1. WELD DEFECTS ( WELD DISCONTINUITIES ), DISTORTION AND ITS CONTROLS
Ramanatha C.
Scientist ‘F’ (Retd.)
Gas Turbine Research Establishment
Bangalore

2. Types of Welding Fusion Welding Manual Metal Arc Welding
Shielded Metal Arc welding
Submerged Arc Welding
Gas Tungsten Arc Welding (GTAW) or (TIG)
Gas Metal Arc Welding (GMAW) or (MIG)
Flux Cored Arc Welding
Resistance Welding- Spot Welding and Seam Welding
Electrogas Welding (EGW)
Electron Beam Welding (EBW)
Laser Beam Welding
High Frequency Welding
Non – Fusion or Solid State Welding
Diffusion Welding
Ultrasonic Welding
Friction Welding

3. Definition
The word “Defect “ should be used carefully, as it implies that a weld is defective and requires corrective measures or rejection, In some cases repairs may be made unnecessarily and solely by implication without a critical engineering assessment. Consequently the engineering community has recently begun to use the word “Discontinuity” or “Flaw” instead of “Defect”, Discontinuity may be defined as interruptions in the desirable physical structure of a weld. The significance of a weld discontinuity should be viewed in the context of the fitness-for-purpose of the welded construction. Fitness-for-purpose is a concept of weld evaluation that seeks a balance between quality, reliability and economy of welding procedure. It is not a constant varies depending in the service requirements of a particular welded structure as well as on the properties of the material involved.

4. Classification of the Causes of Discontinuities
Design
related
Structural Details
Choice of the wrong type of weld joint For a given application
Undesirable changes in cross section
Welding process Related
Undercut
Slag inclusions
Porosity
Tungsten inclusion
Backing piece left on
Shrinkage voids
Oxide inclusions
Lack of fusion (LOF)
Lack of penetration (LOP)
Craters
Melt – through
Spatter
Arc Strikes (Arc burns)
Underfill
Metallurgical
Cracks
Fissures
Fisheye
Segregation
Lamellar Tearing

5. Design Related Discontinuity
Misalignment (hi-lo)
Amount of a joint is out of alignment at the root.
Causes:
  1. Carelessness.
2. Due to joining of different thicknesses
(Transition thickness)
Prevention:
  1. Workmanship
2. Transition angles not to exceed 2.5 to 1

6. Welding Process Related Discontinuities
Discontinuities resulting from welding process include:
Undercut: A groove melted into the base metal adjacent to the toe or root of a weld and left unfilled by weld metal.
Slag inclusion: Non metallic solid material entrapped in weld or between weld metal and base metal.
Porosity: Cavity-type discontinuities formed by gas entrapment during solidification.
Tungsten inclusions: Particles from Tungsten electrodes. Which results from improper gas Tungsten arc welding procedures.
Backing piece left on: Failure to remove material placed at the root of a weld joint to support molten weld metal.
Shrinkage voids: Cavity type discontinuities normally formed by shrinkage during solidification.
Oxide inclusions: Particles of surface oxides which have not melted and are mixed into the weld metal.

7. Welding Process Related Discontinuities Contd….
Lack of Fusion: A condition in which fusion is less than complete.
Lack of Penetration: A condition in which joint penetration is less than that specified.
Craters: Depression at the termination of a weld head or in the molten weld pool
Melt-through: A condition resulting when the arc melts through the bottom of a joint welded from one side.
Spatter: Metal particles expelled during welding which do not form a part of the weld.
Arc strikes (arc burns): Discontinuities consisting of any localized remelted metal, or change in the surface profile of any part of a weld or base metal resulting from an arc.
Underfill: A depression on the face of the weld or root surface of the adjacent base metal.

8. INCOMPLETE PENETRATION
This type of defect is found in any of three ways:
1)   When the weld bead does not penetrate the entire thickness of the base plate.
2)   When two opposing weld beads do not interpenetrate.
3)   When the weld bead does not penetrate the toe of a fillet weld but only bridges
across it.
Welding current has the greatest effect on penetration. Incomplete penetration is usually caused by the use of  too low a welding current and can be eliminated by simply increasing the amperage. Other causes can be the use of too slow a travel speed and an incorrect torch angle. Both will allow the molten weld metal to roll in front of the arc, acting as a cushion to prevent penetration. The arc must be kept on the leading edge of the weld puddle.

9. LACK OF FUSION
Lack of fusion, also called cold lapping or cold shuts, occurs when there is no fusion between the weld metal and the surfaces of the base plate.
Causes:
1. Poor welding technique.
2. Use of a very wide weld joint.
3. Very low travel speed and attempting to make too large a weld in
a single pass.
4. Low welding voltage.
5. Presence of Oxide layer.

10. INCOMPLETE PENETRATION & EXCESSIVE PENETRATION

11. Lack of Fusion

12. UNDERCUTTING
Undercutting is a defect that appears as a groove in the parent metal directly along the edges of the weld. It is most common in lap fillet welds, but can also be encountered in fillet and butt joints.
Causes:
1. Improper welding parameters; particularly the travel speed
and arc voltage.
2. Excessive welding currents.

13. Undercutting

14. Porosity
Porosity is gas pores found in the solidified weld bead. These pores may vary in size and are generally distributed in a random manner. However, it is possible that porosity can only be found at the weld centre. Pores can occur either under or on the weld surface.
The most common causes of porosity are atmosphere contamination, excessively oxidized work piece surfaces, inadequate deoxidizing alloys in the wire and the presence of foreign matter.
Atmospheric contamination can be caused by:
1)   Inadequate shielding gas flow.
2)   Excessive shielding gas flow. This can cause aspiration of air into the gas stream.
3)   Severely clogged gas nozzle or damaged gas supply system (leaking hoses, fittings, etc.)
4)   An excessive wind in the welding area. This can blow away the gas shield.

15. Porosity

16. Weld Defects, Their Possible causes and corrective action POROSITY
A. Inadequate shielding gas coverage
Corrective Actions
Increase the shielding gas flow to displace all air from the weld area
Decrease the shielding gas flow to avoid turbulence and entrapment of air in the gas
Remove Spatter from the interior of the gas nozzle.
Eliminate drafts (from fans, open doors, etc.) blowing into the welding arc.
Use a slower travel speed.
Reduce the nozzle-to-work distance.
Hold the gun at the end of the weld until the molten crater solidifiers

17. Weld Defects, Their Possible causes and corrective action POROSITY ( Contd..) Possible Causes Corrective actions
B. Electrode contamination C. Workpiece contamination D. Arc Voltage too high E. Excess nozzle -to- work distance
Use only clean and dry electrode wire
Eliminate pickup of lubricant on electrode in the wire feeder or conduit
Remove all grease, oil, rust, paint and dirt from work surfaces before welding.
Use a more highly deoxidizing electrode.
Reduce operating Voltage
Reduce electrode extension.

18. Weld Defects, Their Possible causes and corrective action Lack of penetration Possible Causes Corrective actions
Improper joint preparation
b. Improper welding Technique
c. Inadequate heat input
Joint design must be adequate to provide access to the bottom of the grove while maintaining proper nozzle-to-work distance and arc characteristics
Reduce root face height
Provide or increase the root opening in butt joints
Position the electrode at the proper travel angle to achieve maximum penetration
Keep the arc on the leading edge of the weld pool
Increase electrode feed to obtain higher welding currents
Maintain proper nozzle-to-work distance

19. Weld Defects, Their Possible causes and corrective action Excessive melt through Possible Causes Corrective actions
Reduce the electrode feed rate and arc voltage
Increase the travel speed
Reduce excessive root opening
Increase root face height
Excessive heat input
Improper joint preparations

20. Overlap

21. Under Fill

22. SPATTER

23. EXCESSIVE CONVEXITY

24. EXCESSIVE CONCAVITY

25. EXCESSIVE WELD REINFORCEMENT

26. UNACCEPTABLE WELD PROFILES

27. Metallurgical Discontinuities
Cracks: Fracture type discontinuities characterized by a sharp tip and high ratio of length and width to opening displacement.
Fissures: Small crack-like discontinuities with only a slight separation (opening displacement) of the fracture surfaces.
Fisheye: A discontinuity found on the fracture surface of a weld in steel that consists of a small pore or inclusion surrounded by a bright, round area.
Segregation: A non-uniform distribution or concentration of impurities or alloying elements which arises during the solidification of the weld.
Lamellar tearing: A type of cracking that occurs in the base metal or heat affected zone (HAZ) of restrained weld joints that is the result of inadequate ductility in through-thickness direction of steel plate.

28. Cracking
This can occur due just to thermal shrinkage or due to a combination of strain accompanying phase change and thermal shrinkage. In the case of welded stiff frames, a combination of poor design and inappropriate procedure may result in high residual stresses and cracking. Where alloy steels or steels with a carbon content greater than about 0.2% are being welded, self cooling may be rapid enough to cause some (brittle) martensite to form. This will easily develop cracks. To prevent these problems a process of pre-heating in stages may be needed and after welding a slow controlled post cooling in stages will be required. This can greatly increase the cost of welded joins, but for high strength steels, such as those used in petrochemical plant and piping, there may well be no alternative.

29. Factors Promoting Hot Cracking
Welding Current Density (High Levels promote cracking)
Heat Distribution (Joint design)
Restraint
Crack sensitivity of electrode material
Dilution of weld metal
Impurities (Eg. Sulphur & Phosphorous)
Preheating (Increase liability to cracking)
Welding Procedures (High Speeds, long arcs increase sensitivity)

30. Types of Cracking Solidification Cracking
This is also called centreline or hot cracking. They are called hot cracks because they occur immediately after welds are completed and sometimes while the welds are being made. These defects, which are often caused by sulphur and phosphorus, are more likely to occur in higher carbon steels.
A schematic diagram of a centreline crack is shown below:

31. Types of cracking (Contd..)
Hydrogen induced cracking (HIC)
It is also referred to as hydrogen cracking or hydrogen assisted cracking, can occur in steels during manufacture, during fabrication or during service. When HIC occurs as a result of welding, the cracks are in the heat affected zone (HAZ) or in the weld metal itself.
Four requirements for HIC to occur are:
a) Hydrogen be present, this may come from moisture in any flux or from other sources. It is absorbed by the weld pool and diffuses into the HAZ.
b) A HAZ microstructure susceptible to hydrogen cracking.
c) Tensile stresses act on the weld
d) The assembly has cooled to close to ambient - less than 150oC
HIC in the HAZ is often at the weld toe, but can be under the weld bead or at the weld root. In fillet welds cracks are normally parallel to the weld run but in butt welds cracks can be transverse to the welding direction.

32. Reheat Cracking Mo-V and Mo-B steels susceptible
Due to high temperature embrittlement of the heat-affected zone and the presence of residual stress
Coarse-grained region near fusion line most susceptible
Prevention by
Low heat input welding
Intermediate stress relief of partially completed welds
Design to avoid high restraint
Restrict vanadium additions to 0.1% in steels
Dress the weld toe region to remove possible areas of stress concentration

33. Lamellar Tearing
Occurs in thick plate subjected to high transverse welding stress
Related to elongated non-metallic inclusions, sulfides and silicates, lying parallel to plate surface and producing regions of reduced ductility
Prevention by
Low sulfur steel
Specify minimum ductility levels in transverse direction
Avoid designs with heavy through-thickness direction stress

34. Residual Stress & Distortion
Residual Stresses also referred to as internal stresses , initial stresses , inherent stresses reaction stresses and locked in stresses, arc stresses that continue to exist in a body even after the removal of all the external loads. Thermal stresses occur in parts during welding due to the localized application of heat. Residual stresses and distortion remain after the welding process is completed. Thermal stresses, residual stresses and distortion sometimes cause cracking and mismatching of joints. Correcting unacceptable distortion is costly and in some cases impossible.
Distortion in a weld results from the expansion and contraction of the weld metal and adjacent base metal during the heating and cooling cycle of the welding process.

35. Causes of residual Stresses
Residual stresses in metal structures occur for many reasons during various manufacturing stages. They may occur during forming and shaping of metal parts by processes such as shearing, bending, machining and grinding. They also occur during fabrication processes such as welding. Residual stresses are classified into two types: 1. Residual stresses produced by mismatch. When bars of different lengths are forcibly connected , tensile stresses are produced in the shorter bar and compressive stresses are produced in the longer bars. 2. Residual stresses produced by uneven distribution of non elastic strains. When materials are heated uniformly, thermal stress is not produced, as they also expand uniformly. However, residual stress is expected when materials are not heated uniformly. Also, residual stresses are produced when unevenly distributed non elastic strains, or plastic strains exist.
Before welding; Free state
After welding; Stressed state

36. Residual Stresses developed during welding
Fig : Residual stresses developed during welding of a butt joint.

37. Distortion after Welding
Fig : Distortion of parts after welding : (a) butt joints; (b) fillet welds. Distortion is caused by differential thermal expansion and contraction of different parts of the welded assembly.

38. Prevention or Minimization of distortion
Several ways can be used to minimize distortion caused by shrinkage:
Do not overweld
Control fitup.
Use intermittent Welding
Use as few weld passes as possible
Place welds near the neutral axis
Balance welds around the neutral axis
Use backstep welding
Anticipate the shrinkage forces
Plan the welding sequence
Remove shrinkage forces after welding
Minimize welding time
Use the smallest leg size permissible when fillet welding.

39. Prevention or Minimization of distortion (Contd..)
For groove welds, use joints that will minimize the volume of weld metal. Consider
double-sided joints instead of single-sided joints.
Weld alternately on either side of the joint when possible with multiple-pass welds.
Use low heat input procedures. This generally means high deposition rates and
higher travel speeds.
Use welding positioners to achieve the maximum amount of flat-position welding.
The flat position permits the use of large- diameter electrodes and high-
deposition-rate welding procedures.
Weld toward the unrestrained part of the member.
Use clamps, fixtures, and strongbacks to maintain fitup and alignment.
Prebend the members or preset the joints to let shrinkage pull them back into
alignment.
Sequence subassemblies and final assemblies so that the welds being made
continually balance each other around the neutral axis of the section.

40. Prevention or Minimization of distortion (Contd..)

41. Prevention or Minimization of distortion (Contd..)

42. Other Techniques for Distortion Control
Water-Cooled Jig Various techniques have been developed to control distortion on specific weldments. In sheet-metal welding, for example, a water-cooled jig is useful to carry heat away from the welded components. Copper tubes are brazed or soldered to copper holding clamps, and the water is circulated through the tubes during welding. The restraint of the clamps also helps minimize distortion.

43. Other Techniques for Distortion Control (Contd..)
Strongback
The "strongback" is another useful technique for distortion control during butt welding of plates. Clips are welded to the edge of one plate and wedges are driven under the clips to force the edges into alignment and to hold them during welding.

44. Other Techniques for Distortion Control (Contd..)
Thermal Stress Relieving (Heat Treatment)
Except in special situations, stress relief by heating is not used for correcting distortion. There are occasions, however, when stress relief is necessary to prevent further distortion from occurring before the weldment is finished.
Preheat
Preheat reduces the temperature differential between the weld region and the base metal
Reduces the cooling rate, which reduces the chance of forming martensite in steels
Reduces distortion and shrinkage stress
Reduces the danger of weld cracking
Allows hydrogen to escape

45. Interaction of Preheat and Composition
CE = %C + %Mn/6 + %(Cr+Mo+V)/5 + %(Si+Ni+Cu)/15
Carbon equivalent (CE) measures ability to form martensite, which is necessary for hydrogen cracking
CE < no preheat or postweld heat treatment
0.35 < CE < preheat
0.55 < CE preheat and postweld heat treatment
Preheat temp. ­ as CE ­ and plate thickness ­

46. Other Techniques for Distortion Control (Contd..)
Post-Weld Heat Treatment
The fast cooling rates associated with welding often produce martensite
During postweld heat treatment, martensite is tempered (transforms to ferrite and carbides)
Reduces hardness
Reduces strength
Increases ductility
Increases toughness
Residual stress is also reduced by the postweld heat treatment
Postweld Heat Treatment and Hydrogen Cracking
Postweld heat treatment (~ 1200°F) tempers any martensite that may have formed
Increase in ductility and toughness
Reduction in strength and hardness
Residual stress is decreased by postweld heat treatment
Rule of thumb: hold at temperature for 1 hour per inch of plate thickness; minimum hold of 30 minutes

47. Thank You