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Faisal Yusof
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Copyright © 2003 TWI Ltd
Welding InspectionResidual Stress and Distortion
Course Reference WIS 5
Faisal Yusof
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Normal StressStress arising from a force perpendicular to the
cross sectional area
Compression
Tension
StressesStresses
Faisal Yusof
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Shear StressStress arising from forces which are parallel to, and lie in the plane of the cross sectional area.
Shear Stress
StressesStresses
Faisal Yusof
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Hoop StressStress 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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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.
Residual StressResidual Stress
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Residual StressResidual 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
Faisal Yusof
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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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DistortionDistortion
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
Faisal Yusof
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DistortionDistortionDistortion 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
Faisal Yusof
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DistortionDistortionFit-up is very important to minimise distortion, fit-up must be
even and uniform to ensure constant shrinkage along the joint
Faisal Yusof
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DistortionDistortionA 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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DistortionDistortionThe 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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DistortionDistortion
Longitudinal shrinkage/distortion
Faisal Yusof
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DistortionDistortion
Transverse shrinkage/distortion
Faisal Yusof
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DistortionDistortion
Angular distortion: weld metal contraction transverse
Faisal Yusof
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DistortionDistortion
Angular distortion: weld metal contraction transverse
Faisal Yusof
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DistortionDistortion
Longitudinal bowing: weld metal contraction longitudinal
Faisal Yusof
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Angular distortion: weld metal contraction transverse
DistortionDistortion
Longitudinal bowing: weld metal contraction longitudinal
Longitudinal shrinkage/distortion Transverse shrinkage/distortion
Faisal Yusof
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DistortionDistortion
Angular distortion: weld metal contraction transverse Longitudinal bowing: weld metal contraction longitudinal
Longitudinal shrinkage/distortion Transverse shrinkage/distortion
Faisal Yusof
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DistortionDistortion
Angular distortion: weld metal contraction transverse Longitudinal bowing: weld metal contraction longitudinal
Longitudinal shrinkage/distortion Transverse shrinkage/distortion
Faisal Yusof
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DistortionDistortion
Angular distortion: weld metal contraction transverse Longitudinal bowing: weld metal contraction longitudinal
Longitudinal shrinkage/distortion Transverse shrinkage/distortion
Faisal Yusof
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DistortionDistortion
Angular Distortion
Bowing Distortion Longitudinal Distortion
Transverse Distortion
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DistortionDistortion
The used of a different joint design
Control of distortion my be achieved in on of the following way
Faisal Yusof
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DistortionDistortionThe 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
Preparation angle 40o
Preparation angle 0o
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DistortionDistortion
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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DistortionDistortionOffsetting:The amount of offsetting required is generally a function of trail and error
Faisal Yusof
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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
DistortionDistortion
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Back-skip welding technique
Back-step welding technique
1. 2. 3. 4.
1. 2. 4.
DistortionDistortion
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
DistortionDistortion
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DistortionDistortionClamping 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.
Faisal Yusof
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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
Faisal Yusof
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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 & DistortionSummary of Residual Stress & Distortion
Faisal Yusof
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Copyright © 2003 TWI Ltd
Any Questions?Any Questions?
Faisal Yusof
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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.