f weld distortion

7/25/2019 f Weld Distortion

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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


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


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


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.

f weld distortion

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