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SUBJECT : ANALYSIS OF MANUFACTURING PROCESSES

DISTORTION IN WELDING

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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

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Introduction: Distortion in WeldingQ. 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

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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 High cost of rectifying deformations

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Significance of Material Properties

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

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Influence of Welding Processes & Procedures

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 Welds•Carried out from clamped to a free end

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TYPES OF WELDING DISTORTI

ONS

Longitudinal Shrinkage

Transverse Shrinkage

Angular Distortion

Longitudinal

Distortions/ Bowing or Bending

Rotational Distortion

Buckling and

Twisting

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Schematic View of Distortions in Welding

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Longitudinal Shrinkage 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

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Longitudinal Shrinkage 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

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Transverse Shrinkage 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, mmFigure: Butt Joint

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Transverse Shrinkage 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

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Longitudinal Vs Transverse Shrinkage

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

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Angular Distortion 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,

distribution of heat source density

Figure: Angular Distortion in Butt Weld-joint

Figure: Angular Distortion in Fillet Weld-Joint

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Angular Distortion 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

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Bowing or Longitudinal Bending

A = cross-sectional area of the weld,mm2

d = distance from C.G. to outermost fibre, mm

L = length of the weld, mmI = 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

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Rotational Distortion 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

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Rotational DistortionProgressively 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

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Buckling Distortions 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 aspect ratio

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Twisting Distortions

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

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Buckling And Twisting 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

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Welding Suitability Index Welding Suitability Index based on Distortion (λƐ)

where,Tm, a, α, E, ơy, refers to material under considerationTm*, a*, α*, E*, ơy

* refers to those of reference material

Tm: Melting Temperature, (°C)a : Thermal Diffusivity, (mm2 / sec)α : Thermal Expansion, (1/°C) *10-6E : Elastic Modulus, (kN/mm2)ơy, : Yield Limit, (N/mm2)

23 0 1 2 3 4 5 6 7 80

0.20.40.60.8

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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 1High Alloy

Steel 1400 5.0-7.5 16 200 250-550 0.86Aluminium

Alloy 600 75-100 24 65 80-280 0.01Titanium Alloy 1800 6 8.5 110 500-700 1.08Copper Alloy 1080 120 18 130 30-420 0.02Nickel Alloy 1435 15 13 215 120-630 0.43

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Measurement of Distortion 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

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Measurement of Distortion Measuring Bending or Angular Distortion

Figure: Measuring Angular Distortions or Bending

Figure: Measuring Angular Distortions

Figure: Measuring Bending

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Measurement of Distortion 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 surfaces

Figure: Distortions in vertically Extended components

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Measurement of Distortion Linear Variable Differential Transformer (LVDT)

Figure: LVDT set-up with Workpiece Dimensions

Figure: Anticipated displacements

Figure: Measured results (FEM vs LVDT)

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Measurement of Distortion Small Scale Distortions using a

Stereoscopic Video Imaging system

Figure: 3d deformation measurement using a stereoscopic video imaging system

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Control of Distortion in Weldments 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

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Control of Distortion in Weldments 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

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Control of Distortion in Weldments 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

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Control of Distortion in Weldments 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

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Control of Distortion in Weldments

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

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Control of Distortion in Weldments 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

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Control of Distortion in Weldments Counter or Opposing Set-up

Figure: Warpage in a T-beam and Suggested Counter setup

Figure: Counter Set-up for Angular Distortion

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Control of Distortion in Weldments Distortion control in Thin Plates and Sheets

Used in light gauges Copper abstract heat from weld

reducing heating and warpage orbuckling 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

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Correction of Distorted Weldments 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 MethodsPresses, Jack Screws ,

Straightening Rolls, Sledges,

Special Fixtures

Thermal MethodsOxy-acetylen

e torc

h

Carbon Arc

Powerful oil or

gas burners

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Future Scope Artificial Neural Networks used to measure the

distortion more precisely Mechanised techniques with proper simulation can

give least distortion in the welded product

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References 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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