Wis5 stress and distortion 15

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Copyright © 2003 TWI Ltd

Welding InspectionResidual Stress and Distortion

Course Reference WIS 5

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Normal StressStress arising from a force perpendicular to the

cross sectional area

Compression

Tension

StressesStresses

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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

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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.


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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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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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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

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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

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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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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: weld metal contraction transverse


Longitudinal bowing: weld metal contraction longitudinal

Longitudinal shrinkage/distortion Transverse shrinkage/distortion

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Angular distortion: weld metal contraction transverse Longitudinal bowing: weld metal contraction longitudinal

Longitudinal shrinkage/distortion Transverse shrinkage/distortion

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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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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



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Offsetting the joints to be welded – so that the metal distorts into the required position.


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

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Offsetting the joints to be welded – so that the metal distorts into the required position.

The use of a balanced welding technique



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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 metal distorts into the required position.

The use of a balanced welding technique

The use of clamps, jigs and fixtures.



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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.

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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

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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

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Any Questions?Any Questions?

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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.

Wis5 stress and distortion 15

Technology

Transcript of Wis5 stress and distortion 15