There are many forms to inspect welds or materials, both destructive and non-destructive.
As the name suggests, destructive weld testing involves the physical destruction of the completed weld to evaluate its characteristics. This method of testing is used frequently for several applications. These applications include welding procedure and welder performance qualification testing, sampling inspection of production welds, research inspection, and failure analysis work. Several destructive weld testing methods determine weld integrity or performance. Typically, they involve sectioning and breaking the welded component and evaluating various mechanical and physical characteristics. We shall briefly examine some of the more common methods of this type of welding inspection.
Non-destructive testing (NDT) is the process of inspecting, testing, or evaluating materials, components, or assemblies for discontinuities or differences in characteristics without destroying the serviceability of the part or system. In other words, when the inspection or test is completed, the part or system can still be used for its intended purpose.
Testing and Inspection Methods
- Tests must be made to qualify the process(es) and the operators.
- The inspection usually involves examining completed welds to establish their quality and their confirmation of specifications.
- Thus, testing and inspection determine whether or not the quality standards of materials and workmanship are being met.
- In welded joints, the complexity is further increased by the nature of the joint, which is far from homogeneous, metallurgically, or chemically. In addition to the base metal, the welded joint consists of weld metal and a heat-affected zone. A variety of properties are thus to be expected throughout the welded joint.
- Mechanical tests qualify welding procedures, welders, and welding processes and determine if electrodes and filler metals meet the specifications.
- Welds in weldments are often tested for soundness, strength, and toughness by mechanical tests.
- Mechanical tests are destructive tests since the weld joint is destroyed in making the test. However, the test specimen (coupon) may be taken from a completed joint in a welded structure or a test piece under the same conditions.
Weld Tension Test
- Welding results in metallurgical (and often compositional) differences in the weld joint, but it is crucial to know the effects of these changes on mechanical properties.
- The tension testing of welds is more involved than base metal because the weld test section is heterogeneous, composed of the deposited weld metal, the HAZ, and the unaffected base metal.
- Tensile test specimens can be either transverse or longitudinal depending on the loading on the welded joint.
- Intension test, strength, elongation, and reduction area are of primary importance.
- If the weld metal strength exceeds that of the base metal, most plastic strain occurs in the base metal, resulting in necking (local reduction in the cross-section area by stretching) and failure outside of the area. In such a case, the test does not indicate weld ductility.
- When the weld strength is considerably lower than the base metal, most plastic strain occurs in the weld.
- Transverse weld specimens may measure joint efficiency in terms of strength but do not provide a good ductility measurement of the weld. However, the transverse sample is generally used.
Weld Tension Test
- Tension-shear tests may be used to evaluate the shear properties of fillet welds.
- Such tests usually represent completed joints in weldments and are prepared using similar procedures.
- The tension-shear test is the most widely used method for determining the strength of resistance spot welds.
Weld Hardness Test
- Brinell, Rockwell, Vickers, and Knoop hardness tests can apply to welds.
- Hardness measurements can provide information about the metallurgical changes caused by welding. For example, in construction steels, rapid cooling from high HAZ temperature may cause the formation of martensite of much higher hardness than the base metal.
- Hardness values in a welded joint are usually sensitive to welding conditions such as the process used, heat input, preheat or interpass temp, electrode compositions, and plate thickness.
- Hardness testing of welds is performed on the joint area’s ground, polished, or etched cross-sections.
- Indentations are made in specific areas of interest, including the deposit’s weld centerline, face or root regions, the HAZ, and the base metal.
- Which hardness test is used depends primarily on the hardness or strength of the material, the size of the welded joint, and the type of information desired.
- The Brinell test produces a large indentation, typically 2 to 5.6 mm in diameter, and it is thus suited for significant welds.
- The Rockwell test produces a much smaller indentation more suited for hardness traverses.
- The Vickers and Knoop tests make relatively small indentations and thus are well-suited for hardness measurements of the various regions of the HAZ and fine-scale traverses.
Weld Bend Test
- Various bend tests are used to evaluate the flexibility and soundness of welded joints.
- Bend specimens may be longitudinal or transverse to the weld axis and bent in simple three- or four-point bending (free bend) or around a mandrel of specified diameter (guided bend).
- A welded plate’s top and bottom surfaces are frequently designated as the face and root surfaces, respectively.
- Face bends have the weld face on the tension side of the bend specimen; with root bends, the weld root is on the convex side.
- In bend testing of thick plates, transverse slices are usually cut from the welded joint and bent so that one of the cut side surfaces becomes the convex side of the bend specimen; these are referred to as side bends.
- Transverse bend tests help qualify welders and welding operators because they often reveal the presence of defects (lack of root fusion or penetration) that are not detected in tension tests.
- The transverse bend test is sensitive to the relative strengths of the weld metal, the HAZ, and the base metal.
- It is normal to machine or grinds flat the face and root of a weld bend test coupon to reduce the stress-raising effect these would have.
- Weld joints with non-uniform properties, such as dissimilar metal joints or where the weld and parent metal strengths are substantially different, can result in the ‘peaking’ of the bend coupon. This is when most of the deformation occurs in the weaker of the two materials, which therefore experiences excessive localized deformation that may result in premature failure.
- It is the most widely used non-destructive testing technique. It is highly effective and is the least expensive inspection method.
- The welding inspector can utilize inspection visual inspection throughout the entire production cycle of a weldment.
- It is an effective quality control method that will ensure procedure conformity and will catch errors at early stages.
- Visual inspection methods can be divided into three sub-groups:
- Visual examinations to be completed before welding should be drawings, material specifications, edge preparation, dimensions, cleanliness of the welding joint, etc.
- Visual examination during welding: welding process, electrode selection, operating conditions, preheat requirements, welder performance, etc.
- Visual examinations to complete the finished weldment should be weld size (using weld gauges), defects (surface cracks, crater cracks, surface porosity, incomplete root penetration, undercut, underfill), warpage, base metal defects, etc.
Non-destructive Testing (NDT)
- Non-destructive testing is also known as non-destructive examinations, evaluation (NDE), or inspection.
- These techniques apply physical principles from detecting flaws or discontinuities in materials without impairing their usefulness.
- In the field of welding, four non-destructive tests are widely used:
- Dye-penetrant testing and Fluorescent-penetrant testing
- Magnetic particle testing
- Ultrasonic testing
- Radiographic testing
Dye Penetrant Testing
- The liquid-penetrant examination is a highly sensitive, non-destructive method for detecting minute discontinuities (flaws) such as cracks, pores, and porosity, which are open to the surface of the inspected material.
- It may be applied to many materials, including ferrous and nonferrous metals, glass, and plastics.
- The applied surface must be cleaned from dirt and film. So, discontinuities must be free from dirt, dust, grease, or paint to enable the penetrant to enter the surface opening.
- A liquid penetrant is applied to the part’s surface to be inspected. The penetrant remains on the surface and seeps into any surface opening. The penetrant is drawn into the surface opening by capillary action. Therefore, the parts may be in any position when tested. After sufficient penetration time elapsed, the surface is cleaned, and excess penetrant is removed.
- The penetrant is usually red; therefore, the indication shows brilliantly against the white background. Nevertheless, even minor defects may be located.
- The liquid-penetrant examination detects surface defects in aluminum, magnesium, and stainless-steel weldments when the magnetic particle examination method cannot be used.
- It is beneficial for locating leaks in all types of welds. For example, welds in pressure and storage vessels and piping for the petroleum industry are examined for surface cracks and porosity.
- Fluorescent-Penetrant Examination:
- The penetrant is fluorescent, and when it is exposed to ultraviolet or black light, it shows a glowing fluorescent type of read-out.
- It provides a greater contrast than the visible dye penetrants.
- They are used for leak detection in magnetic and nonmagnetic weldments.
- A fluorescent penetrant is applied to one side of the joint, and a portable ultraviolet light is used on the reverse side to examine the weld for leaks.
- Inspect the root pass of highly critical pipe welds.
- Radiography is a non-destructive examination method that uses invisible X-ray or Gamma radiation to examine the interior of materials.
- It gives a permanent film record of defects that is relatively easy to interpret.
- Although this is a slow and expensive non-destructive examination method, it is a positive method for detecting porosity, inclusions, cracks, and voids in the interior of castings, welds, and other structures.
- X-ray generated by electron bombardment of tungsten and gamma rays emitted by radioactive elements is penetrating radiation whose intensity is modified by passage through a material.
- The amount of energy absorbed by a material depends on its thickness and density. Energy not absorbed by the material will cause exposure to the radiographic film. As a result, those areas will be dark when the film is developed.
- Areas of material where the thickness has been changed by discontinuities, such as porosity or cracks, will appear as dark outlines on the film.
- All discontinuities are detected by viewing the shape and variations in the density of the processed film.
- It is used for the examination of weldments in all types of materials.
- The pipeline industry ensures good weld quality.
- It is a non-destructive examination method that employs mechanical vibrations similar to sound waves but of a higher frequency.
- A beam of ultrasonic energy is directed into the specimen to be examined. This beam travels through a material with only a tiny loss, except when intercepted and reflected by a discontinuity or a material change.
- Ultrasonic examination is capable of finding surface and subsurface discontinuities.
- The system uses a transducer, which changes electrical energy into mechanical energy. The transducer is excited by a high-frequency voltage that causes a crystal to vibrate mechanically. The crystal probe becomes the source of ultrasonic mechanical vibrations.
- These vibrations are transmitted into the test piece through a coupling fluid; usually, a film of oil called a couplant.
- When the pulse of ultrasonic waves strikes a discontinuity in the test piece, it is reflected in its point of origin.
- The transducer serves as a receiver for the reflected energy.
- The initial signal or main bang, the returned echoes from the discontinuities, and the echo of the rear surface of the test material are all displayed by a trace on the screen of a cathode-ray oscilloscope.
- Transducer, pulse rate generator, amplifier, timer, and cathode-ray oscilloscope (all are portable).
- Practically any metal or material
- Restricted only very complex weldments and joining of plates with thicknesses of 10mm to 12mm or more
- Cracks, gas pockets, and slag
|Examination Technique||Equipment||Defects Detected||Advantages||Disadvantages||Other Considerations|
|Visual: VT||Pocket magnifier, welding viewer, flashlight, weld gauge, scale||Weld penetration, fit-up, cleanliness, roughness, spatter, undercut, overlap, weld contour, and size, welding procedure||Easy to use, fast, inexpensive, usable at all stages of production||For surface conditions only, dependent on subjective opinion inspector||The most universally used examinations method|
|Liquid Penetrate: PT||Fluorescent or visible penetrating liquids and developers; ultraviolet light for the fluorescent type||Defects open to the surface only; good for leak detection||Detects tiny, tight, surface imperfections, easy to apply and interpret; inexpensive; used on magnetic or nonmagnetic materials||Time-consuming in the various steps of the process; usually, no permanent record||Often used on root pass of highly critical gas welds if material improperly cleaned; some indications may be misleading|
|Magnetic Particle: MT||Iron particles, wet or dry, or fluorescent; particular power source; ultraviolet light for the fluorescent type||Surface and near-surface discontinuities, cracks, etc.; porosity, slag||Indicates discontinuities not visible to the naked eye; useful in checking edges before welding also repairs; no size restriction||Used on magnetic materials only; surface roughness may distort magnetic field; usually, no permanent record||The examination should be from two perpendicular directions to catch discontinuities that may be parallel to one set of magnetic lines of force|
|Radiographic: RT||X-ray or gamma-ray; source: film processing equipment, film viewing equipment, penetrometers||Most internal discontinuities and flaws; limited by the direction of discontinuity||Provides permanent record; indicates surface and internal flaws; applicable on all materials.||Usually unsuitable for fillet weld inspection, film exposure and processing are critical, slow, and expensive.||A most popular technique for subsurface inspection; required by some codes and specifications|
|Ultrasonic: UT||Ultrasonic units and probes; reference and comparison patterns||Can locate all internal flaws discovered by other methods with the addition of exceptionally minor flaws||Extremely sensitive; restricted only by very complex weldments; can be used on all materials.||Demands highly developed interpretation skills.||Required by some codes and specifications|