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Distortion in welding

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

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Distortion in welding

  2. 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. 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. 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. 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. 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. 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. 8 Schematic View of Distortions in Welding
  9. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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)
  23. 23. 23 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. 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. 25. Measurement of Distortion 25  Measuring Bending or Angular Distortion Figure: Measuring Angular Distortions or Bending Figure: Measuring Angular Distortions Figure: Measuring Bending
  26. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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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