Testing and Inspection of Welds

There are many forms to inspect welds or materials, both destructive and non-destructive.???

Destructive weld testing, as the name suggests, 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 qualification 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 the examination of 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 are used to qualify welding procedures, welders, and welding processes and determine if electrodes and filler metals meet the requirements of the specification.
• 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. The test specimen (coupon) may be taken from a completed joint in a welded structure or a test piece welded at the same conditions.

Weld Tension Test

• Welding results in metallurgical (and often compositional) differences in the weld joint, but it is important to know the effects of these changes on mechanical properties.
• The tension testing of welds is somewhat more involved than for base metal because the weld test section is heterogeneous, composed of the deposited weld metal, the HAZ, and the unaffected base metal.
• Tensile test specimen 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 of the plastic strain occurs in the base metal, with resultant 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 that of the base metal, most of the plastic strain occurs in the weld.
• Transverse weld specimens may provide a measure of joint efficiency in terms of strength but do not provide a good ductility measurement of the weld. However, the transverse specimen is generally used.

Weld Tension Shear Test

• Tension-shear tests may be used to evaluate the shear properties of fillet welds.
• Such tests are usually intended to represent completed joints in weldments and so 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 such conditions of welding as the process used, heat input, preheat or interpass temp, electrode compositions, and plate thickness.
• Hardness testing of welds is performed on the ground, polished, or polished and etched cross-section of the joint area.
• Indentations are made in the specific areas of interest, including the weld centerline, face or root regions of the deposit, 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.

1. The Brinell test?produces a large indentation, typically 2 to 5.6 mm in diameter, and it is thus suited for large welds.
2. The Rockwell test?produces a much smaller indentation more suited for hardness traverses.
3. 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 types of 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 may be bent in simple three- or four-point bending (free bend) or around a mandrel of specified diameter (guided bend).
• The top and bottom surfaces of a welded plate 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 are useful in qualifying 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 grind, flat the face and root of a weld bend test coupon to reduce the stress raising effect that 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 '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.

Visual Inspection

• It is the most widely used non-destructive testing technique. It is extremely 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:

1. Visual examinations to be completed before welding should be drawings, material specifications, edge preparation, dimensions, cleanliness of the welding joint, etc.
2. Visual examination during welding: welding process, electrode selection, operating conditions, preheat requirements, welder performance, etc.
3. Visual examinations to be completed of 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 or evaluation (NDE) or inspection.
• These techniques apply physical principles from the detection of flaws or discontinuities in materials without impairing their usefulness.
• In the field of welding, four non-destructive tests are widely used:

1. Dye-penetrant testing and Fluorescent-penetrant testing
2. Magnetic particle testing
3. Ultrasonic testing
4. 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, 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 surface of the part 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 a red color; therefore, the indication shows up brilliantly against the white background. Even small defects may be located.

Application

• The liquid-penetrant examination is used to detect surface defects in aluminum, magnesium, and stainless-steel weldments when the magnetic particle examination method cannot be used.
• It is very useful 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 then used on the reverse side of the joint to examine the weld for leaks.
• Inspect the root pass of highly critical pipe welds.

Radiographic Examination

• 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, 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 shape and variations in the density of the processed film.

Applications:

• It is used for the examination of weldments in all types of materials.
• Pipeline industry to ensure proper weld quality.

Ultrasonic examinations

• 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 small loss, except when intercepted and reflected by a discontinuity or by a change in material.
• 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.

Equipment:

• Transducer, pulse rate generator, amplifier, timer, and cathode-ray oscilloscope (all are portable).

Applications:

• 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