While engineers push the boundaries of material capacities to their limits in the design, traceability of proper material becomes ever more important. In addition to compatibility issues, several other reasons can exist for material specification including design, corrosion resistance, and compliance to codes and standards such as ASME Boiler and Pressure Vessel Code.
Quality procedures are put in place to document materials as they are received and as they move through the production process, but what happened to those raw materials before they arrived at the receiving dock? Each time raw material changes hands - from the mill to service centers, from processing plants (e.g., pipe, tube and fittings) to subcontractors – the opportunity for error increases, resulting in questionable material quality.
The Solution
With Positive Material Identification (PMI) the alloy composition, and thus, the identity of materials can be determined. If a material certificate is missing or/and you need to be certain about the type of material used, PMI as an NDT method is the best solution. Positive Material Identification is particularly used for high quality metals like stainless steel and high alloy metals.
Magnetic flux leakage (MFL) is a magnetic method of nondestructive testing that is used to detect corrosion and pitting in steel structures, most commonly pipelines and storage tanks. The basic principle is that a powerful magnet is used to magnetize the steel. At areas where there is corrosion or missing metal, the magnetic field "leaks" from the steel. In an MFL tool, a magnetic detector is placed between the poles of the magnet to detect the leakage field. Analysts interpret the chart recording of the leakage field to identify damaged areas and hopefully to estimate the depth of metal loss. This article currently focuses mainly on the pipeline application of MFL, but links to tank floor examination are provided at the end. MFL principle As an MFL tool navigates the pipeline a magnetic circuit is created between the pipewall and the tool. Brushes typically act as a transmitter of magnetic flux from the tool into the pipewall, and as the magnets are oriented in opposing directions, a flow of flux is created in an elliptical pattern. High Field MFL tools saturate the pipewall with magnetic flux until the pipewall can no longer hold any more flux. The remaining flux leaks out of the pipewall and strategically placed tri-axial Hall effect sensor heads can accurately measure the three dimensional vector of the leakage field. Given the fact that magnetic flux leakage is a vector quantity and that a hall sensor can only measure in one direction, three sensors must be oriented within a sensor head to accurately measure the axial, radial and circumferential components of an MFL signal. The axial component of the vector signal is measured by a sensor mounted orthogonal to the axis of the pipe, and the radial sensor is mounted to measure the strength of the flux that leaks out of the pipe. The circumferential component of the vector signal can be measured by mounting a sensor perpendicular to this field. Earlier MFL tools recorded only the axial component but high-resolution tools typically measure all three components. To determine if metal loss is occurring on the internal or external surface of a pipe, a separate eddy current sensor is utilized to indicate wall surface location of the anomaly. The unit of measure when sensing an MFL signal is the gauss or the tesla and generally speaking, the larger the change in the detected magnetic field, the larger the anomaly.
Phased Array (PA) ultrasonics is an advanced method of ultrasonic testing that has applications in medical imaging and industrial nondestructive testing, originally pioneered by Albert Macovski of Stanford University.[1] In medicine a common application of phased array is the imaging of the heart (images of the fetus in the womb are usually made by curvilinear array, a multi-element probe that does not actually phase the signals). When applied to steel the PA image shows a slice that may reveal defects hidden inside a structure or weld.
Principle of operation
The PA probe consists of many small ultrasonic elements, each of which can be pulsed individually. By varying the timing, for instance by pulsing the elements one by one in sequence along a row, a pattern of interference is set up that results in a beam at a set angle. In other words, the beam can be steered electronically. The beam is swept like a search-light through the tissue or object being examined, and the data from multiple beams are put together to make a visual image showing a slice through the object.
The Time Of Flight Diffraction (TOFD) is a Advanced NDT method developed in the 70's by AEA. AEA Sonomatic specializes in this method. This method differs from traditional pulse echo technique in that it monitors diffracted signals at the edges of defects which are directly related to the true position and size of the defect, as opposed to the reflection on defects according to a reference reflector. The TOFD technique uses two probes in a transmitter-receiver arrangement. When sound is introduced into the material via the transmitter the defect will oscillate. Each defect edge works as a source point of ultrasound signals. These very weak signals are called diffracted waves and their appearance does not relate to the orientation of flat or spherical defects. These diffracted signals are received via the receiver probe. The diffracted signals are evaluated with the Microplus-Systems to clear gray scale B-scan or D-scan images (transversal- or longitudinal projection of the object being tested). The amplitude of the signal is not displayed , but the position of the signals on the time scale are. Thus it is possible to determine the defect location exactly - length, and defect height. Therefore the Probability of Detection (POD) increases greatly (up to 90 % !) for flat or spherical defects when compared to traditional techniques. By use of today's advanced computer techniques it is possible to evaluate signals very rapidly. That makes it possible to perform scans with a speed of hundreds of millimeters per second. In practice speed is limited only by the mechanic. Applications for TOFD The main TOFD applications are: o In-service defect monitoring. o Defect detection, documentation and evaluation during the production. The dead zone under the outside surface has always been a limitation of TOFD. Defects close to the surface could not be detected (surface breaking cracks are detectable).. AEA Sonomatic succeeded in reducing this zone to 2 mm! For that reason TOFD can be applied down to 6 mm wall thickness. On the upper end it is usual to inspect up to 350 mm wall thickness. Advantages of TOFD
• TOFD defect detection does not depend on the defect orientation, in contrast to the pulse echo technique . • In contrast to the radiography method, planar defects and cracks, which are not perpendicular to the measured surface can be detected . • Defect height can be exactly determined. • Higher POD improves risk reduction and calculation. • The evacuation of areas because of radiation is not necessary. That means less interruption in the production process less during pre-service or in-service inspections and fewer logistical problems for the manufacturer. • The inspection results are immediately available, as is a permanent record and a permanent print as longitudinal or transversal projection of the weld is available. • When Engineering Critical Assessment (ECA) is applied, only the relevant defect has to be cut, thereby preventing needless repairs which could harm the integrity of the weld. • Because of the high test speed the costs are less than those for radiography for wall thickness above 25 mm. • The inspection can be performed above 200° C. • Since the Microplus-System is easy to transport, it is possible to perform test on all feasibly accessible areas. • TOFD saves costs, if applied during construction, since it is possible to distinguish pre-service and in-service defects. That means the unit can stay longer in production, and is safe.
HT: The hardness of steel is generally determined by testing its resistance to deformation. A number of methods are employed including Brinell, Vickers and Rockwell. The steel to be tested is indented by a hardened steel ball or diamond under a given load and the size of the impression is then measured. For steel there is an empirical relationship between hardness and tensile strength and the hardness number is often used as a guide to the tensile strength, e.g. 229 Brinell = 772N/mm2 (50 tons/sq.in).
RT: A source of ionising radiation positioned at on one side of item to be inspected, and a photographic film placed in close proximity to the other side. The radiation is partly absorbed during transmission and differences in material thickness or absorption qualities are recorded on the film giving a full-size image showing internal detail. The higher the Material density more radiation absorbtion will occur. Processed films are called radiographs, Industrial radiography requires X-rays or gamma rays to reveal hidden flaws in solid objects. X-ray radiography is generated electrically by means of a high voltage X-ray tube. Gamma rays are produced by the natural disintegration of nuclei in a radioactive isotope. Common types being Iridium 192 and Cobalt 60. Radiography's main benefits are that it provides a non-destructive method of detecting hidden flaws in materials and fabrications and provides a permanent record. Radiography is particularly good at detecting volumetric flaws such as voids, gas pores and solid inclusions, It is also good at determining the nature and dimensions (length and width) of flaws - however it cannot be used to measure through-thickness of defects.
EC: Eddy Current can be used in two different manners, Firstly for finding surface and subsurface flaws, and Secondly for determining different metallurgical characteristics. Eddy current is based on the principle of measuring changes in the impedance of an electromagnetic coil as it is scanned over a surface of conductive material.
An alternating current in the coil produces a magnetic field that is induced in the material. To counter the coil's primary magnetic field, eddy currents are produced in the material. Eddy currents produce a secondary magnetic field H'B to oppose the coil's primary magnetic field HB. When the coil is scanned over a discontinuity, the secondary magnetic field is distorted, thereby changing the loading on the coil. Changes in coil loading directly affect the coil impedance, these changes are signified on the trace as a (possible) flaw
PT: This method involves applying a visible or fluorescent dye to the surface. After application by immersion or spray the dye enters any discontinuities via capillary action. The component is wiped dry and any subsequent seepage from fissures is detected by drawing the liquid out into a white absorbent coating applied after drying off (See Diagrams Below). This method is suitable for any non ferrous components or material that is non absorbent. Typical applications are forged, cast or welded products.
MT: Magnetic particle inspection (MPI) can be used for the detection of surface and near-surface flaws in ferromagnetic materials. Using a permanent magnet, electromagnet, flexible cables or hand-held prods a magnetic field is applied to the item under test. If a flaw is present the magnetic flux is distorted and 'leaks'. Fine magnetic particles, (normally in spray form in carrier fluid) can be applied to the surface of the specimen, are attracted to the area of flux leakage creating a visible flaw indication. It is recommended that the inspection surface is magnetised in at least two perpendicular directions at 90° to each other, due to lack of disturbance to the magnetic field if the crack runs parallel to the magnetic field
UT: This method of testing for flaws utilises sound waves which are introduced into the component via an ultrasonic source, as the sound travels through material reflections or echoes occur from the back surface. In addition any internal discontinuity will reflect the sound wave and generate a signal into the receiver. The time lags of the echoes are measured to determine the thickness of the material and the distance to the discontinuity.
It is very difficult to weld or mold a solid object that has the risk of breaking in service, so testing at manufacture and during use is often essential. During the process of casting a metal object, for example, the metal may shrink as it cools, and crack or introduce voids inside the structure. Even the best welders (and welding machines) do not make 100% perfect welds. Some typical weld defects that need to be found and repaired are lack of fusion of the weld to the metal and porous bubbles inside the weld, both of which could cause a structure to break or a pipeline to rupture. During their service lives, many industrial components need regular non-destructive tests to detect damage that may be difficult or expensive to find by everyday methods. For example: • Aircraft skins need regular checking to detect cracks; • Underground pipelines are subject to corrosion and stress corrosion cracking; • Pipes in industrial plants may be subject to erosion and corrosion from the products they carry; • Reinforced concrete structures may be weakened if the inner reinforcing steel is corroded; • Pressure vessels may develop cracks in welds; • The wire ropes in suspension bridges are subject to weather, vibration, and high loads, so testing for broken wires and other damage is important. Finished machined parts, such as bearings, that have newly been assembled can be tested for missing pieces, such as a ball or roller bearing, or grease within the housing non-destructively with a checkweigher. A roller motor for a conveyor can be tested for the proper level of oil, without disassembling the finished product. Thousand of manufactured products can benefit from this form of testing. Over the past centuries, swordsmiths, blacksmiths, and bell-makers would listen to the ring of the objects they were creating to get an indication of the soundness of the material. The wheel-tapper would test the wheels of locomotives for the presence of cracks, often caused by fatigue — a function that is now carried out by instrumentation and referred to as the acoustic impact technique. Use of X-rays for NDT is a common way of examining the interior of products for voids and defects, although some skill is needed in using radiography to examine samples and interpret the results. Soft X-rays are needed for examining low density material like polymers, composites and ceramics.
NDT: Non Destructive testing. ANDT: Advanced Non Destructive Testing. PCN: Personnel Certification in Non-destructive testing. ASNT: American Society for Non-Destructive Testing. CWI: Certified Welding Inspector. AWS: American Welding society. NACE: National Association of Corrosion Engineers. ASTM: American Society for Testing and Materials RT: Radiographic testing. UT: Ultrasonic Testing. MT: Magnetic Testing. PT: Penetrate testing. ToFD: Time of Flight Diffraction. ECT: Eddy Current Testing. IRIS: Internal Rotary Inspection System. VT: Visual Testing. LT: Leak sealing. HT: Hardness Testing. MFL: Magnetic Flux Leakage. PMI: Positive material identification. RFT: Remote Field Testing AE: Acoustic Emission
ASME: The American Society of Mechanical Engineers is a professional body, specifically an engineering society, focused on mechanical engineering
Heat Treatment is most often associated with increasing the strength of materials, it also can be used to improve machining, enhance formability, and restore ductility after a cold working operation. Given varied applications, heat treatment is a valuable manufacturing process can improve product performance and yield other desirable characteristics.
Heat treatment is a method used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering and quenching. It is noteworthy that while the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.
