welding dis trot ion

8/3/2019 Welding Dis Trot Ion

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M.S.Rogers .S.Rogers Copyright © 2003 TWI Ltd

CSWIP Welder TrainingResidual Stress and Distortion




Normal Stress

Stress arising from a force perpendicular to the crosssectional area

Compression

Tension

Stresses




Shear Stress

Stress arising from forces which are parallel to, and liein the plane of the cross sectional area.

Shear Stress

Stresses




Hoop Stress

Stress acting circumferentially around a pipe due tointernal pressure.

Hoop Stress

Stresses




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




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



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



Distortion

Factors which affect distortion are: material properties,

welding process, welding variables, the amount of restraint, joint geometry and the welding procedure



Distortion

Fit-up is very important to minimise distortion, fit-up must be

even and uniform to ensure constant shrinkage along the joint



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



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



Distortion

Longitudinal shrinkage/distortion



Distortion

Transverse shrinkage/distortion




Distortion

Angular distortion: weld metal contraction transverse




Distortion





Distortion

Longitudinal bowing: weld metal contraction longitudinal





Distortion







Distortion

Angular Distortion

Bowing Distortion Longitudinal Distortion

Transverse Distortion




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




Material properties and condition

Heat input

The amount of restrain

The amount of weld metal deposited

Distortion

Factors which affect distortion




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

5

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Distortion




Distortion

The used of a different joint design

Control of distortion my be achieved in on of the

following way




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






Distortion


Offsetting the joints to be welded – so that the

metal distorts into the required position.


following way




Distortion

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





Offsetting the joints to be welded – so that the


The use of a balanced welding technique


following way

Distortion




Back-skip welding technique

Back-step welding technique

1. 2. 3. 4.

1. 2. 4.

Distortion

3.





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

Distortion




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.




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




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




Any Questions?Any Questions?

Q ti



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.

welding dis trot ion

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