Published on 01-Apr-2024

Non-destructive Testing of Welds

Non-destructive Testing of Welds

Image Credit @Zetec

Introduction 

The structural integrity of metal joints is crucial in ensuring the quality and integrity of metal objects, structures, and machines. The increased use of metals through the ages led to the onset of primitive welding techniques like pressure welding of metal joints. With the advent of the Iron Age, welding techniques were used to create joints in iron parts. Hammering techniques to create weld joints were eventually developed as people developed smithing skills.

Acetylene was synthesized in the 1800s by adding water to potassium carbide, which resulted in the formation of a luminous flame. This concept was eventually harnessed in the form of an arc. Electrodes made of metal were used to create an arc in welding processes devised in the 1890s. 

Post this discovery Carbon Arc Welding was discovered in 1880, soon followed by Metal Arc Welding in 1890. The 1900s bought the discovery of coated metal electrodes which expanded the domain of welding technologies as they increased arc stability.

Newer varieties of welding joints such as spot, seam, projection, and butt welding could be created with the discovery of resistance welding techniques. Gas welding also caught momentum during this period. The world war created an increased demand for metal-based products, which in turn required extensive welding procedures. This high demand led to intense research, development, and utilization of welding techniques all over the world.

Welding processes began to be automated post-1920s, to be used by manufacturing units, especially in the automobile sector, and for repair and restoration of working parts. Atomic Hydrogen welding processes were developed after much research following the exponentially increasing requirements for efficient metal joints. This process was used for niche procedures and the manufacture of tooling equipment. 

Newer automatic processes were further developed in the 1930s, such as Stud welding, smothered arc welding, and submerged arc welding. The 1940s, led to the inception of welding processes such as gas metal arc welding and gas tungsten arc welding, the latter of which became one of the most popular welding techniques of modern times. This was followed by the discovery of Electrogas welding, Plasma arc welding, and Electron beam welding processes in the 1950s to 1960s.

Welding is a machine process that has remained vital in the engineering industry and the growth of human technology for centuries. Research, requirements, and novel techniques in this domain are constantly being updated. Techniques like Laser welding and friction welding are some of the most recent welding techniques in the industry and the utilization and need for such technology and manufacturing processes will exponentially increase with time.

Welding processes are crucial to many industries like Construction, Manufacturing, etc. and defects in the weld can cause catastrophic losses to human life and structures built with great effort and expense. Hence the integrity and quality of weld joints require thorough analysis and monitoring, for which non-destructive testing is the most efficient and meticulous technique.


Types Of Welding Joints

Welding has historically been dominated by the manual crafting of metal joints, which requires years of experience and high levels of skill from the operator. The different types of joints utilized may affect the structural integrity and function of the workpiece to be machined.

The operator is required to have a thorough knowledge of the material characteristics, its behavior under temperature and pressure, the load to be applied on the joint, the welding apparatus, and the appropriate joint for the workpiece function.

The type of fitting and alignment of the pieces of metals to be joined by welding affect the expense, load-bearing capability, and overall quality of the product. Some of the welding joints that are preferred as per the standards set by the American Welding Society are:

  • Butt Joint
  • Tee Joint
  • Corner Joint
  • Lap Joint
  • Edge Joint

Types Of Welding Techniques

The basic concept followed by welding techniques is to increase the temperature of the joining points of two metals (similar or not) to a temperature high enough to melt them and cause fusion. 

This process sometimes requires the application of pressure, or the use of filler materials such as tin, lead, silver, aluminum, nickel, gold, copper, etc. Filler materials are available in the form of a paste or as a solid.

Welding processes can be divided into two types, namely Plastic processes, and Fusion processes. 

Plastic processes include processes like Force welding and resistance welding, out of which the latter can be further divided into Spot welding, Butt welding, Seam welding, and Percussion welding.

Fusion processes include processes like Thermit welding, Arc welding, and Gas welding. Arc welding, in turn, includes Metal Arc welding, Atomic Hydrogen welding, and Carbon Arc welding techniques.

Welding techniques can further be classified as per the method of heating the metal, which is as follows:

  • TIG welding
  • MIG welding
  • Flux cord arc welding
  • Plasma Arc welding
  • Electron Beam welding
  • Laser Beam welding
  • Stick welding
  • Gas welding
  • Atomic Hydrogen welding
  • Electroslag welding

Further Reading, Preheating In Welding 

Types Of Defects In Welding 

The welding process is highly procedural and is affected by errors in technique and lack of knowledge that may result in using an unsuitable filler material for the base or parent material. 

These erroneous elements in the weld zone, which are created during the welding process itself may cause failure and defects when they cross a certain pre-set limit. The presence of such deformities does not permit the finished product to operate at its intended capacity.

The American Society of Mechanical Engineers standards have classified the causes of welding defects as per their percentages as follows:

Welding defects can be debilitating to the functionality of the workpiece. However, not all deformities or deviations from normal can be considered a defect. The discontinuity should pass acceptance criteria to be considered a defect.

In the event of the detection of a defect, the workpiece or test subject should be rejected or reworked, if possible. Restorative measures can be taken to ensure the error does not occur again and reworking of the defective workpiece should be conducted only after approval from a trainer or certified expert.

Based on the appearance of the defect, the various welding defects can be classified as follows:

  • Cracks
  • Crater Cracks
  • Hot Cracks
  • Cold Cracks
  • Cavities
  • Blow Holes
  • Porosities
  • Shrinkage 
  • Pipes
  • Incomplete Fusion and Penetration
  • Lack of fusion
  • Lack of penetration
  • Solid Inclusions
  • Slag 
  • Metal Oxides
  • Tungsten
  • Wagon Cracks
  • Imperfect Shape
  • Undercut
  • Underfill
  • Overlaps
  • Excessive Penetration
  • Improper Bead Shape
  • Miscellaneous Defects
  • Arc Strike
  • Excessive Splatter
  • Rough Surface
  • Uneven Ripples

Defects such as surface cracks, overlaps, excessive penetration, arc strike, splatter, etc show up on the exterior or the surface of the workpiece and can be detected by non-destructive testing techniques that can detect surface defects.

Whereas defects such as lack of penetration, porosities, sub-surface defects, blow holes, inclusions, etc show significant characteristics on the interior of the workpiece and are considered hidden defects. Methods that can penetrate the workpiece without damaging it or affecting its operation are used for the NDT inspection of such defects.


Types Of Non-Destructive Testing Methods For Weld Analysis

Welding processes, and the industries that utilize such techniques and technology, rely heavily on reliability, accuracy, and costs. Errors in welds may often lead to catastrophic failures, losses, or product recalls. The waste of man hours, raw materials, and resources in such scenarios also lays a heavy load on the productivity and profits of such industries.

Non-destructive testing methods are characterized by their ability to ensure continued operation, production, and/or load-bearing without any hindrances post the NDT analysis process. Development and research in such techniques have allowed operators to recognize minute flaws that may worsen operations and defects in production if needed.

Some popular methods for non-destructive testing of weld defects are as follows:

Radiographic Testing:

This method utilizes X-rays or gamma rays on the weld zone in workpieces. For the longest time, this method was the only available method for the analysis of welds, as it provided data on the presence of deformities on the surface as well as the subsurface region of the weld. The radiographic waves are highly penetrative and effective and can be easily recorded digitally or on a radiographic film.

Also Read, Radiography Testing- A Complete Guide

Ultrasonic Testing:

The ultrasonic method was not introduced for weld defect detection until the 1960s. This method quickly picked up as a favorable NDT testing method for welds as radiography can be hazardous to humans and precautionary measures are often cumbersome in the long run.

Further Reading, Advanced Ultrasonic Testing Methods

Visual Testing:

This is the most basic method for testing defects and deformities. The operators observe the surface of the material with the naked eye. It requires years of experience and thorough knowledge of the workpiece material and its behavior to recognize and infer deformities in this testing method. The operator may use aids such as scales, microscopes, etc to get a better inference from this NDT analysis. This method is very limited as it cannot help to detect sub-surface defects.

Magnetic Particle Testing:

Magnetic particle testing is most appropriate for ferromagnetic materials and is another preferred option for detecting defects in welds. This material magnetizes the workpiece, and a magnetic powder is applied on the surface. The magnetic powder is attracted to points of flux leakage, which occur in regions with cracks or deformities. The limitation of this method is that it can only efficiently detect surface and a few subsurface defects.

Liquid Penetrant Inspection:

This method, also called the Dye Penetrant Inspection method uses a liquid or powder dye on the surface of the material, which after a certain dwelling period seeps into the cracks and deformities. Post-cleansing the excess dye, the liquid that has penetrated the defects indicates the location and characteristics of the dye (to a certain extent). The visibility of the dye should also be factored in as efficient visual inspection of the dye marking can only be carried out under white light of specific frequencies or ultraviolet light.

Electromagnetic or Eddy current Testing:

An eddy current testing apparatus uses a probe that houses a coil carrying an alternating current. This alternating current generates a magnetic field around the coil. On bringing the probe within the proximity of the test subject, eddy currents are generated in the workpiece. If there is a presence of defects in the weld, the magnetic coupling in the testing probe is changed. The variation in the impedance of the coil hence helps in inferring a defect signal.


Industrial Standards And Criteria For Weld Defects 

Industry codes and standards help organizations and operators navigate the welding process more efficiently and set criteria for defects based on the detecting apparatus, the material of the workpiece, the welding method, etc. This aids in preventing the failure of machinery and improves efficiency and helps organizations conform to industry standards. 

Some of those Standards are as follows:

American Welding Society- A02.4-Standard symbols for welding, brazing, and nondestructive examination

American Welding Society -A03.0-Standard welding terms and definitions

American Welding Society -A05.1-Specification for carbon steel electrodes for shielded metal arc welding

American Welding Society -A05.18-Specification for carbon steel electrodes and rods for gas-shielded arc welding

American Welding Society- B01.10-Guide for the non-destructive examination of welds

American Welding Society- B02.1-Specification for Welding Procedure and Performance Qualification

American Welding Society- D01.1-Structural welding (steel)

American Welding Society- D01.2-Structural welding (aluminium)

American Welding Society- D01.3-Structural welding (sheet steel)

American Welding Society- D01.4-Structural welding (reinforcing steel)

American Welding Society -D01.5-Bridge welding

American Welding Society- D01.6-Structural welding (stainless steel)

American Welding Society- D01.7-Structural welding (strengthening and repair)

American Welding Society -D01.8-Structural welding seismic supplement

American Welding Society- D01.9-Structural welding (titanium)

American Welding Society- D08.1-Automotive spot-welding

American Welding Society -D08.6-Automotive spot-welding electrodes supplement

American Welding Society -D08.7-Automotive spot-welding recommendations supplement

American Welding Society -D08.8-Automotive arc welding (steel)

American Welding Society- D08.9-Automotive spot weld testing

American Welding Society- D08.14-Automotive arc welding (aluminium)

American Welding Society- D09.1-Sheet metal welding

American Welding Society- D10.10-Heating practices for pipe and tube

American Welding Society- D10.11-Root pass welding for pipe

American Welding Society- D10.12-Pipe welding (mild steel)

American Welding Society- D10.13-Tube brazing (copper)

American Welding Society- D10.18-Pipe welding (stainless steel)

American Welding Society- D11.2-Welding (cast iron)

American Welding Society- D14.1-Industrial mill crane welding

American Welding Society- D14.3-Earthmoving & agricultural equipment welding

American Welding Society- D14.4-Machinery joint welding

American Welding Society- D14.5-Press welding.

American Welding Society- D14.6-Industrial mill roll surfacing.

American Welding Society- D15.1-Railroad welding

American Welding Society- D15.2-Railroad welding practice supplement

American Welding Society- D16.1-Robotic arc welding safety

American Welding Society- D16.2-Robotic arc welding system installation

American Welding Society- D16.3-Robotic arc welding risk assessment

American Welding Society- D16.4-Robotic arc welder operator qualification

American Welding Society- D17.1-Aerospace fusion welding

American Welding Society- D17.2-Aerospace resistance welding

American Welding Society- D18.1-Hygienic tube welding (stainless steel)

American Welding Society- D18.2-Stainless steel tube discoloration guide

American Welding Society- D18.3-Hygienic Equipment welding

American Petroleum Institute (API) -RP 577-Welding Inspection and Metallurgy

American Petroleum Institute (API) -1104-Welding of pipelines and related facilities

International Standards Organization-5817:2014-Quality levels of imperfections in fusion-welded joints

International Standards Organization -13919‑1-Quality levels for beam welded joints in steel 

International Standards Organization -4063: 11- Metal-arc welding without gas protection

International Standards Organization -4063: 12- Submerged-arc welding

International Standards Organization -4063: 13- Gas-shielded metal-arc welding.

International Standards Organization -4063: 14- Gas-shielded arc welding with non-consumable tungsten electrodes.

International Standards Organization -4063: 15- Plasma arc welding. 

International Standards Organization -4063: 31-Oxy-fuel gas welding (for steel only).

American Society of Mechanical Engineers-BPVC Section II-Part C: Specifications for Welding Rods, Electrodes, and Filler Metals

American Society of Mechanical Engineers-BPVC Section V-Non-destructive Examination

American Society of Mechanical Engineers-BPVC Section IX-Welding and Brazing Qualifications

American Society of Mechanical Engineers-BPVC B16.25 - Buttwelding ends.

UIG-97, Page – 345 - American Society of Mechanical Engineers- BPVC Section VIII Div 1, 2017 Edition.)- Acceptance Criteria for Visual Inspection

UW-51: Sub Para b (Page 148 and 149) and Mandatory Appendix 4 (Page 400 and Page 403345 - American Society of Mechanical Engineers- BPVC Section VIII Div 1, 2017 Edition.)- Acceptance criteria for Radiography Test (RT)

Appendix 12 (Page 435) - American Society of Mechanical Engineers -BPVC Section VIII Div 1, 2017 Edition.)- Acceptance criteria for Ultrasonic Test (UT)

Acceptance criteria for Liquid Penetrant Test (LPT/LPI/DPT)

Appendix 8 (Page 417) - American Society of Mechanical Engineers BPVC Section VIII Div 1, 2017 Edition.)

Mandatory Appendix 6 (Page 412) - American Society of Mechanical Engineers BPVC Section VIII Div 1, 2017 Edition- Acceptance criteria for Magnetic Particle Test (MPI/MPT/MT)


Conclusion

The existence and lifetime of a welded workpiece are highly dependent on the status of its weld defects. The absence of defects ensures a smooth operation without energy losses and gradually increasing internal defects which may cause a sudden failure in the workpiece. 

Welds are used to hold together integral parts of machines, tools, and objects of varying sizes and load-bearing abilities.

Responsible organizations, manufacturers, and consumers ensure the quality and structural integrity of the welds in vital zones that are prone to failure or analyze workpieces for potential failure zones to ensure safety. 

Non-destructive testing methods have been tried and tested for decades for this purpose and further research has only reinforced the need for the usage of NDT techniques to detect weld defects.

Newer technologies have been introduced to automate these testing procedures and industries should continue using such methodologies to save time, money, and resources.




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