Job Knowledge 33
This
article covers several key issues on distortion in arc welded
fabrications, especially basic types of and factors affecting the degree
of distortion.
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 below:
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° 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.
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