Welding Distortion Control
WeldingDistortion Control
What is distortion ?
Undesirable change in Undesirable change in
original shape is called original shape is called
DISTORTIONDISTORTION
Before distortion
After distortion Distortion occurs due to heat input and
mechanical forces.
• Uniform heating of a steel bar through out of its entire volume -
considerable expansion take place in all direction.
• Now, if cooling of the bar is allowed evenly - retain its original
shape and size without distortion.
DURING HEATED CONDITION
X
X + 9X
BEFORE HEATING AND AFTER COOLING
Experiment No 1:
So, we can say that,
“ Uniform heating and cooling of a
component that can expand and
contract does not cause any
appreciable distortion ”
• Repeat experiment no:1 but heat the steel bar in
clamp condition and see the changes in shape and size
after cooling.
STEEL BAR AFTER HEATING & COOLING DOWN
STEEL BAR BEFORE HEATING
CLAMPING JAWS
CLAMPING JAWS
Experiment No 2 :
RKS,HZW
So, we can conclude that,
• Restraint hinders free expansion and
contraction and causes material to
deform resulting in
Distortion
Gas cutting/heating welding
Heat input
HEATING Heated area expands
Expansion restrained by surrounding solid area
Compressive stresses are developed
Further compressive stress leads to plastic deformation
Material bulges at the spot towards heat source side
HEAT SOURCE
Distortion in case of spot heating?
COOLING
Spot area tends to contract.
Contraction restrained by surrounding hot area.
Material goes back to original position with plastic deformation.
Resulting distortion
Distortion in case of spot heating?
WELD BEAD
ORIGINAL POSITION
AFTER WELDING
Longitudinal distortion WELD BEAD
LONGITUDINALDISTORTION
LONGITUDINAL SHRINKAGE• (A) BUTT WELDS IN CS/LAS
LS = 3. I .L / 100,000 t
LS = longitudinal shrinkage (mm) I = welding current(amp) L = length of weld (mm) t = plate thickness (mm)
It is contraction along the length of weld bead
It is maximum along weld bead and decreases at points away from the bead.
In C/S of shell it lead to reduction in diameter at
the
weld
Distortion in Butt welds
Longitudinal Distortion
EXAMPLE (LS IN BUTT WELDS)• Calculate LS for 6mm thick CS plate
welded by SMAW using 200 A current.
• Solution : LS = 3. 200. L / 100,000 x 6
= L/1000 mm
LONGITUDINAL SHRINKAGE
LS = 25 Aw/ Ap
Aw = Weld X-sectional area
Ap = Resisting X-sectional area
ApAw
• (B) FILLET WELD
EXAMPLE OF LS IN FILLET WELD
100
75
6
6
8x8
All dimensions in mm
LS = 1.52 mm
It is the shrinkage perpendicular to the weld.
It leads to the development of high residual stress and
also cracking in case of highly restrained joint.
It is not uniform along the length of the plate
It is lesser at that end of plate where bead is started.
Distortion in Butt welds
Transverse Distortion
ORIGINAL POSITION
AFTER WELDING
Transverse distortion
WELD BEAD
WELD BEAD TRANSVERSE DISTORTION
TRANSVERSE SHRINKAGE IN SINGLE PASS BUTT JOINTS
S = 0.2 Aw / t + 0.05 d
Where
S = Transverse Shrinkage (mm)
Aw = Cross sectional Area of Weld (mm2)
t = Thickness of Plates (mm)
d = Root Opening (mm)
TRANSVERSE SHRINKAGE DURING MULTIPASS WELDING
TS = TS0 + b (log w - log w0)
Where
TS = Total Transverse Shrinkage
TS0 = Transverse Shrinkage after first pass
w = Total weight of weld metal
w0 = weight of first pass weld metal
b = a coefficient
Effect of Various Procedures on Transverse Shrinkage of Butt
WeldsProcedures
Root Gap
Joint design
Electrode dia.
Degree of constraint
Peening
Gouging & repairs
Effect on TSTS increases with increase in RGSingle Vee produces more TS thandouble VTS decreases with increase in electrodedia.TS decreases with Degree of constraint
TS decreases by peeningTS increases by these operations.
TRANSVERSE SHRINKAGE IN FILLET JOINTS
1. For T joints with two continuous fillets.
TS = Leg of fillet Weld (l) x 1.02
Bottom Plate thickness (tb)
All dimensions in mm.tb
l x l
2. For intermittent fillet welds , a correcting
factor of proportional length of fillet weld to
total length of joint should be used.
TRANSVERSE SHRINKAGE IN FILLET JOINTS
(3) For fillet welds in a lap joints between
plates of equal thickness (two welds)
TS = Leg of fillet Weld (l) x 1.52
Plate thickness (t)All dimensions in mm.
TRANSVERSE SHRINKAGE IN FILLET JOINTS
l
l
tt
ORIGINAL POSITION
AFTER WELDING
Angular distortion
It is the bending transverse to the weld.
Due to non-uniform heating and cooling along
the thickness of plate.
This is the main source of mismatch and
dimensional inaccuracy in large welded
structures
Distortion in Butt weldsAngular Distortion
Angular Distortion in Butt Joints
t1
t2
t3
g
g = 3 mmt3 = 2 mmt
t1 + 1/2 t3
t= 0.6
1. Use Both Side Welding Technique in place of Single Side Welding
AD = 0.0076 . W . l1.3
t2
Where
AD= Angular Distortion, mm
W=flange width, mm
l = weld leg length, mm
t = flange thickness, mm
Angular Distortion in Fillet Welds
W W
ADAD
t
lt
RKS,HZW
Example of Angular Distortion in Fillet Welds
Find the angular distortion in a double fillet weld of a T-joint between a flange 1000 mm wide and a vertical member when the thickness of both the members is 6 mm and the weld leg length = 8 mm
Solution. AD = =
0.0076 x 1000 x (8)1.3
(6)23.15 mm.
Multiple Restrained Fillet Welds
ADØ
LAD
L
1
4= Ø
xL
1
2
2Ø
AD = Angular distortion, mm.L = span length, mm.Ø = angular change, radians
x = distance from weld to the point where distortion is to be determined, mm.
RKS,HZW
Example of AD in Multiple Restrained Fillet Welds
In multiple restrained fillet welds the span length is 1 m and the angular change is 90 at a distance of 400 mm from the span end, find the distortion.
Solution.
By putting L = 1000 mm, Ø = 90 = 0.1571 rad.
x = L/2 - 400 = 100 mm in the Formula,
AD = 14.164 mm.
Distortion in ‘T’-jointsAngular distortion
Before welding After welding
Distortion in ‘T’-jointsLongitudinal distortion(a) pulling effect towards neutral axis
A
ASection A - A
Distortion in ‘T’-StiffenerLongitudinal distortion(b) pulling effect of welds above neutral axis.
Section A - A A
A
To prevent distortion :-
(A)
Reduce the effective shrinkage force.
Reduce effective shrinkage force (A-1) Keep the angle of weld joint to the
barest minimum.
keep the angle of weld joint 45 deg.
MINIMUM ANGLE, LESS WELDING , LESS HEAT INPUT
Hence less distortion
50 deg. +/- 5 deg.
50 deg. +/- 5 deg.
keep the angle of weld joint 45 deg.
keep fillet size 18 mm/6 mm
19 mm +3/-1
7 mm +3/-1
(A-4) Minimize no of passes larger size of electrodes
Reduce effective shrinkage force
MORE NO OF PASSES LESS NO OF PASSES
(A-5) Place welds near the neutral axis
N. A.
Reduce effective shrinkage force
To prevent distortion :-
(B)
Make shrinkage work for us
WEDGE
CLAMPS ALONG EDGE
Make shrinkage work for us
(B.1) Pre cambering OR Pre bending in plate
Make shrinkage work for us(B-2) Keep over dimensions OR over
bend before welding
To prevent distortion :-
(C)
Balance shrinkage force with other forces
4
3
12
1
3
52
4
6 1
3
5
6
4
2
(C-1)
Do Sequence welding
Balance shrinkage forces with other forces.
Balance shrinkage forces with other forces
(C-2) Back step welding
1 2 3 4
Welding progresion
Two identical parts should be tacked back to back together before welding as shown
PART -II
PART -IEND PLATES TACKED
(C-3) Back to back clamping for welding
Balance shrinkage forces with other forces
SADDLE-I
SADDLE-IIWELDING TACKS
Back to back welding of saddles
Good working
methods for
welding distortion
in our routine work
Bulging of tube sheet of heat exchanger
TUBE SHEET BULGES DURING SHELL TO TUBE SHEET WELDING
• Welding of shell to tube sheet
LEADS TO
• Improper seating of gasket and leakage
• Non uniform projection of tube ends from tube sheet face
CONTROLLED BY • Back to back
• Weld optimum fillet size
TUBE SHEET
SHELL
Distortion of shell long seams Typical weld sequence and distortion observed
118
4 m
m D
IA
58T MIN LAS.
3200
D/4
D
INSIDE
OUTSIDE
600
2/3T
1/3T
T
6000.2mm GAP
3
JOINT DETAIL WELD SEQUENCE
1
2
3
SMAW
SAW
SAW
BACKGOUGING
Distortion of shell long seams
1 2 31 SET-UP STAGE 4 + 2 + 2 +2 AFTER SEAL RUN 6 + 4 + 5 +3 AFTE R O/S WELDING 8 + 6 + 8 +4 AFTER BACK GOUGING 6 + 5 + 5 +5 AFTER I/S WELDING 4 + 2.5 + 4 +
LOCATION STAGE
( D/4 TEMPLATE READING
• Caused by longitudinal shrinkage of weld
• Reduction in diameter around circumferential seam
• Reduction in shell length
Controlled by • Provide compression spiders on both sides of C/S• Design weld joint to have minimum weld metal deposit • Use restricted heat input ( minimum no. of passes )
CIRCSEAM JIINT SHELL
SUGARCANE EFFECT
Distortion of circumferential seams in shell
C/S
Gauge for checking long seam distortion in plate stage welding
PICK IN OR PICK OUT = A-B OR C-D
(MAXIMUM DIFFERENCE TO BE CONSIDERED)
GAUGE FOR CHECKING
ABCD
Gauge for checking distortion of ‘T’- joint welding
PRE-TILT OF T-STIFFENER = A - C
SAGGING OF T-STIFFENER = A - B
GAUGE FOR CHECKING
A
B
C
Distortion in flange to pipe welding
FLANGE
BEFORE WELDING
AFTER WELDING
PIPE
FLANGE WARPS
FLANGE FLANGE
PIPE PIPE
TEMP. SUPPORTS
• Heavy fillet weld on flange to pipe joint leads to warping of flange
• Causing no machining allowance on flange face thickness CONTROL : back to back welding
• Temporary set up two flanges back to back as shown
Sinking in of nozzle on shell Controlling sinking
• Provide rigid internal jacks
/supports with moon plates
/compression spider
• Maintain optimum weld
preparation and fit up to avoid
extra weld deposit
• Keep excess nozzle projection
at set up stage to compensate
for sinking
STRIP CUTTING FROM PLATE
The strip tends to bow outwards as shown
Distortion ( bow ) results due to unequal heating of
the metal
During cutting when hot, the bow is more on cooling
& the bow diminishes slightly
Finally the strip never returns to it’s intended shape
PLATE
STRIP
Distortion During Oxy-acetylene Cutting
Controlling distortion during oxy-acetylene cutting
METHOD I
Two Torches Technique
• Mark strip of required width leaving 10 mm distance
• Move two torches simultaneously carrying out cutting operation
SCRAP 10 mm
TORCH I
TORCH II
STRIP
PLATE
DIRECTION OF MOVEMENT FOR TORCHES
Controlling distortion during oxy - acetylene cutting
Method II
• Mark the strips with kerf allowance on the plate
• Drill small hole in kerf allowance at distance 20 mm away from the edge
• Start cut from drilled hole in kerf to the end such that the strip is attached to main plate
• Cut the balance strip attached to the plate
HOLE
PLATE
STRIPS
KERF
Controlling distortion during oxy - acetylene cutting
Aim : To get undistorted segment from the plate of size as shownSpecific Steps • Mark leaving 30mm Dist. from edge• Start with pierce cut as shown instead of starting from the edge • Follow the path as shown
30 mm
30 mm
12 mm THK PLATE
50 mmPIERCE START
R250 mm
Reduction in distortion• Less weld edge preparation.
• Less welding current as per WPS.
• Higher base metal thickness.
• Lesser welding passes
• Do not over weld
• More distortion in stainless steel then carbon
steel.
• Less offset-Lesser welding-Lower distortion
• Provide intermittent welding
• Place weld near the neutral axis
• Balancing weld around neutral axis
• Back-step welding
• Sequence welding
• Pre bending OR Pre cambering
• Back to back clamping
• Double operator welding technique
Reduction in distortion