Wis5 stress and distortion 15

Faisal Yusof
TE
Copyright © 2003 TWI Ltd
Welding Inspection
Residual Stress and Distortion
Course Reference WIS 5

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TE
 Normal Stress
Stress arising from a force perpendicular to the
cross sectional area
Compression
Tension
StressesStresses

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TE
 Shear Stress
Stress arising from forces which are parallel to, and lie
in the plane of the cross sectional area.
Shear Stress
StressesStresses

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TE
 Hoop Stress
Stress acting circumferentially around a pipe due to
internal pressure.
Hoop Stress
StressesStresses

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 Metal contract during solidification and subsequent
cooling.
 If this contraction is prevented or inhibited
 residual stress will develop.
 The tendency to develop residual stresses increases
when the heating and cooling is localised.
 Welding is very localised heating and the presence of
liquid and solid metal in contact can be expected to
induce very high levels of residual stresses.
Residual StressResidual Stress

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TE
 Residual stresses are very difficult to measure with
any real accuracy.
 Residual stresses are self balancing internal forces
and not stresses induced whilst applying external
load
 Stresses are more concentrated at the surface of the
component.
 The removal of residual stresses is termed stress
relieving.

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TE
Longitudinal
 Along the weld – longitudinal residual stresses
Transverse
 Across the weld – transverse residual stresses
Short Transverse
 Through the weld – short transverse residual stresses
Residual stresses occur in welds in the following directions

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 Material properties and condition
 Heat input
 The amount of restraint
 The amount of weld metal deposited
DistortionDistortion
Factors which affect distortion

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TE
 Distortion will occur in all welded joints if the material
are free to move i.e. not restrained
 Restrained materials result in low distortion but high
residual stress
 More than one type of distortion may occur at one time
 Highly restrained joints also have a higher crack
tendency to joints of a low restraint
 The action of residual in welded joints is to cause
distortion

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TE
Distortion is a very complex matter more than 20 factors
influence its magnitude and for this reason it is very difficult
to predict its exact amount of distortion that may occur

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TE
Fit-up is very important to minimise distortion, fit-up must be
even and uniform to ensure constant shrinkage along the
joint

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TE
A mechanised welding process is preferable to a manual
process because its consistent operation allows for better
distortion control. The required volume of weld metal should
be deposited in the shortest time to minimise heat input

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TE
The shorter the welding time the less heat is transmitted in to
the material and less distortion will occur. Keep welding
times down by carefully selecting the welding process,
electrode type and size, welding current and travel speed

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TE
Longitudinal shrinkage/distortion

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Transverse shrinkage/distortion

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Angular distortion: weld metal contraction transverse

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Longitudinal bowing: weld metal contraction longitudinal

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Angular Distortion
Bowing Distortion Longitudinal Distortion
Transverse Distortion

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The used of a different joint design
Control of distortion my be achieved in on of the
following way

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TE
The volume of weld metal in a joint will affect the amount
of local expansion and contraction, hence the more weld
deposited the higher amount of distortion
Preparation angle 60o

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TE
Offsetting the joints to be welded – so that the
metal distorts into the required position.
following way

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TE
Offsetting:
The amount of offsetting required is generally a function of
trail and error

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TE
Offsetting the joints to be welded – so that the
The use of a balanced welding technique
following way

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TE
Back-skip welding technique
Back-step welding technique
1. 2. 3. 4.
1. 2. 4.
3.

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Presetting the joints to be welded – so that the
The use of a balanced welding technique
The use of clamps, jigs and fixtures.
following way

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TE
Clamping and jigging:
The materials to be welded are prevented from moving by
the clamp or jig the main advantage of using a jig is that the
elements in a fabrication can be precisely located in the
position to be welded. Main disadvantage of jigging and
high restraint is the high levels of residual stresses.

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TE
1. Residual stresses are locked in elastic strain, which is
caused by local expansion and contraction in the weld
area.
2. Residual stresses should be removed from structures
after welding.
3. The amount of contraction is controlled by, the volume of
weld metal in the joint, the thickness, heat input, joint
design and the materials properties
4. Offsetting may be used to finalise the position of the joint.
5. If plates or pipes are prevented from moving by tacking,
clamping or jigging etc (restraint), then the amount of
residual stresses that remain will be higher.
Summary of Residual Stress & DistortionSummary of Residual Stress & Distortion

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TE
6. The movement caused by welding related stresses is
called distortion.
7. The directions of contractional stresses and distortion is
very complex, as is the amount and type of final distortion,
however we can say that there are three directions:
a. Longitudinal b. Transverse c.Short transverse
8. A high percentage of residual stresses can be removed by
heat treatments.
9. The peening of weld faces will only redistribute the
residual stress, and place the weld face in compression.
Summary of Residual Stress & DistortionSummary of Residual Stress & Distortion

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TE
Any Questions?Any Questions?

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TE
QU 1. What causes residual stress in welds?
QU 2. State three directions which residual stresses form in a
welded joint
QU 3. Give four methods of controlling distortion.
QU 4. Sketch a balanced welding techniques.
QuestionsQuestions
QU 5. State four factors which affect distortion.

Wis5 stress and distortion 15

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