welding dis trot ion
TRANSCRIPT
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M.S.Rogers .S.Rogers Copyright © 2003 TWI Ltd
CSWIP Welder TrainingResidual Stress and Distortion
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Normal Stress
Stress arising from a force perpendicular to the crosssectional area
Compression
Tension
Stresses
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Shear Stress
Stress arising from forces which are parallel to, and liein the plane of the cross sectional area.
Shear Stress
Stresses
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Hoop Stress
Stress acting circumferentially around a pipe due tointernal pressure.
Hoop Stress
Stresses
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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 Stress
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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 stressrelieving.
Residual Stress
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Residual Stress
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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Distortion
Distortion is a very complex matter more than 20 factors
influence its magnitude and for this reason it is very difficultto predict its exact amount of distortion that may occur
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Distortion
Factors which affect distortion are: material properties,
welding process, welding variables, the amount of restraint, joint geometry and the welding procedure
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Distortion
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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DistortionA mechanised welding process is preferable to a manualprocess because its consistent operation allows for better
distortion control. The required volume of weld metal shouldbe deposited in the shortest time to minimise heat input
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DistortionThe 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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Distortion
Longitudinal shrinkage/distortion
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Distortion
Transverse shrinkage/distortion
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Distortion
Angular distortion: weld metal contraction transverse
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Distortion
Angular distortion: weld metal contraction transverse
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Distortion
Longitudinal bowing: weld metal contraction longitudinal
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Angular distortion: weld metal contraction transverse
Distortion
Longitudinal bowing: weld metal contraction longitudinal
Longitudinal shrinkage/distortion
Transverse shrinkage/distortion
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Distortion
Angular distortion: weld metal contraction transverse
Longitudinal bowing: weld metal contraction longitudinal
Longitudinal shrinkage/distortion
Transverse shrinkage/distortion
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Distortion
Angular distortion: weld metal contraction transverse
Longitudinal bowing: weld metal contraction longitudinal
Longitudinal shrinkage/distortion
Transverse shrinkage/distortion
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Distortion
Angular distortion: weld metal contraction transverse
Longitudinal bowing: weld metal contraction longitudinal
Longitudinal shrinkage/distortion
Transverse shrinkage/distortion
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Distortion
Angular Distortion
Bowing Distortion Longitudinal Distortion
Transverse Distortion
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Distortion
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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Material properties and condition
Heat input
The amount of restrain
The amount of weld metal deposited
Distortion
Factors which affect distortion
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The following movements can be detected in a simple butt
weld
1.Contraction in the weld metal along the welds length
2.Bowing – due to the greater volume of weld metal at the top
of the weld
3.Peaking due to the V angle
4.Ripple (in sheet) away from the weld
5.Contraction in the weld metal transverse to the weld, angular
distortion
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Distortion
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Distortion
The used of a different joint design
Control of distortion my be achieved in on of the
following way
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DistortionThe volume of weld metal in a joint will affect the amountof local expansion and contraction, hence the more welddeposited the higher amount of distortion
Preparation angle 60o
Preparation angle 40o
Preparation angle 0o
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Distortion
The used of a different joint design
Offsetting the joints to be welded – so that the
metal distorts into the required position.
Control of distortion my be achieved in on of the
following way
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Distortion
Offsetting:The amount of offsetting required is generally a function oftrail and error
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The used of a different joint design
Offsetting the joints to be welded – so that the
metal distorts into the required position.
The use of a balanced welding technique
Control of distortion my be achieved in on of the
following way
Distortion
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Back-skip welding technique
Back-step welding technique
1. 2. 3. 4.
1. 2. 4.
Distortion
3.
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The used of a different joint design
Presetting the joints to be welded – so that the
metal distorts into the required position.
The use of a balanced welding technique
The use of clamps, jigs and fixtures.
Control of distortion my be achieved in on of the
following way
Distortion
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Distortion
Clamping and jigging:The materials to be welded are prevented from moving bythe clamp or jig the main advantage of using a jig is that theelements in a fabrication can be precisely located in theposition to be welded. Main disadvantage of jigging andhigh restraint is the high levels of residual stresses.
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1. Residual stresses are locked in elastic strain, which is
caused by local expansion and contraction in the weldarea.
2. Residual stresses should be removed from structures
after welding.
3. The amount of contraction is controlled by, the volume ofweld 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 & Distortion
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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 & Distortion
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Any Questions?Any Questions?
Q ti
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QU 1. What causes residual stress in welds?
QU 2. State three directions which residual stresses form in awelded joint
QU 3. Give four methods of controlling distortion.
QU 4. Sketch a balanced welding techniques.
Questions
QU 5. State four factors which affect distortion.