Distortion & control in welding parameters.pptx

Whole lot about distortion, it's types,control

1. DISTORTION IN WELDING Prepared by - Urvisha Kharva Department of Metallurgical & Material Engineering MSU Vadodara

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 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 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 4

5. Significance of Material Properties 5 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 ( ơ y ) 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 Welds

7. 7 TYPES OF WELDING DI S T O R T I O NS Long it ud in al S h ri n ka g e Transverse Shrinkage Angular D i storti o n Longitudin al D i storti o ns/ Bowing or Bending R o tati o n a l Distortion B u ckl i ng and Twisting Types of distortion Transverse Shrinkage

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 Figure: Butt Joint

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, mm 2 t = plate thicknes, mm Figure: Butt Joint

12. Longitudinal Vs Transverse Shrinkage 13 Longitudinal Shrinkage Transverse Shrinkage Butt W elds 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. 1000 th 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

13. 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, distribution of heat source density Figure: Angular Distortion in Butt Weld- joint Figure: Angular Distortion in Fillet Weld- Joint

14. 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

15. Bowing or Longitudinal welding 16 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 A = cross-sectional area of the weld,mm 2 d = distance from C.G. to outermost layer , m m L = length of the weld, mm I = Moment of Inertia of the section, mm 4

16. 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

17. 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

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

19. 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

20. 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

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

22. Measurement of Distortion 26 Circumferential measurements on spherical and cylindrical shells are performed by string wrapped around the structure immer s i 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 ng in water Figure: Distortions in Circumferential surfaces Figure: Distortions in vertically Extended components

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

24. Control of Distortion 29 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

25. Control of Distortion 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 30 phenomenon due to tack w 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

26. 31 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 Sequen c e Figure: Cascade Sequence Control of Distortion

27. 32 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 Control of Distortion

28. 34 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 Control of Distortion

29. 35 Counter or Opposing Set-up Figure: Warpage in a T-beam and Suggested Counter setup Figure: Counter Set-up for Angular Distortion Control of Distortion

30. Correction of Distorted Weldments 37 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 Mech a nical Methods Presses, Jack Screws , Straightening Rolls, Sledges, Special Fixtures Thermal Methods Oxy- ace t y len e torch Carbon Arc P o w e rf u l oil or gas burners

31. 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

32. 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

33. Thank you