DISTORTION: PREVENTION BY DESIGN


This article advises on best practice for limiting distortion in arc-welded structures.


GENERAL GUIDELINES ARE given below as ‘best practice’ for limiting distortion when considering the design of arc-welded structures.


Design principles At the design stage, welding distortion can often be prevented, or at least restricted, by considering:


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 not only the amount of welding is kept to a minimum, but also that the smallest amount of weld metal is deposited. Welding can often be eliminated at the design stage by forming the plate or using a standard rolled section, as shown in Fig. 1 below.


a)


As most welds are deposited away from the neutral axis, distortion can be minimised by designing the fabrication so that the shrinkage forces of an individual weld are balanced by placing another weld on the opposite side of the neutral axis. Whenever possible, welding should be carried out alternately on opposite sides, instead of completing one side first. In large structures, if distortion is occurring preferentially on one side,


it may be possible to take corrective action e.g. by increasing welding on the other side to control the overall distortion.


Reducing the volume of weldmetal To minimise distortion, as well as for economic reasons, the volume of weld metal should be limited to the design requirements. For a single-sided joint, the cross-section of the weld should be kept as small as possible to reduce the level of angular distortion, as illustrated in Fig. 3.


b)


Fig. 3: Reducing the amount of angular distortion and lateral shrinkage by a) reducing the volume of weld metal and b) using a single-pass weld


a) b)


Fig. 1: Elimination of welds by a) forming the plate and b) use of rolled or extruded section


If possible, the design should use intermittent welds rather than a continuous run, to reduce the amount of welding. For example, in attaching stiffening plates, a substantial reduction in the amount of welding can often be achieved whilst maintaining adequate strength.


Weld placement Placing and balancing of welds are important in designing for minimum distortion. The closer a weld is positioned to the neutral axis of a fabrication, the lower the leverage effect of the shrinkage forces and the final distortion. Examples of good and poor designs are shown in Fig. 2.


Neutral axis


Neutral axis


Joint preparation angle and root gap should be minimised for the weld to be made satisfactorily. To facilitate access, it may be possible to specify a larger root gap and smaller preparation angle. By cutting down the difference in the amount of weld metal at the root and the face of the weld, the degree of angular distortion will be correspondingly reduced. Butt joints made in a single pass using deep penetration have little


angular distortion, especially if a closed butt joint can be welded (Fig. 3). For example, thin section material can be welded using plasma and laser welding processes and thick section can be welded, in the vertical position, using electrogas and electroslag processes. Although angular distortion can be eliminated, there will still be longitudinal and transverse shrinkage. In thick section material, as the cross sectional area of a double-V joint preparation is often only half that of a single-V preparation, the volume of weld metal to be deposited can be substantially reduced. The double-V joint preparation also enables balanced welding about the middle of the joint to eliminate angular distortion. As weld shrinkage is proportional to the amount of weld metal,


poor joint fit-up and over-welding will increase the amount of distortion. Angular distortion in fillet welds is particularly affected by over-welding. As design strength is based on throat thickness, over-welding to produce a convex weld bead does not increase the allowable design strength but will increase the shrinkage and distortion.


Poor 20 Good Fig. 2: Distortion may be reduced by placing the welds around the neutral axis


Reducing the number of runs There are conflicting opinions on whether it is better to deposit a given volume of weld metal using a small number of large weld passes or a large number of small passes. Experience shows that, for a single-sided butt joint or a single-side fillet weld, a large single weld deposit gives less angular distortion than if the weld is made with a number of small runs. Generally, in an unrestrained joint, the degree of angular distortion is approximately proportional to the number of passes.


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