Welding Distortion Control
What is distortion ? Undesirable change in
Before distortion
original shape is called DISTORTION After distortion ➨
Distortion occurs due to heat input and mechanical forces.
Experiment No 1: DURING HEATED CONDITION
X
BEFORE HEATING AND AFTER COOLING
X + 9X
• 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.
So, we can say that, “ Uniform heating and cooling of a component that can expand and contract
does
not cause any
appreciable distortion ”
Experiment No 2: CLAMPING JAWS
STEEL BAR BEFORE HEATING
CLAMPING JAWS
STEEL BAR AFTER HEATING & COOLING DOWN
• Repeat experiment no:1 but heat the steel bar in clamp condition and see the changes in shape and size after cooling.
So, we can conclude that, • Restraint hinders free expansion and contraction and
causes material
deform resulting in
Distortion
to
Heat input
Gas cutting/heating
welding
Distortion in case of spot heating? HEAT SOURCE
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
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
Longitudinal distortion
WELD BEAD
ORIGINAL POSITION
WELD BEAD
LONGITUDINAL DISTORTION
AFTER WELDING
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)
Distortion in Butt welds Longitudinal Distortion ☛ 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
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 • (B) FILLET WELD LS = 25 Aw/ Ap Aw = Weld X-sectional area Ap = Resisting X-sectional area
Ap Aw
EXAMPLE OF LS IN FILLET WELD 6 All dimensions in mm
75
8x8
6 100 LS = 1.52 mm
Distortion in Butt welds Transverse Distortion ☛
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.
Transverse distortion WELD BEAD
TRANSVERSE DISTORTION
ORIGINAL POSITION
WELD BEAD AFTER WELDING
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 Welds
Degree of constraint
Effect on TS TS increases with increase in RG Single Vee produces more TS than double V TS decreases with increase in electrode dia. TS decreases with Degree of constraint
Peening Gouging & repairs
TS decreases by peening TS increases by these operations.
Procedures Root Gap Joint design Electrode dia.
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) lxl
All dimensions in mm.
tb
TRANSVERSE SHRINKAGE IN FILLET JOINTS 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 l t
t l
(3) For fillet welds in a lap joints between plates of equal thickness (two welds) TS =
Leg of fillet Weld (l) Plate thickness (t) All dimensions in mm.
x 1.52
Angular distortion AFTER WELDING
ORIGINAL POSITION
Distortion in Butt welds
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
Angular Distortion in Butt Joints 1. Use Both Side Welding Technique in place of Single Side Welding
t1
t
t2
t3
g
t1 + 1/2 t3 t
= 0.6
g = 3 mm t3 = 2 mm
Angular Distortion in Fillet Welds
W
W t AD l
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
AD t
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. 0.0076 x 1000 x (8)1.3 AD = = 3.15 mm. (6)2
Multiple
Restrained Fillet Welds Ø AD
L
AD L
=
1 Ø 4
x L
1 2 Ø 2
AD = Angular distortion, x = distance from weld to mm. the point where distortion is L = span length, mm. to be determined, mm. Ø = angular change, radians 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’joints
Angular distortion
Before welding
After welding
Distortion in ‘T’-joints Longitudinal distortion (a) pulling effect towards neutral axis
A
Section A - A
A
Distortion in ‘T’Stiffener
Longitudinal distortion
(b) pulling effect of welds above neutral axis.
A
Section 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. 50 deg. +/- 5 deg.
keep the angle of weld joint 45 deg. ➨
MINIMUM ANGLE, LESS WELDING , LESS HEAT INPUT Hence less distortion
7 mm +3/-1
keep fillet size 18 mm/6 mm 19 mm +3/-1
50 deg. +/- 5 deg.
keep the angle of weld joint 45 deg.
Reduce effective shrinkage force (A-4) Minimize no of passes larger size of electrodes
MORE NO OF PASSES
LESS NO OF PASSES
Reduce effective shrinkage force (A-5) Place welds near the neutral axis
N. A.
To prevent distortion :-
(B) Make shrinkage work for us
Make shrinkage work for us (B.1) Pre cambering OR Pre bending in plate
WEDGE
CLAMPS ALONG EDGE
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
Balance shrinkage forces with other forces. 6 2
5 5
(C-1) 4
3
4
Do Sequence welding
3 2
6
1
1 4 1 2 3
✸ Balance shrinkage forces with other forces (C-2) Back step welding
1
2
3
Welding progresion
4
✸ Balance shrinkage forces with other forces (C-3) Back to back clamping for welding PART -I END PLATES TACKED
PART -II Two identical parts should be tacked back to back together before welding as shown
Back to back welding of saddles SADDLEI
WELDING TACKS
SADDLE-II
Good working methods for welding distortion in our routine work
Bulging of tube sheet of heat exchanger
TUBE SHEET
SHELL
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
Distortion of shell long seams Typical weld sequence and distortion observed
1184 mm DIA
58T MIN LAS. D/4
D 3200 JOINT DETAIL 600
WELD SEQUENCE 2
OUTSIDE /3T
2
1
3
T
/3T
INSIDE
600
0.2mm GAP
BACK GOUGING
SAW
1
SMAW
3
SAW
Distortion of shell long seams
( D/4 TEMPLATE READING
STAGE
1 2 3 4 5
1 S E T-U P S TA G E 4 + A F TE R S E A L R U N 6 + A F TE R O / S W E L D IN G 8 + A F TE R B A C K G O U G IN 6G + A F TE R I/ S W E L D IN G 4 +
L O C A T IO N 2 3 2 + 2 + 4 + 5 + 6 + 8 + 5 + 5 + 2.5 + 4 +
Distortion of circumferential seams in shell • Caused by longitudinal shrinkage of weld • Reduction in diameter around circumferential seam CIRCSEAM JIINT
C/S SUGARCANE EFFECT SHELL
• 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 )
Gauge for checking long seam distortion in plate stage welding GAUGE FOR CHECKING
D
C
B
A
PICK IN OR PICK OUT = A-B OR C-D (MAXIMUM DIFFERENCE TO BE CONSIDERED)
Gauge for checking distortion of ‘T’- joint welding GAUGE FOR CHECKING A
B
C
PRE-TILT OF T-STIFFENER = A - C SAGGING OF T-STIFFENER = A - B
Distortion in flange to pipe welding FLANGE WARPS
TEMP. SUPPORTS
FLANGE
FLANGE FLANGE
PIPE
PIPE
•
PIPE
AFTER WELDING
BEFORE WELDING
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
Distortion During Oxyacetylene Cutting PLATE
STRIP
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
Controlling distortion during oxyacetylene cutting PLATE
TORCH I
STRIP
DIRECTION OF MOVEMENT FOR TORCHES SCRAP
10 mm
TORCH II
METHOD I Two Torches Technique •
Mark strip of required width leaving 10 mm distance
•
Move two torches simultaneously carrying out cutting operation
Controlling distortion during oxy - acetylene cutting PLATE STRIPS
KERF
HOLE
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
Controlling distortion during oxy - acetylene cutting 30 mm
12 mm THK PLATE
R250 mm 30 mm
PIERCE START
50 mm
Aim : To get undistorted segment from the plate of size as shown Specific Steps • Mark leaving 30mm Dist. from edge • Start with pierce cut as shown instead of starting from the edge • Follow the path as shown
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
Reduction in 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
