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Distortion - prevention bydesign

Strongbacks on girder flange to prevent cross bowing.Courtesy John Allen

General guidelines are given below as 'best practice' for limitingdistortion when considering the design of arc welded structures.

Design principles

At the design stage, welding distortion can often be prevented, or at least restricted, byconsidering:

elimination of welding weld placement

reducing the volume of weld metal reducing the number of runs

use of balanced welding

Elimination of welding

As distortion and shrinkage are an inevitable result of welding, good design requires that notonly the amount of welding is kept to a minimum, but also the smallest amount of weld metal is

deposited. Welding can often be eliminated at the design stage by forming the plate or using astandard rolled section, as shown in ig !.

Fig. 1 Elimination of welds by: a) forming the plate b) !seof rolled or e"tr!ded section

"f possible, the design should use intermittent welds ratherthan a continuous run, to reduce the amount of welding. ore#ample, in attaching stiffening plates, a substantial reductionin the amount of welding can often be achieved whilstmaintaining adequate strength.

#eld placement

$lacing and balancing of welds are important in designing for minimum distortion. %he closer aweld is positioned to the neutral a#is of a fabrication, the lower the leverage effect of theshrinkage forces and the final distortion. amples of poor and good designs are shown in ig.

Fig. $ Distortion may be red!ced by placing the welds aro!nd the ne!tral a"is

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As most welds are deposited away from the neutral a#is,distortion can be minimised by designing the fabrication sothe shrinkage forces of an individual weld are balanced byplacing another weld on the opposite side of the neutral a#is.Whenever possible, welding should be carried out alternatelyon opposite sides, instead of completing one side first. "n

large structures, if distortion is occurring preferentially on oneside, it may be possible to take corrective actions, fore#ample, by increasing welding on the other side to controlthe overall distortion.

%ed!cing the vol!me of weld metal

%o minimise distortion, as well as for economic reasons, the volume of weld metal should belimited to the design requirements.or a single(sided )oint, the cross(section of the weld should be kept as small as possible to

reduce the level of angular distortion, as illustrated in ig *.

Fig. & %ed!cing the amo!nt of ang!lar distortion and lateral shrinkage by: a) red!cingthe vol!me of weld metal b) !sing single pass weld

+oint preparation angle and root gap should be minimisedproviding the weld can be made satisfactorily. %o facilitate access,it may be possible to specify a larger root gap and smallerpreparation angle. y cutting down the difference in the amount of

weld metal at the root and the face of the weld, the degree ofangular distortion will be correspondingly reduced. utt )oints madein a single pass using deep penetration have little angulardistortion, especially if a closed butt )oint can be welded -ig *.or e#ample, thin section material can be welded using plasmaand laser welding processes and thick section can be welded, inthe vertical position, using electrogas and electroslag processes.Although angular distortion can be eliminated, there will still belongitudinal and transverse shrinkage.

"n thick section material, as the cross sectional area of a double(/ )oint preparation is often only

half that of a single(/ preparation, the volume of weld metal to be deposited can besubstantially reduced. %he double(/ )oint preparation also permits balanced welding about themiddle of the )oint to eliminate angular distortion.

As weld shrinkage is proportional to the amount of weld metal, both poor )oint fit(up and over(welding will increase the amount of distortion. Angular distortion in fillet welds is particularlyaffected by over(welding. As design strength is based on throat thickness, over(welding toproduce a conve# weld bead does not increase the allowable design strength but it will increasethe shrinkage and distortion.

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%ed!cing the n!mber of r!ns

%here are conflicting opinions on whether it is better to deposit a given volume of weld metalusing a small number of large weld passes or a large number of small passes. perienceshows that for a single(sided butt )oint, or a single(side fillet weld, a large single weld depositgives less angular distortion than if the weld is made with a number of small runs. Generally, in

an unrestrained )oint, the degree of angular distortion is appro#imately proportional to thenumber of passes.

0ompleting the )oint with a small number of large weld deposits results in more longitudinal andtransverse shrinkage than a weld completed in a larger number of small passes. "n a multi(passweld, previously deposited weld metal provides restraint, so the angular distortion per passdecreases as the weld is built up. 1arge deposits also increase the risk of elastic bucklingparticularly in thin section plate.

'se of balanced welding

alanced welding is an effective means of controlling angular distortion in a multi(pass butt weldby arranging the welding sequence to ensure that angular distortion is continually beingcorrected and not allowed to accumulate during welding. 0omparative amounts of angulardistortion from balanced welding and welding one side of the )oint first are shown schematicallyin ig 2. %he balanced welding technique can also be applied to fillet )oints.

Fig. ( alanced welding to red!ce the amo!nt of ang!lardistortion

"f welding alternately on either side of the )oint is not possible,

or if one side has to be completed first, an asymmetrical )ointpreparation may be used with more weld metal beingdeposited on the second side. %he greater contractionresulting from depositing the weld metal on the second sidewill help counteract the distortion on the first side.

est practice%he following design principles can control distortion:

eliminate welding by forming the plate and using rolled or e#truded sections minimise the amount of weld metal

do not over weld

use intermittent welding in preference to a continuous weld pass place welds about the neutral a#is

balance the welding about the middle of the )oint by using a double(/ )oint in preferenceto a single(/ )oint

Adopting best practice principles can have surprising cost benefits. or e#ample, for a designfillet leg length of 3mm, depositing an 4mm leg length will result in the deposition of 567additional weld metal. esides the e#tra cost of depositing weld metal and the increase risk ofdistortion, it is costly to remove this e#tra weld metal later. 8owever, designing for distortion

control may incur additional fabrication costs. or e#ample, the use of a double(/ )ointpreparation is an e#cellent way to reduce weld volume and control distortion, but e#tra costsmay be incurred in production through manipulation of the workpiece for the welder to accessthe reverse side.