Radiographic test (RT) is a technique used for volumetric flaw detection. Short wavelength electromagnetic radiation such as X-rays penetrate the through thickness of the component and on to photographic films placed on the other side of the component. The film gets exposed to varying levels of radiation depending on the thickness of the penetrated material, and forms the latent images. Discontinuities within the component also affects the amount of radiation received by the film. Exposed films are then processed in dark room and developed radiographic images are viewed on illuminated screen for interpretation.
The most basic form of ultrasonic testing (UT) utilizes a high frequency, high energy cone of sound from a probe to “scan” the interior of a component for volumetric flaws or discontinuities. The sound wave is generated within the probe and transmitted into the component. The sound wave travels within the component and reflects off any internal features, including the walls of the component and any defects within it. As sound travels at a known, fixed speed within a given medium, the operator can predict when should an echo off the component wall be reflected back to the probe. An echo that is detected earlier than expected is therefore a sign of a discontinuity at that location. By moving the probe in a methodical manner across the surface of the component, an operator can not only measure the thickness of the component (Ultrasonic Thickness Gauge Measurement), but also detect any defects within it.
Due to the portable nature of the equipment, and the high accuracy in flaw detection and sizing, UT is often deployed to detect internal discontinuities in welded plate and pipes on site, and for measuring wall thicknesses of in-service pipes, boilers and pressure vessels.
Immersion Ultrasonic Testing is a variation of the standard UT technique. Instead of using a thin layer of oil or water-based gel to aid the transmission of soundwaves from the probe to the component, the soundwave is transmitted either via a jet of water (also known as a squirter) or by immersing the component into a tank of water. Immersion UT is therefore often automated or mechanized, and can carry out rapid, repeatable, and very accurate UT scans to collect data which can be further analyzed. This is unlike the standard, Manual A-Scan method, which does not normally leave a record of the scan.
Some applications for Tank Immersion type Immersion UT include the rapid detection of laminations, determination of Crack Area Ratio after HIC testing, as well as rapid inspection of welded plate coupons. Applications of water jet type Immersion UT include wall thickness measurement of small diameter, thin wall pipes such as those typically used in heat exchangers.
Magnetic Particle Test (MT), is an NDT method that utilizes magnetic fields to attract small, ferromagnetic iron fillings towards fine surface breaking discontinuities in order to create a larger or higher contrast visible indication. The technique relies on magnetic fields that are either introduced into the component or generated within the component itself via an electrical current. As such, it can only be used on ferromagnetic or electrically conductive materials. MT is a very effective method of detecting surface cracks in welds, castings and forgings, and often more tolerant of poor surface conditions, making it ideal for inspecting in-service components. For high sensitivity requirements, such as in detecting fine fatigue cracks, the more sensitive Florescent MT technique can be used over the standard colour contrast MT, provided a suitably dark environment can be created on site.
Liquid penetrant test (PT) is used to detect surfacing breaking discontinuities on clean, dry, non-porous materials using dyes. The dye is called the penetrant, and may either be visible or fluorescent, which then has to be conducted under ambient light (visible method) and black light (Fluorescent method) respectively. It is a relative simple technique where the penetrant is applied to the component surface and is allowed to seep into any surface breaking discontinuities, such as cracks, via capillary action. Excess penetrant is removed via cleaning, and a developer is applied, causing the penetrant to resurface, forming a high contrast indication of the discontinuity.
The Feritscope operates by generating an alternating magnetic field in the sample which is proportional to the ferrite content and which the instrument can detect. The instrument, using a non-destructive magnetic method measures the relative permeability of a material in the alternating magnetic field of its probe. This provides a ferrite content reading, which is largely uninfluenced by various extrinsic parameters (lift-off, strain in specimen, curvature of specimen surface, thickness of welded material).
In-situ Metallography refers to the examination of the microstructure of a metallic component on site. It is typically considered a non-destructive method because the area of interest does not need to be extracted for examination in a lab. However, in practice, In-situ Metallography will require the removal of any paint coatings and the application of light grinding followed by etching before examination can be carried out. Examination of the polished and etched surface can then be carried out immediately by using a portable microscope with attached digital camera or alternatively, by making a replica of the microstructure using special resins or plastics followed by examining the replica in a laboratory.
In-situ Metallography can be used to determine the ferrite to austenite ratio in duplex stainless steel, estimate grain size in carbon and stainless steels as well as examining the general macrostructure and microstructure of a component. It has applications in examination of boilers, heat exchangers and reactors during annual shutdown maintenance, check the effects of post weld heat treatment processes on site, and to verify weld microstructure of very large fracture mechanics (CTOD) specimens prior to notching.
Professional Testing Services Sdn Bhd
PT. Professional Technology Specialist