Distortion in Weldling: types, measurements, suitability index, control and correction

1. SUBJECT : ANALYSIS OF MANUFACTURING
PROCESSES
DISTORTION IN
WELDING
1

2. CONTENTS
 Introduction: Distortion in Welding
 Significance of Material Properties
 Influence of Welding Processes & Procedures
 Types of Welding Distortions
 Welding Suitability Index based on Distortion
 Measurement of Distortion
 Control of Distortion in Weldments
 Correction of Distorted Weldments
 Future Scope in Measuring Weld Distortions
2

3. Introduction: Distortion in
Welding
Q. What is Distortion ?
 Any unwanted physical change or departure from
specifications in a fabricated structure or component, as
a consequence of welding
Figure: Distortion in Sheet due to Welding Figure: Simulation for T-Joint Welding
3

4. Introduction: Distortion in
Welding
 Main Causes of Distortion
 Non-Uniform Expansion and Contraction, i.e. Shrinkage
due to plastic thermal strain, of the weld metal and base
metal during the heating and cooling cycle
 Internal stresses formed in base metal due to removing
restraints given to welds by fixed components surrounding
it
So, both Welding processes & procedures and Material
properties
affect the extent of distortion
 Effects of Distortion:
 Complicate further fabrication
 Reduced application of the structure
4

5. Significance of Material
Properties5
Properties of Materials Effects
(Requirements for Less
Distortion)
Coefficient of Thermal
Expansion (α)
Lower coefficient of thermal expansion
Thermal Conductivity (K) High Thermal Conductivity leads to low
thermal gradients
Yield Strength (ơy) Lower the yield strength of the parent
material, lower the residual stresses causing
distortions
Modulus of Elasticity (E) Higher the Modulus of Elasticity (stiffness) of
the parent material

6. Influence of Welding Processes &
Procedures
6
Factors affecting
Volume of Heated
Metal
Effects
(Requirements for Less
Distortion)
Welding Processes •Concentrated heat source
•High welding speeds
•Deep penetration
•Single Pass Welding, Least Weld runs
Amount of Weld Metal •Minimum amount of weld metal
Welding Speed Maximum Welding speed Minimizes heat
spread and built-up, Solidification of weld
metal should be controlled
Edge Preparation and Fit-
up
Uniform Edge Preparations to allow consistent
shrinkage along the joint, Close Fit-Ups
Welding Procedure • Mechanised, Single Pass, High Speed

7. 7
TYPES OF
WELDING
DISTORTIO
NS
Longitudin
al
Shrinkage
Transverse
Shrinkage
Angular
Distortion
Longitudin
al
Distortions/
Bowing or
Bending
Rotational
Distortion
Buckling
and
Twisting

8. 8
Schematic View of Distortions in
Welding

9. Longitudinal Shrinkage
9
 Shrinkage in the direction of the weld axis
 Cause:
 Preheat or fast cooling problem
 Shrinkage stresses in high constraint areas
 Prevention:
 Weld toward areas of less constraint
 Weld short length
 Also preheat to even out the cooling rates
 Straightening press, jacks, clamps should be
used
Figure: Longitudinal
Shrinkage

10. Longitudinal Shrinkage
10
 Butt Welds
• ẟL= longitudinal shrinkage, mm
• I = welding current, amps
• T = length of the weld, mm
• t=plate thickness, mm
 Fillet Welds
• ẟL = longitudinal Shrinkage
• Aw = Cross-sectional area of the weld metal
• Ap = Cross-sectional area of the resisting structure
Figure: Butt Joint
Figure: T-joint with two fillet
welds

11. Transverse Shrinkage
11
 Shrinkage running into or inside a weld, transverse to the weld axis
direction
 Cause: Weld metal hardness problem,
Constraints applied to weld-joints
Figure: Transverse
Shrinkage
 Butt Welds :
 ẟt = transverse Shrinkage
 ∆w = Cross-sectional area of weld,
mm2
 t = plate thicknes, mm
Figure: Butt Joint

12. Transverse Shrinkage
12
 Fillet Weld :
 For a T-joint with two fillet welds :
 ẟt = transverse Shrinkage
 l= leg of fillet weld, mm
 t = plate thickness, mm
 For fillet weld(s) in Lap Joint :
 ẟt = transverse Shrinkage
 l= leg of fillet weld, mm
 t = plate thickness, mm
Figure: T-joint with two fillet
welds
Figure: Fillet weld in Lap Joint

13. Longitudinal Vs Transverse
Shrinkage13
Longitudinal Shrinkage Transverse Shrinkage
Butt
Welds
• 3mm per 3m of weld • 1.5 to 3mm per weld for 60°
V joint, depending on number
of runs
• Amount of transverse shrinkage in a butt weld is much more (i.e.
1000th times of the weld length) than the longitudinal shrinkage
Fillet
Welds
• 0.8mm per 3m of weld • 0.8mm per weld where the
leg length does not exceed 3/4
plate thickness
• Increasing the leg length of fillet welds increases shrinkage

14. Angular Distortion
14
 Weld tends to be wider at the top than
the bottom, causing more solidification
shrinkage and thermal contraction
 For Double-V Edge Butt weld-joint, it
depends upon root face and root gap
 Fillet weld-joints, it depends upon
flange width, weld leg length and
flange thickness
 Depends Upon :
 Width and depth of fusion zone relative
to plate thickness
 Type of joint
 Weld pass sequence
 Thermo-mechanical material properties
 Heat input per unit length of weld,
Figure: Angular Distortion in Butt Weld-
joint
Figure: Angular Distortion in Fillet Weld-
Joint

15. Angular Distortion
15
 Occurs at butt, lap, T, corner joints due to single-sided as well as
asymmetrical double-sided welding
 Prevention:
 Reducing volume of weld metal
 Using double-V joint and alternate welding
 Placing welds around neutral axis
 Presetting: By compensating the amount of distortion to occur in
welding
 Elastic pre-springing can reduce angular changes after restraint
is removed.
 Preheating and post weld treatment

16. Bowing or Longitudinal
Bending16
A = cross-sectional area of the weld,mm2
d = distance from C.G. to outermost fibre, mm
L = length of the weld, mm
I = Moment of Inertia of the section, mm4
Figure: Longitudinal Bending
 Weld line does not coincide with neutral axis of a weld
structure
 Longitudinal shrinkage of the weld metal induces bending
moments
 Amount of distortion depends on :
 Shrinkage moment
 Resistance of the member to bending

17. Rotational Distortion
17
 In this, sheets being butt welded either come closer to each other or
the distance between them is widened
 Depends upon:
 Thickness of parent material
 Temperature difference between a molten pool and the unheaten parent
material (difference in heat flow)
 Speed of Welding,
 Heat Source
Figure: Rotational Distortions

18. Rotational Distortion
18
Progressively welding
material at
widely different heat inputs
Expanding & Contracting Zones in
arc butt welding
Here, Manual welds are termed as slow welds, while Automatic
welds are termed as fast welds

19. Buckling Distortions
19
 When thin plates are welded, considerable residual stresses occur in
areas away from the weld and cause “Buckling”
 Occurs when Specimen Length exceeds the Critical Length for a given
thickness
 Amount of deformation of Buckling distortion is much greater than that
in Bending
 Buckling due to welding of a panel increases directly as the thickness
decreases
Figure: Bucking Distortion Figure: Relationship for buckling
distortion of butt weld for different

20. Twisting Distortions
20
When a weld is made along the centre
of a member, the weld area tends to
shrink and become shorter
To satisfy the conditions of a member that
has outer edges longer than its centreline,
the member must twist
 Twisting is the due to low torsional resistance on thin
materials

21. Buckling And Twisting
21
 Prevention:
 Minimize Shrinkage by decreasing volume of weld
metal and highest compatible speed
 Keep the length of the welded member as short as
practical
 Incorporate torsional resistances to twisting as much
feasible

22. Welding Suitability Index
22
 Welding Suitability Index based on Distortion
(λƐ)
where,
Tm, a, α, E, ơy, refers to material under consideration
Tm*, a*, α*, E*, ơy
* refers to those of reference material
Tm: Melting Temperature, (°C)
a : Thermal Diffusivity, (mm2 / sec)
α : Thermal Expansion, (1/°C) *10-6
E : Elastic Modulus, (kN/mm2)
ơ : Yield Limit, (N/mm2)

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0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4 5 6 7 8
Welding Suitability Indices in
Distortion
Welding Suitability
Indices in Distortion
Base Metal
Melting
Temperature,
Tm (°C)
Thermal
Diffusivity, a
(mm2 / sec)
Thermal
Expansion, α
(1/°C) *10-6
Elastic
Modulus, E
(kN/mm2)
Yield Limit,
ơy, (N/mm2)
Welding
Suitability
Indices in
Distortion
Low Alloy Steel 1520 7.5-9.5 11 210 200-700 1
High Alloy Steel 1400 5.0-7.5 16 200 250-550 0.86
Aluminium Alloy 600 75-100 24 65 80-280 0.01
Titanium Alloy 1800 6 8.5 110 500-700 1.08
Copper Alloy 1080 120 18 130 30-420 0.02
Nickel Alloy 1435 15 13 215 120-630 0.43

24. Measurement of Distortion
24
 Distortion in the post weld cooled state is determined by
applying length and angular measuring techniques
 Transverse and Longitudinal Shrinkage are determined by
Measuring Tape
 Angular Shrinkage is measured on a measuring plate by
means of straight edge set agaisnt the component (as shown
in below figure)
Figure: Measuring Longitudinal
& Transverse Shrinkage
Figure: Measuring Angular Distortions

25. Measurement of Distortion
25
 Measuring Bending or Angular Distortion
Figure: Measuring Angular Distortions or
Bending
Figure: Measuring Angular Distortions
Figure: Measuring Bending

26. Measurement of Distortion
26
 Circumferential
measurements on spherical
and cylindrical shells are
performed by string wrapped
around the structure
 Vertically extended
components, e.g. Pillars,
supports and tank walls,
inclinations and deflections
are measured by means of
strings hanging exactly
vertically and tensioning
weight immersing in water
Figure: Distortions in Circumferential Figure: Distortions in vertically Extended

27. Measurement of Distortion
27
 Linear Variable Differential Transformer (LVDT)
Figure: LVDT set-up with Workpiece
Dimensions
Figure: Anticipated displacements
Figure: Measured results (FEM vs LVDT)

28. Measurement of Distortion
28
 Small Scale Distortions using a Stereoscopic
Video Imaging system
Figure: 3d deformation measurement using a stereoscopic video imaging
system

29. Control of Distortion in
Weldments29
 Welding Residual stresses and Welding Distortion behave in
a contrary way
 Least root gap:
 As small as possible, but sufficient for good penetration
 Excessive gaps should be avoided
 Included angle should not exceed 60°
 For heavy sections, double-V preparation should be preferred

30. Control of Distortion in
Weldments30
 Tack Welding
 Sufficiently long tack welds
transmit shrinkage forces
 Tack weld length should be
two-three times the plate
thickness
 Preheating, slag removal and
further defect removal
methods are employed to
counter undesired
phenomenon due to tack weld
 Narrow Groove Section in
Welding
 Least as possible to produce least
heat concentration
 U shape groove is preferable than
Vee shape
 Symmetrical weld groove reduces
angular shrinkage, but residual
stresses are increased
 Double-sided fillet weld is selected
over single-sided fillet weld

31. Control of Distortion in
Weldments31
 Direction of Welding :
 Away from the point of restraint and towards the point of maximum
freedom
 Weld Metal Deposited :
 No excess metal should be deposited
 Block Sequence and Cascade Sequence :
 To deposit long welds of high thickness
 Layer deposited until the effective throat thickness is achieved
Figure: Block
Sequence
Figure: Cascade
Sequence

32. Control of Distortion in
Weldments32
 Welding Sequnce :
 For large surface area consisting of several
plates, transverse seams should be welded
first followed by longitudinal seams
 In welding I- or H- beam joints within each
web plate and flange are to welded first,
followed by butt joints between web plates
and flanges of a beam
Figure: Welding Sequence
for large plates
Figure: Welding Sequence for I or H Beam

33. Control of Distortion in
Weldments33
 For cylindrical vessel, longitudinal seams
should be welded first, followed by the
circumferential seams
 In welding frames of different length and thicknesses, least
distortionwould result if weld 1 & 2 are done simultaneously followed by
3 & 4, as shown in given figure
Figure: Welding Sequence for cylindrical
vessel
Figure: Various Welding
Sequence for Welding
Frames

34. Control of Distortion in
Weldments34
 Back- Step Welding Sequence :
 Measure to counteract the wedge shaped-opening and closing(rotational
distortion)
 Reduces transverse and longitudinal shrinkage
 Used widely in fabrication of large structures, such as ships, storage
tanks
Figure: Back-Step Welding Sequence

35. Control of Distortion in
Weldments35
 Counter or Opposing Set-up
Figure: Warpage in a T-beam and
Suggested Counter setup
Figure: Counter Set-up for Angular Distortion

36. Control of Distortion in
Weldments36
 Distortion control in Thin Plates and Sheets
 Used in light gauges
 Copper abstract heat from weld
reducing heating and warpage or
buckling of the plates
 Water-cooled jig, Copper Clamps,
Copper tubes used
Figure: Water Cooled Jig for rapid removal of
heat to control distortion in welding shheet
metal
 Fixing :
 Fixing parts, to be joined by welding, in a frame or rigidly as possible
 To reduce back-spring shrinkage

37. Correction of Distorted
Weldments37
 If a weldment warps despite the precautions taken, there are
ways and means of correcting the defect using one of the
following two methods:
Methods for Correction of
Distorted Weldments
Mechanical
Methods
Presses, Jack Screws
, Straightening Rolls,
Sledges, Special
Fixtures
Thermal
Methods
Oxy-
acetylen
e torch
Carbon
Arc
Powerful
oil or
gas
burners

38. Future Scope
38
 Artificial Neural Networks used to measure the distortion
more precisely
 Mechanised techniques with proper simulation can give
least distortion in the welded product

39. References
39
 R. S. Parmar, Welding Engineering and Technology, Khanna
Publishers, 2010
 Zhili Fen, Processes and mechanisms of welding residual
stress and distortion, 2005, Pg 209-216
 airproducts.com

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