Distortion Control methods

DISTORTIONDISTORTION

NON UNIFORM EXPANSION AND NON UNIFORM EXPANSION AND CONTRACTION OF WELD METAL CONTRACTION OF WELD METAL AND PARENT METAL AND PARENT METAL

TYPES OF DISTORTION TYPES OF DISTORTION LONGITUDINAL LONGITUDINAL TRANSVERSE TRANSVERSE ANGULAR ANGULAR BUCKLING BUCKLING TWISTINGTWISTING BENDINGBENDING LOCALLOCAL

TYPE OF DISTORTIONTYPE OF DISTORTIONSHRINKAGE IS PARALLEL TO THE WELD AXIS-- SHRINKAGE IS PARALLEL TO THE WELD AXIS--

LONGITUDINALLONGITUDINALSHRINKAGE IS PERPENDICULAR TO WELD AXISSHRINKAGE IS PERPENDICULAR TO WELD AXIS

—TRANSVERSE—TRANSVERSECHANGE IN THE ANGLE -ANGULARCHANGE IN THE ANGLE -ANGULARWHILE WELDING THIN SHEETS USING MMAW-- WHILE WELDING THIN SHEETS USING MMAW--

BUCKLINGBUCKLINGIN BOX STRUCTURES AND DUCTS--TWISTINGIN BOX STRUCTURES AND DUCTS--TWISTINGIN PANEL ASSEMBLIES--BENDINGIN PANEL ASSEMBLIES--BENDINGIN A PARTICULAR PLACE A DENT OCCURS-IN A PARTICULAR PLACE A DENT OCCURS-

DENTSDENTS

Longitudinal distortion Types of distortion

d

d

N A

Transverse distortion

Types of distortion

Angular distortion

Types of distortion

Buckling distortion

Types of distortion

WHY TO BE MINIMIZEDWHY TO BE MINIMIZED

MISMATCHINGMISMATCHING REPAIR/ REWORK TIME AND COSTREPAIR/ REWORK TIME AND COST MAY AFFECT THE FUNCTIONAL MAY AFFECT THE FUNCTIONAL

REQUIREMENTS AND LOAD CARRYING REQUIREMENTS AND LOAD CARRYING CAPACITYCAPACITY

AESTHETICS IS LOSTAESTHETICS IS LOST

FACTORS INFLUENCING FACTORS INFLUENCING PROCESS RELATED PARAMETERSPROCESS RELATED PARAMETERS GEOMETRY RELATED PARAMETERSGEOMETRY RELATED PARAMETERS MATERIAL RELATED PARAMETERSMATERIAL RELATED PARAMETERS FABRICATION RELATED PARAMETERSFABRICATION RELATED PARAMETERS

Factors Affecting DistortionFactors Affecting Distortion If a metal component was If a metal component was uniformly heated and cooled there would uniformly heated and cooled there would

be uniform tri-axial expansion during be uniform tri-axial expansion during heating followed byheating followed by

uniform tri-axial contraction during uniform tri-axial contraction during cooling and cooling and

no thermal distortionno thermal distortion evident upon return evident upon return to ambient temperature. to ambient temperature.

PROCESS PROCESS HEAT INPUT RATEHEAT INPUT RATE NO OF PASSESNO OF PASSES PROCESS EFFIENCYPROCESS EFFIENCY PENETRATIONPENETRATION

GEOMETRY GEOMETRY LENGTHLENGTH THICKNESSTHICKNESS AREA OF CROSS SECTIONAREA OF CROSS SECTION SYMMETRIYSYMMETRIY

MATERIAL MATERIAL CO-EFFICIENT OF THERMAL EXPANSIONCO-EFFICIENT OF THERMAL EXPANSION THERMAL CONDUCTIVITYTHERMAL CONDUCTIVITY MICRO STRUCTUREMICRO STRUCTURE YIELD STRENGTH YIELD STRENGTH

Carbon steel expands less than stainless steel Carbon steel expands less than stainless steel which expands less than aluminium which expands less than aluminium

Carbon steel expands < stainless steel expands Carbon steel expands < stainless steel expands <aluminium <aluminium

Lower thermal expansion lower distortion Lower thermal expansion lower distortion

Higher thermal expansion higher distortion Higher thermal expansion higher distortion

Specific heat per unit volumeSpecific heat per unit volume

Specific heat, Specific heat, cc, is the amount of energy , is the amount of energy required to raise the temperature of one required to raise the temperature of one kilogram of the substance by one kelvin kilogram of the substance by one kelvin and is expressed as expressed as joule per and is expressed as expressed as joule per kilogram Kelvin. kilogram Kelvin.

Lower specific heat lower distortion Lower specific heat lower distortion Higher specific heat higher distortion Higher specific heat higher distortion

Thermal conductivity coefficientThermal conductivity coefficient

The coefficient of thermal conductivity, is a The coefficient of thermal conductivity, is a measure of the rate at which heat flows measure of the rate at which heat flows through a material.through a material.

CONTROL OF DISTORTION CONTROL OF DISTORTION DESIGN STAGEDESIGN STAGE FABRICATION STAGEFABRICATION STAGE CORRECTION STAGECORRECTION STAGE

FABRICATION FABRICATION ROOT GAP ROOT GAP TACK WELDTACK WELD JOINT DESIGNJOINT DESIGN FIXTURESFIXTURES PRE-HEATINGPRE-HEATING

DESIGN STAGEDESIGN STAGE MINIMUM WELDINGMINIMUM WELDING PROPER JOINT PREPARATION &FITUPPROPER JOINT PREPARATION &FITUP INTERMITTENT WELDS IN LIEW OF INTERMITTENT WELDS IN LIEW OF

CONTINUOUS WELDSCONTINUOUS WELDS AVOID OVER SIZE WELDINGAVOID OVER SIZE WELDING PLACE WELD NEAR NUETRAL AXISPLACE WELD NEAR NUETRAL AXIS

Prevention of Distortion by DesignPrevention of Distortion by Design

EliminationElimination of welding by of welding by Forming the plate,Forming the plate, Use of rolled or extruded Use of rolled or extruded

sections,sections, intermittent welds, attaching intermittent welds, attaching

stiffening plates,stiffening plates,

Weld placementWeld placement Place the welds around the Place the welds around the

neutral axis,neutral axis, Balance welds intermittently Balance welds intermittently

on either side of joint,on either side of joint, Weld alternatively on Weld alternatively on

opposite sides of the joint,opposite sides of the joint,

Prevention of Distortion by DesignPrevention of Distortion by Design

Reducing volume of weld MetalReducing volume of weld Metal Shrinkage is proportional to the Shrinkage is proportional to the

amount of weld deposit,amount of weld deposit, Reduce angle of V – joint,Reduce angle of V – joint, Reduce no. of passes to reduce Reduce no. of passes to reduce

angular distortion,angular distortion, Cross-sectional area of double –V Cross-sectional area of double –V

nearly half that of single-V,nearly half that of single-V,Use of balanced weldingUse of balanced welding The adjacent figure gives an The adjacent figure gives an

effective means of controlling effective means of controlling distortion in multi-pass butt distortion in multi-pass butt welding by arranging the welding welding by arranging the welding sequencesequence

Joint designJoint design Locate weld near NALocate weld near NA

FABRICATION STAGEFABRICATION STAGE WELD FEWER PASSES WITH LARGER SIZE WELD FEWER PASSES WITH LARGER SIZE

ELECTRODE.ELECTRODE. BACK STEP TECHNIQUE.BACK STEP TECHNIQUE. BACK SKIP TECHNIQUEBACK SKIP TECHNIQUE WELD FROM CENTRE TOWARDS END.WELD FROM CENTRE TOWARDS END. USE JIGS & FIXTURES.USE JIGS & FIXTURES. WELDING ALTERNATIVELY ON BOTH SIDES OF WELDING ALTERNATIVELY ON BOTH SIDES OF

BUTT WELDS.BUTT WELDS. BALANCING INTERMITTENT WELDS.BALANCING INTERMITTENT WELDS. PRESET THE COMPONENTS.PRESET THE COMPONENTS. PREBENDING.PREBENDING. BREAK WORK INTO SIMPLE COMPONENTS.BREAK WORK INTO SIMPLE COMPONENTS.

Use of pre-setting & pre-bendingUse of pre-setting & pre-bending

Pre-setting of parts Pre-setting of parts To achieve correct alignment To achieve correct alignment after welding in order to prevent after welding in order to prevent angular distortion in angular distortion in a) fillet joints b) butt jointsa) fillet joints b) butt joints

Also pre-set Also pre-set Tapered gap Tapered gap to to prevent closureprevent closure

Pre-bending of parts Pre-bending of parts Using Using press breaks and wedges to press breaks and wedges to accommodate angular distortion accommodate angular distortion in thin platesin thin plates

1 2 3 4

Back Step Welding

Intermittent WeldingPositioning welds

12

3

5

4

6

78

1

2

3

4

5

6

7

8

Distortion control methods

1 2 3 4 5 6 7 8 9 10

10

123456

87

9

11

Weld Pass

Minimize number of passes

Prevention of Distortion by Fabrication Prevention of Distortion by Fabrication TechniqueTechnique

Longitudinal stiffeners welded along Longitudinal stiffeners welded along each side of the butt-welded seam each side of the butt-welded seam prevents bowingprevents bowing

Back-to-back assembly – clamping Back-to-back assembly – clamping two identical components back-to-two identical components back-to-back for tacking and weldingback for tacking and welding

Use balanced welding about the Use balanced welding about the neutral axisneutral axis

Keep the weld deposit to the Keep the weld deposit to the minimum specified size minimum specified size

Use MIG in preference to MMAUse MIG in preference to MMA Keep time between runs to a Keep time between runs to a

minimum and deposit weld metal as minimum and deposit weld metal as quickly as possiblequickly as possible

Use least number of runs to fill the Use least number of runs to fill the jointsjoints

CORRECTION STAGECORRECTION STAGE THERMAL / FLAME STRAIGHTENING.THERMAL / FLAME STRAIGHTENING. PRESSING / MECH. STRAIGHTNING.PRESSING / MECH. STRAIGHTNING. PREHEATING.PREHEATING.

Distortion correction techniquesDistortion correction techniques

Mechanical techniquesMechanical techniques HammeringHammering PressingPressing

Distortion Correction TechniquesDistortion Correction Techniques

Thermal technique by Thermal technique by localised Heatinglocalised Heating

Spot heating – used mainly to Spot heating – used mainly to remove buckling in thin sheet remove buckling in thin sheet

Line heating – components Line heating – components heated along the line of weld heated along the line of weld to correct angular distortionto correct angular distortion

Restrict the area of heating Restrict the area of heating to avoid over-shrinkage of to avoid over-shrinkage of the componentthe component

Distortion Correction TechniquesDistortion Correction Techniques

Wedge shaped heatingWedge shaped heating Used to correct distortion in Used to correct distortion in

large complex fabrication large complex fabrication including box-shaped structuresincluding box-shaped structures

For the heat to penetrate evenly For the heat to penetrate evenly through the plate thickness, through the plate thickness, heat from the baseheat from the base

Limit the temperature to 650Limit the temperature to 65000C C to prevent metallurgical damage to prevent metallurgical damage and to avoid over shrinkageand to avoid over shrinkage