17. Residual stress and distortion-EWS course.ppt

Residual stress and distortion-EWS course

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STRESS, RESIDUAL STRESS &
DISTORTION
SECTION 17
TWI
CSWIP 3.1
WIS 5
WELDING INSPECTION

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STRESS.
Stresses can be divided into two main types:
•NORMAL STRESS –arising from a force
applied perpendicular to the cross
sectional area of a material.
•TENSILE STRESS –normal stress
resulting from tension.
•COMPRESSIVE STRESS –normal stress
resulting from compression.
•SHEAR STRESS –arising from forces
parrallel to the plane of the cross
sectional area

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NORMAL & SHEAR STRESS .
COMPRESSION
TENSION
NORMAL STRESSES
SHEAR STRESSES

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HOOP STRESS.
HOOP STRESS
Stress acting around the internal circumference of a pipe due to internal pressure

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RESIDUAL STRESS
Residual stresses are self balancing
internal forces which can be undesirable
because:
•they can lead to distortion
•they can affect dimensional stability of
the welded assembly
•they can increase the risk of brittle
fracture
•they can facilitate certain types of
corrosion

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Residual stress
Heating and
cooling causes
expansion and
contraction

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Residual stress
In case of a heated
bar, the resistance
of the surrounding
material to the
expansion and
contraction leads
to formation of
residual stress

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Types of residual stress
Transverse residual stress after welding
Tension
Compression
Maximum stress = YS at
room temperature
The longer the weld, the higher the tensile stress!

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Types of residual stress
Longitudinal residual stress after welding
TensionCompression
YS at room
temperature
The higher the heat input the wider the tensile zone!

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Types of residual stress
Residual stress after PWHT
TensionCompression
YS at room
temperature
YS at PWHT
temperature
After PWHT, peak residual stress is less
than a quarter of its initial level!

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RESIDUAL STRESS
Residual stresses in welded components
can be relaxed by PWHT.
Stress relieving between 550 -700°C
will help to relax residual stresses but
will not eliminate them

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DISTORTION.
Shrinkage during welding consists of
three components:
1. Contraction of liquid metal
3. Contraction of solid metal
2. Change of volume on solidification

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DISTORTION.
Factors affecting distortion:
•parent material properties
•amount of restrain
•joint design
•fit-up
•welding sequence
•heat input

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DISTORTION.
Types of distortion:
•Longitudinal shrinkage
•Transverse shrinkage
•Angular distortion
•Bowing

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Types of distortion
Longitudinal shrinkage

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Types of distortion
Transverse shrinkage

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Types of distortion
Longitudinal shrinkage

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Types of distortion
Angular distortion

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Types of distortion
Bowing and dishing

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FACTORS AFFECTING DISTORTION.
Joint design:
•weld metal volume
•type of joint -butt vs. fillet, single vs. double side
Amount of restrain:
•thickness -as thickness increase, so do the
stresses
•high level of restrain lead to high stresses
•preheat may increase the level of stresses (pipe
welding!)
Fit-up:
•misalignment may reduce stresses in some cases
•root gap -increase in root gap increases shrinkage

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FACTORS AFFECTING DISTORTION.
Welding sequence:
•number of passes -every pass adds to the total
contraction
•heat input -the higher the heat input, the greater
the shrinkage
•travel speed -the faster the welding speed, the
less the stress
•build-up sequence

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DISTORTION.
Methods of reducing distortion:
•Design
•Preheating
•Forced restrain
•Joint design
•Welding technique / sequence
•Heat input

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STRESS & DISTORTION .
QUESTIONS ?

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DISTORTION PREVENTION.
Distortion prevention by design
Consider eliminating the welding!!
a) by forming the plate
b) by use of rolled or extruded sections

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DISTORTION PREVENTION.
Distortion prevention by design
•consider weld
placement
•reduce weld metal
volume and/or
number of runs

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DISTORTION PREVENTION.
Distortion prevention by design
•use of balanced welding

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DISTORTION PREVENTION.
Distortion prevention by pre-setting
a) pre-setting of fillet
joint to prevent
angular distortion
b) pre-setting of butt
joint to prevent
angular distortion
c) tapered gap to
prevent closure

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DISTORTION PREVENTION.
Distortion prevention by pre-bending
using strongbacks and wedges

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DISTORTION PREVENTION.
Distortion prevention by restraint techniques
a) use of welding jigs
b) use of flexible
clamps

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DISTORTION PREVENTION.
Distortion prevention by restraint techniques
c) use of strongbacks
with wedges
d) use of fully welded
strongbacks

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Distortion prevention
Distortion prevention by fabrication
techniques
•control welding techniques by
a) Back-step welding
b) Skip welding

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FACTORS AFFECTING DISTORTION.
Parent material properties:
•thermal expansion coefficient -the greater the
value, the greater the residual stress
•yield strength -the greater the value, the greater
the residual stress
•Young’s modulus -the greater the value (increase
in stiffness), the greater the residual stress
•thermal conductivity -the higher the value, the
lower the residual stress
•transformation temperature -during phase
transformation, expansion/contraction takes
place. The lower the transformation temperature,
the lower the residual stress

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Residual stress
In case of a heated
bar, the resistance
of the surrounding
material to the
expansion and
contraction leads
to formation of
residual stress

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Residual stress
Origins of residual stress in welded joints
Hot weld
Cold weld unfused
Cold weld fused

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Distortion
Origins of distortion in welded joints:

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Distortion prevention
-Transverse Shrinkage
•Fillet Welds0.8mm per weld where the leg length
does not exceed 3/4 plate thickness
•Butt weld1.5 to 3mm per weld for 60°V joint,
depending on number of runs
-Longitudinal Shrinkage
•Fillet Welds0.8mm per 3m of weld
•Butt Welds3mm per 3m of weld
Allowances to cover shrinkage
Distortions prevention by design

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Distortion prevention
Distortion prevention by fabrication
techniques
•tack welding
a) tack weld straight
through to end of joint
b) tack weld one end, then
use back-step technique
for tacking the rest of
the joint
c) tack weld the centre,
then complete the tack
welding by the back-
step technique

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Distortion prevention
Distortion prevention by fabrication
techniques
•back to back assembly
a) assemblies tacked
together before
welding
b) use of wedges for
components that
distort on separation
after welding

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Distortion prevention
Distortion prevention by fabrication
techniques
•use of stiffeners
•control welding process by:
-deposit the weld metal as quickly as possible
-use the least number of runs to fill the joint

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Distortion prevention
Distortion prevention by fabrication
techniques
•reduce the number of
runs required to make a
weld (e.g. angular
distortion as a function
of number of runs for a
10 mm leg length weld)

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Distortion prevention
Distortion prevention by fabrication techniques
•control welding techniques by use
balanced welding about the neutral
axis
•control welding techniques by keeping
the time between runs to a minimum

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Distortion prevention
Distortion -Best practice for fabrication
corrective techniques
•using tack welds to set up and maintain the joint gap
•identical components welded back to back so welding can be
balanced about the neutral axis
•attachment of longitudinal stiffeners to prevent longitudinal
bowing in butt welds of thin plate structures
•where there is choice of welding procedure, process and
technique should aim to deposit the weld metal as quickly as
possible; MIG in preference to MMA or gas welding and
mechanised rather than manual welding
•in long runs, the whole weld should not be completed in one
direction; back-step or skip welding techniques should be
used

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Distortion corrective techniques
Distortion -mechanical corrective
techniques
Use of press to correct bowing in T butt joint

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Distortion corrective techniques
Distortion -Best practice for mechanical
corrective techniques
•Use packing pieces which will over correct the distortion so
that spring-back will return the component to the correct
shape
•Check that the component is adequately supported during
pressing to prevent buckling
•Use a former (or rolling) to achieve a straight component or
produce a curvature
•As unsecured packing pieces may fly out from the press, the
following safe practice must be adopted:
-bolt the packing pieces to the platen
-place a metal plate of adequate thickness to intercept the
'missile'
-clear personnel from the hazard area

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Distortion corrective techniques
Distortion -thermal corrective techniques
Localised heating to
correct distortion
Spot heating for
correcting buckling

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Distortion corrective techniques
Distortion -thermal corrective techniques
Line heating to correct angular
distortion in a fillet weld
Use of wedge shaped heating
to straighten plate

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Distortion corrective techniques
Distortion -thermal corrective techniques
Wedge shaped heating to correct distortion
a) standard
rolled steel
section
b) buckled edge of
plate
c) box fabrication
General guidelines:
•Length of wedge = two-thirds of the plate width
•Width of wedge (base) = one sixth of its length (base to apex)

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Distortion corrective techniques
Distortion -thermal corrective techniques
•use spot heating to remove buckling in thin sheet structures
•other than in spot heating of thin panels, use a wedge-shaped
heating technique
•use line heating to correct angular distortion in plate
•restrict the area of heating to avoid over-shrinking the
component
•limit the temperature to 60°to 650°C (dull red heat) in steels
to prevent metallurgical damage
•in wedge heating, heat from the base to the apex of the
wedge, penetrate evenly through the plate thickness and
maintain an even temperature