Testing and Inspection Methods Of Welding Joints
TESTING AND INSPECTION METHODS:
- Destructive Tests
- Tension test
- Nick-Break test
- Bend test
- Hardness test
- Fatigue test
- Impact test
1) WELD HARDNESS TEST
- Brinell, Rockwell, Vickers and Knoop hardness tests can be applicable to welds.
- Hardness testing of welds is performed on ground, polished, or polished and etched cross-section of the joint area.
- Indentations are made in the specific areas of interests, including the weld center line, face or root regions of the deposit, the HAZ, and the base metal.
2) TENSILE TEST
- This is used to find tensile strength and ductility properties of the weld. Two end of specimen is held in a tensile testing machine and load is applied on it. During the test elongation of gauge length marked initially are measure by load dial.
- The extension caused will remain as it is and specimen can not regain its original shape even after removal of load.
- It is important that the tensile properties of the base metal, the weld metal, the bond between the base and the weld, and the heat-affected zone conform to the design requirements.
- Tensile strength of the welded joint is obtained by pulling specimens to failure. Tensile strength is determined by dividing the maximum load required during testing by the cross-sectional area.
- The result will be in units of tension per cross-sectional area. This test is nearly always required as part of the mechanical testing when qualifying welding procedure specifications for groove welds.
3) WELD BEND TEST
- The transverse bend test is sensitive to the relative strengths of the weld metal, the HAZ, and the base metal.
- This is when the most of the deformation takes place in the weaker of the two materials which therefore experiences excessive localized deformation that may result in premature failure.
Nondestructive tests (NDT)
- Visual examinations
- Radiography testing
- Gamma-ray X-Ray
- Magnetic testing
- Ultrasonic inspection
- Penetrant examinations
- Stethoscope test
- Eddy-Current Inspection
- Destructive test
1) EDDY CURRENT EXAMINATION
- A coil carrying an alternating current is placed close to the item to be examined, inducing an eddy current in the specimen. Defects in the specimen will interrupt this eddy current flow and these perturbations can be detected by a second, search coil. The coils can be placed either side of a thin plate-like sample or can be wound to give side- by-side coils in a single probe.
- These may be shaped to fit in the bore or around the outside of pipes and tubes and in these applications the process lends itself to automation. The process has been developed over recent years to make it more portable and simpler to use.
- It is also possible for the depth of surface cracks to be determined. It is of limited use for interrogating welds, however, being most commonly used in the examination of continuously welded tube.
2) ULTRASONIC EXAMINATION
- A ‘sound’ wave emitted from a transmitter is bounced off an object and this reflection captured by a receiver. The direction and distance of the object can be determined by measuring the elapsed time between transmission and detection of the ‘echo’.
- In welded components this is usually done by moving a small probe, containing both transmitter and receiver, over the item to be examined and displaying the echo on an oscilloscope screen.
- The probe transmits a beam of ultrasound that passes through the metal and is reflected back from any defects.
- Deeply buried defects such as lack of fusion, lack of penetration and cracks in addition to volumetric defects such as slag entrapment and porosity are all easily detected.
• It is very good for the detection of planar defects and cracks.
• It can easily determine defect depth.
• It is readily portable.
• Access is required to one side only.
• There are none of the health and safety problems associated with the radiographic technique.
• Very skilled operators are required.
• Surface breaking defects are difficult to detect.
• Accurate sizing of small (<3 mm) defects is difficult or impossible.
3) RADIO-GRAPHIC EXAMINATION
- The rays travel in straight lines and cannot be deflected or reflected by mirrors or lenses; they have wavelengths that enable the radiation to penetrate many materials, including most metals. They will, however, damage living tissue and therefore present some health and safety problems.
- The radiation, either X-rays from a suitable source or gamma rays from a radioactive isotope, is absorbed as it passes through the material. This absorption increases as the density of the material increases so that if a photographic film is placed on the side
opposite the radiation source, any less dense areas will appear as darker areas on the film to give a shadow picture of the internal features of the test sample once the film has been processed.
- Thus voids, porosity, slag, cracks defects can be detected
- A welded joint a suitable source of radiation, a film in a light-proof cassette and some method of processing the film are required.
- The radiation can be produced from an Xray tube, the energy generally being described by the voltage and current at which the tube is operated.
- These may vary from 20 kV to 30MV and 10 to 30mA, Gamma radiation is produced by the decay of a naturally occurring or manufactured radioactive isotope. The isotopes decay over a period of time, a measure of the longevity of the source being the half life, the length of time taken for the source to decay to half of its initial intensity.
• A permanent record is available.
• Both buried and surface defects can be detected and the technique is particularly good for finding volumetric defects such as slag and porosity.
• The equipment is portable, particularly the gamma ray sources.
• All materials can be examined.
• The capital cost of equipment is high. Health and safety considerations – large areas may need to be closed off during
radiography or enclosures must be provided in which the radiography is carried out.
• There are problems in detecting planar defects and fine cracks if these are normal to the beam.
• There is a limitation on the thickness that can be radio graphed and defects easily detected.
• Skilled and experienced radiographers are required.
4) WELD PENETRANT EXAMINATION
- Liquid-penetrated examination is a highly sensitive, nondestructive method for detecting minute discontinuities(flaws) such as cracks, and porosity, which are open to the surface of the material being inspected..
- The applied surface must be cleaned from dirt and film. So, discontinuities must be free from dirt, rust, 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. The parts may be in any position when tested. After sufficient penetration time elapsed, the surface is cleaned and excess penetrate is removed.
- The penetrate is usually a red color; therefore, the indication shows up brilliantly against the white background. Even small defects maybe located
• It can be used on both ferrous and non-ferrous metals.
• It is very portable.
• Large areas can be examined very quickly.
• It can be used on small parts with complex geometry.
• It is simple, cheap and easy to use and interpret.
• It will only detect defects open to the surface.
• Careful surface preparation and cleanliness are required.
• It is not possible to retest a component indefinitely.
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