to the other. During this heating and
cooling cycle, many factors aect shrin!age of the metal and lead
to distortion, such as physical and mechanical properties that
change as heat is applied. #or example, as the temperature of the
weld area increases, yield strength, elasticity, and thermal
conductivity of the steel plate decrease, while thermal expansion
and specic heat increase (#ig. $%&). These changes, in turn,
aect heat 'ow and uniformity of heat distribution.
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#ig. $%& hanges in
the properties of
steel with increases
in temperature
complicate analysisof what happens
during the welding
cycle % and, thus,
understanding of the
factors contributing
to weldment
distortion.
Reasons for Distortion
To understand how and why distortion occurs during heating and
cooling of a metal, consider the bar of steel shown in #ig. $%. *s
the bar is uniformly heated, it expands in all directions, as shown
in #ig. $%(a). *s the metal cools to room temperature it contracts
uniformly to its original dimensions.
#ig. $% +f a steel bar
is uniformly heated
while unrestrained,
as in (a), it will
expand in all
directions and return
to its original
dimentions on
cooling. +f
restrained, as in (b),
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during heating, it
can expand only in
the vertical direction
% become thic!er. n
cooling, thedeformed bar
contracts uniformly,
as shown in (c), and,
thus, is permanently
deformed. This is a
simplied
explanation of basic
cause of distortion in
welding assemblies.
But if the steel bar is restrained %as in a vise % while it is heated,
as shown in #ig. $%(b), lateral expansion cannot ta!e place. But,
since volume expansion must occur during the heating, the bar
expands in a vertical direction (in thic!ness) and becomes thic!er.
*s the deformed bar returns to room temperature, it will still tend
to contract uniformly in all directions, as in #ig. $% (c). The bar is
now shorter, but thic!er. +t has been permanently deformed, ordistorted. (#or simplication, the s!etches show this distortion
occurring in thic!ness only. But in actuality, length is similarly
aected.)
+n a welded "oint, these same expansion and contraction forces
act on the weld metal and on the base metal. *s the weld metal
solidies and fuses with the base metal, it is in its maximum
expanded from. n cooling, it attempts to contract to the volume
it would normally occupy at the lower temperature, but it is
restrained from doing so by the ad"acent base metal. Because of
this, stresses develop within the weld and the ad"acent base
metal. *t this point, the weld stretches (or yields) and thins out,
thus ad"usting to the volume re-uirements of the lower
temperature. But only those stresses that exceed the yield
strength of the weld metal are relieved by this straining. By the
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time the weld reaches room temperature % assuming complete
restraint of the base metal so that it cannot move % the weld will
contain loc!ed%in tensile stresses approximately e-ual to the yield
strength of the metal. +f the restraints (clamps that hold the
wor!piece, or an opposing shrin!age force) are removed, theresidual stresses are partially relieved as they cause the base
metal to move, thus distorting the weldment.
Shrinkage Control - What You Can Do to Minimize
Distortion
To prevent or minimie weld distortion, methods must be used
both in design and during welding to overcome the eects of the
heating and cooling cycle. /hrin!age cannot be prevented, but it
can be controlled. /everal ways can be used to minimie
distortion caused by shrin!age0
1. Do not overweld
The more metal placed in a "oint, the greater the shrin!age
forces. orrectly siing a weld for the re-uirements of the "oint not
only minimies distortion, but also saves weld metal and time.
The amount of weld metal in a llet weld can be minimied by theuse of a 'at or slightly convex bead, and in a butt "oint by proper
edge preparation and tup. The excess weld metal in a highly
convex bead does not increase the allowable strength in code
wor!, but it does increase shrin!age forces.
1hen welding heavy plate (over & inch thic!) bevelling or even
double bevelling can save a substantial amount of weld metal
which translates into much less distortion automatically.
+n general, if distortion is not a problem, select the mosteconomical "oint. +f distortion is a problem, select either a "oint in
which the weld stresses balance each other or a "oint re-uiring
the least amount of weld metal.
. !se intermittent welding
*nother way to minimie weld metal is to use intermittent rather
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than continuous welds where possible, as in #ig. $%2(c). #or
attaching stieners to plate, for example, intermittent welds can
reduce the weld metal by as much as 23 percent yet provide the
needed strength.
#ig. $%2 Distortion
can be prevented or
minimied by
techni-ues that
defeat % or use
constructively % the
eects of theheating and cooling
cycle.
". !se as few weld #asses as #ossi$le
#ewer passes with large electrodes, #ig. $%2(d), are preferable to a
greater number of passes with small electrodes when transverse
distortion could be a problem. /hrin!age caused by each pass
tends to be cumulative, thereby increasing total shrin!age when
many passes are used.
%. &la'e welds near the neutral a(is
Distortion is minimied by providing a smaller leverage for the
shrin!age forces to pull the plates out of alignment. #igure $%2(e)
illustrates this. Both design of the weldment and welding
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se-uence can be used eectively to control distortion.
#ig. $%2 Distortioncan be prevented or
minimied by
techni-ues that
defeat % or use
constructively % the
eects of the
heating and cooling
cycle.
). *alan'e welds around the neutral a(is
This practice, shown in #ig. $%2(f), osets one shrin!age force with
another to eectively minimie distortion of the weldment. 4ere,
too, design of the assembly and proper se-uence of welding are
important factors.
+. !se $a'kste# welding
+n the bac!step techni-ue, the general progression of welding
may be, say, from left to right, but each bead segment is
deposited from right to left as in #ig. $%2(g). *s each bead
segment is placed, the heated edges expand, which temporarily
separates the plates at B. But as the heat moves out across the
plate to , expansion along outer edges D brings the plates bac!
together. This separation is most pronounced as the rst bead is
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laid. 1ith successive beads, the plates expand less and less
because of the restraint of prior welds. Bac!stepping may not be
eective in all applications, and it cannot be used economically in
automatic welding.
#ig. $%2 Distortion
can be prevented or
minimied by
techni-ues that
defeat % or use
constructively % the
eects of the
heating and cooling
cycle.
,. nti'i#ate the shrinkage for'es
5resetting parts (at rst glance, + thought that this was referring
to overhead or vertical welding positions, which is not the case)
before welding can ma!e shrin!age perform constructive wor!.
/everal assemblies, preset in this manner, are shown in #ig. $%
2(h). The re-uired amount of preset for shrin!age to pull the
plates into alignment can be determined from a few trial welds.
5rebending, presetting or prespringing the parts to be welded, #ig.
$%2(i), is a simple example of the use of opposing mechanical
forces to counteract distortion due to welding. The top of the weld
groove % which will contain the bul! of the weld metal % is
lengthened when the plates are preset. Thus the completed weld
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is slightly longer than it would be if it had been made on the 'at
plate. 1hen the clamps are released after welding, the plates
return to the 'at shape, allowing the weld to relieve its
longitudinal shrin!age stresses by shortening to a straight line.
The two actions coincide, and the welded plates assume thedesired 'atness.
*nother common practice for balancing shrin!age forces is to
position identical weldments bac! to bac!, #ig. $%2("), clamping
them tightly together. The welds are completed on both
assemblies and allowed to cool before the clamps are released.
5rebending can be combined with this method by inserting
wedges at suitable positions between the parts before clamping.
+n heavy weldments, particularly, the rigidity of the members andtheir arrangement relative to each other may provide the
balancing forces needed. +f these natural balancing forces are not
present, it is necessary to use other means to counteract the
shrin!age forces in the weld metal. This can be accomplished by
balancing one shrin!age force against another or by creating an
opposing force through the xturing. The opposing forces may be0
other shrin!age forces6 restraining forces imposed by clamps, "igs,
or xtures6 restraining forces arising from the arrangement of
members in the assembly6 or the force from the sag in a memberdue to gravity.
. &lan the welding se/uen'e
* well%planned welding se-uence involves placing weld metal at
dierent points of the assembly so that, as the structure shrin!s
in one place, it counteracts the shrin!age forces of welds already
made. *n example of this is welding alternately on both sides of
the neutral axis in ma!ing a complete "oint penetration groove
weld in a butt "oint, as in #ig. $%2(!). *nother example, in a lletweld, consists of ma!ing intermittent welds according to the
se-uences shown in #ig. $%2(l). +n these examples, the shrin!age
in weld 7o. & is balanced by the shrin!age in weld 7o. .
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#ig. $%2 Distortioncan be prevented or
minimied by
techni-ues that
defeat % or use
constructively % the
eects of the
heating and cooling
cycle.
lamps, "igs, and xtures that loc! parts into a desired position
and hold them until welding is nished are probably the most
widely used means for controlling distortion in small assemblies
or components. +t was mentioned earlier in this section that the
restraining force provided by clamps increases internal stresses in
the weldment until the yield point of the weld metal is reached.
#or typical welds on low%carbon plate, this stress level would
approximate 83,999 psi. ne might expect this stress to causeconsiderable movement or distortion after the welded part is
removed from the "ig or clamps. This does not occur, however,
since the strain (unit contraction) from this stress is very low
compared to the amount of movement that would occur if no
restraint were used during welding.
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0. Remove shrinkage for'es after welding
5eening is one way to counteract the shrin!age forces of a weld
bead as it cools. :ssentially, peening the bead stretches it and
ma!es it thinner, thus relieving (by plastic deformation) the
stresses induced by contraction as the metal cools. But thismethod must be used with care. #or example, a root bead should
never be peened, because of the danger of either concealing a
crac! or causing one. ;enerally, peening is not permitted on the
nal pass, because of the possibility of covering a crac! and
interfering with inspection, and because of the undesirable wor!%
hardening eect. Thus, the utility of the techni-ue is limited, even
though there have been instances where between%pass peening
proved to be the only solution for a distortion or crac!ing problem.
Before peening is used on a "ob, engineering approval should beobtained.
*nother method for removing shrin!age forces is by thermal
stress relieving % controlled heating of the weldment to an
elevated temperature, followed by controlled cooling. /ometimes
two identical weldments are clamped bac! to bac!, welded, and
then stress%relieved while being held in this straight condition.
The residual stresses that would tend to distort the weldments are
thus minimied.1. Minimize welding time
/ince complex cycles of heating and cooling ta!e place during
welding, and since time is re-uired for heat transmission, the time
factor aects distortion. +n general, it is desirable to nish the
weld -uic!ly, before a large volume of surrounding metal heats up
and expands. The welding process used, type and sie of
electrode, welding current, and speed of travel, thus, aect the
degree of shrin!age and distortion of a weldment. The use of
mechanied welding e-uipment reduces welding time and the
amount of metal aected by heat and, conse-uently, distortion.
#or example, depositing a given%sie weld on thic! plate with a
process operating at &23 amp, 3 volts, and $ ipm re-uires
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with a process operating at $&9 amp, $3 volts, and < ipm re-uires
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during butt welding of plates, as in #ig. $%$8(a). lips are welded
to the edge of one plate and wedges are driven under the clips to
force the edges into alignment and to hold them during welding.
#ig. $%$8 Aariousstrongbac!
arrangements to
control distortion
during butt%welding.
3hermal Stress Relieving
:xcept in special situations, stress relief by heating is not used for
correcting distortion. There are occasions, however, when stress
relief is necessary to prevent further distortion from occurringbefore the weldment is nished.
Summar56 Che'klist to Minimize Distortion
#ollow this chec!list in order to minimie distortion in the design
and fabrication of weldments0
Do not overweld
ontrol tup
se intermittent welds where possible and consistent with design
re-uirements
se the smallest leg sie permissible when llet welding
#or groove welds, use "oints that will minimie the volume of weld
metal. onsider double%sided "oints instead of single%sided "oints
1eld alternately on either side of the "oint when possible with
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multiple%pass welds
se minimal number of weld passes
se low heat input procedures. This generally means high
deposition rates and higher travel speeds
se welding positioners to achieve the maximum amount of 'at%position welding. The 'at position permits the use of large%
diameter electrodes and high%
deposition%rate welding procedures
Balance welds about the neutral axis of the member
Distribute the welding heat as evenly as possible through a
planned welding se-uence and weldment positioning
1eld toward the unrestrained part of the member
se clamps, xtures, and strongbac!s to maintain tup and
alignment5rebend the members or preset the "oints to let shrin!age pull
them bac! into alignment
/e-uence subassemblies and nal assemblies so that the welds
being made continually balance each other around the neutral
axis of the section
#ollowing these techni-ues will help minimie the eects of
distortion and residual stresses.