General guidelines are given below as 'best practice' for limiting distortion when considering the design of arc welded structures.
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 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.
a) forming the plate;
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.
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 poor and good designs are shown in Fig 2.
As most welds are deposited away from the neutral axis, distortion can be minimised by designing the fabrication so 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 actions, for example, by increasing welding on the other side to control the overall distortion.
Reducing the volume of weld metal
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.
a) reducing the volume of weld metal;
b) using single pass weld
Joint preparation angle and root gap should be minimised providing the weld can 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 permits balanced welding about the middle of the joint to eliminate angular distortion.
As weld shrinkage is proportional to the amount of weld metal, both 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 it will increase the shrinkage and distortion.
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.
Completing the joint with a small number of large weld deposits results in more longitudinal and transverse shrinkage than a weld completed in a larger number of small passes. In a multi-pass weld, previously deposited weld metal provides restraint, so the angular distortion per pass decreases as the weld is built up. Large deposits also increase the risk of elastic buckling particularly in thin section plate.
Use of balanced welding
Balanced welding is an effective means of controlling angular distortion in a multi-pass butt weld by arranging the welding sequence to ensure that angular distortion is continually being corrected and not allowed to accumulate during welding. Comparative amounts of angular distortion from balanced welding and welding one side of the joint first are shown schematically in Fig 4. The balanced welding technique can also be applied to fillet joints.
If welding alternately on either side of the joint is not possible, or if one side has to be completed first, an asymmetrical joint preparation may be used with more weld metal being deposited on the second side. The greater contraction resulting from depositing the weld metal on the second side will help counteract the distortion on the first side.
The following design principles can control distortion:
- eliminate welding by forming the plate and using rolled or extruded 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 axis
- balance the welding about the middle of the joint by using a double-V joint in preference to a single-V joint
Adopting best practice principles can have surprising cost benefits. For example, for a design fillet leg length of 6mm, depositing an 8mm leg length will result in the deposition of 57% additional weld metal. Besides the extra cost of depositing weld metal and the increase risk of distortion, it is costly to remove this extra weld metal later. However, designing for distortion control may incur additional fabrication costs. For example, the use of a double-V joint preparation is an excellent way to reduce weld volume and control distortion, but extra costs may be incurred in production through manipulation of the workpiece for the welder to access the reverse side.
This article was prepared by Bill Lucas with help from Rick Leggatt and Gene Mathers.