Distortion
Types and causes
This article covers several key issues on distortion in
arc welded fabrications, especially basic types of and factors affecting the
degree of distortion.
Dishing of the steel plate between longitudinal stiffeners can be seen
clearly on the bow of this ship (Courtesy MOD).
What causes distortion?
Because welding involves highly localised heating of joint edges to fuse the
material, non-uniform stresses are set up in the component because of expansion
and contraction of the heated material. Initially, compressive stresses are
created in the surrounding cold parent metal when the weld pool is formed due to
the thermal expansion of the hot metal (heat affected zone) adjacent to the weld
pool. However, tensile stresses occur on cooling when the contraction of the
weld metal and the immediate heat affected zone is resisted by the bulk of the
cold parent metal.
The magnitude of thermal stresses induced into the material can be seen by
the volume change in the weld area on solidification and subsequent cooling to
room temperature. For example, when welding CMn steel, the molten weld metal
volume will be reduced by approximately 3% on solidification and the volume of
the solidified weld metal/heat affected zone (HAZ) will be reduced by a further
7% as its temperature falls from the melting point of steel to room temperature.
If the stresses generated from thermal expansion/contraction exceed the yield
strength of the parent metal, localised plastic deformation of the metal occurs.
Plastic deformation causes a permanent reduction in the component dimensions and
distorts the structure.
What are the main types of distortion?
Distortion occurs in six main forms:
- Longitudinal shrinkage
- Transverse shrinkage
- Angular distortion
- Bowing and dishing
- Buckling
- Twisting
The principal features of the more common forms of distortion for butt and
fillet welds are shown.
Contraction of the weld area on cooling results in both transverse and
longitudinal shrinkage.
Non-uniform contraction (through thickness) produces angular distortion in
addition to longitudinal and transverse shrinkage.
For example, in a single V butt weld, the first weld run produces
longitudinal and transverse shrinkage and rotation. The second run causes the
plates to rotate using the first weld deposit as a fulcrum. Hence, balanced
welding in a double side V butt joint can be used to produce uniform contraction
and prevent angular distortion.
Similarly, in a single side fillet weld, non-uniform contraction produces
angular distortion of the upstanding leg. Double side fillet welds can therefore
be used to control distortion in the upstanding fillet but because the weld is
only deposited on one side of the base plate, angular distortion will now be
produced in the plate.
Longitudinal bowing in welded plates happens when the weld centre is not
coincident with the neutral axis of the section so that longitudinal shrinkage
in the welds bends the section into a curved shape. Clad plate tends to bow in
two directions due to longitudinal and transverse shrinkage of the cladding;
this produces a dished shape. Dishing is also produced in stiffened plating.
Plates usually dish inwards between the stiffeners, because of angular
distortion at the stiffener attachment welds (see main photograph).
In plating, long range compressive stresses can cause elastic buckling in
thin plates, resulting in dishing, bowing or rippling.
Distortion due to elastic buckling is unstable: if you attempt to flatten a
buckled plate, it will probably 'snap' through and dish out in the opposite
direction.
Twisting in a box section is caused by shear deformation at the corner joints
This is caused by unequal longitudinal thermal expansion of the abutting edges.
Increasing the number of tack welds to prevent shear deformation often reduces
the amount of twisting.
How much shall I allow for weld shrinkage?
It is almost impossible to predict accurately the amount of shrinking.
Nevertheless, a 'rule of thumb' has been composed based on the size of the weld
deposit. When welding steel, the following allowances should be made to cover
shrinkage at the assembly stage.
Transverse Shrinkage
Fillet Welds 0.8mm per weld where the leg length does not
exceed 3/4 plate thickness
Butt weld 1.5 to 3mm per weld for 60 degree V joint, depending on
number of runs
Longitudinal Shrinkage
Fillet Welds 0.8mm per 3m of weld
Butt Welds 3mm per 3m of weld
Increasing the leg length of fillet welds, in particular, increases
shrinkage.
What are the factors affecting distortion?
If a metal is uniformly heated and cooled there would be almost no
distortion. However, because the material is locally heated and restrained by
the surrounding cold metal, stresses are generated higher than the material
yield stress causing permanent distortion. The principal factors affecting the
type and degree of distortion, are:
- Parent material properties
- Amount of restraint
- Joint design
- Part fit-up
- Welding procedure
Parent material properties
Parent material properties which influence distortion are coefficient of
thermal expansion and specific heat per unit volume. As distortion is determined
by expansion and contraction of the material, the coefficient of thermal
expansion of the material plays a significant role in determining the stresses
generated during welding and, hence, the degree of distortion. For example, as
stainless steel has a higher coefficient of expansion than plain carbon steel,
it is more likely to suffer from distortion.
Restraint
If a component is welded without any external restraint, it distorts to
relieve the welding stresses. So, methods of restraint, such as 'strong-backs'
in butt welds, can prevent movement and reduce distortion. As restraint produces
higher levels of residual stress in the material, there is a greater risk of
cracking in weld metal and HAZ especially in crack-sensitive materials.
Joint design
Both butt and fillet joints are prone to distortion. It can be minimised in
butt joints by adopting a joint type which balances the thermal stresses through
the plate thickness. For example, a double-sided in preference to a single-sided
weld. Double-sided fillet welds should eliminate angular distortion of the
upstanding member, especially if the two welds are deposited at the same time.
Part fit-up
Fit-up should be uniform to produce predictable and consistent shrinkage.
Excessive joint gap can also increase the degree of distortion by increasing the
amount of weld metal needed to fill the joint. The joints should be adequately
tacked to prevent relative movement between the parts during welding.
Welding procedure
This influences the degree of distortion mainly through its effect on the
heat input. As welding procedure is usually selected for reasons of quality and
productivity, the welder has limited scope for reducing distortion. As a general
rule, weld volume should be kept to a minimum. Also, the welding sequence and
technique should aim to balance the thermally induced stresses around the
neutral axis of the component.
The article was prepared by Bill Lucas in collaboration with Geert Verhaeghe
and Rick Leggatt.
Copyright by TWI, 1999
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REPRODUCED - COURTESY TWI-UK