The following points highlight the six main non-destructive tests conducted for identifying metals and alloys. The tests are: 1. X-Ray Radiography 2. Gamma Radiography 3. Magnetic Particle Inspection 4. Ultrasonic Testing 5. Electrical Method 6. Damping Test.
1. X-Ray Radiography:
Radiography technique is based upon exposing the components to short wavelength radiations in the form of X-rays (wavelength less than 0.001 x 10-8 cm to about 40 x 10-8 cm) or gamma (γ) rays (wavelength about 0-005 x 10-8 to 3 x 10-8 cm) from a suitable source such as an X-ray tube or cobalt-60.
These tests are used to detect defects such as blow holes, cracks, shrinkage cavities and slag inclusions. These defects are of special importance in components designed to withstand high temperatures and pressure employed in power plants atomic reactors, chemical and pressure vessels and oil refining equipments; because they (i.e., defects) cause stress concentration which may frequently lead to part failure.
In X-ray radiography, the portion of the casting where defects are suspected is exposed to X-ray emitted from the X-ray tube. A cassette containing X-ray film is placed behind and in contact with the casting perpendicular to the rays. During exposure, X-rays penetrate the casting and thus affect the X-ray film.
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Since most defects (such as blow holes, porosity, cracks etc.) possess lesser density than the sound metal of the casting they transmit X-ray better than the sound metal does, therefore, film appears to be more dark where defects are in line of X-ray beam. The exposed and developed X-ray film showing light and dark areas is termed as Radiograph (or Exograph).
Fig. [2.35 (a), (b), (c)] shows the radiographs of sound casting and casting containing blow holes and porosity respectively.
2. Gamma Radiography:
The principle of detecting defects is same as X-ray radiography. Gamma-rays are emitted during the disintegration of radio- active material and like X-rays are electromagnetic radiation. Gamma-rays are of shorter in wave length and consequently are more penetrating.
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Gamma-ray radiography differs from X-ray radiography in the following aspects:
1. The apparatus necessary for gamma-ray radiography is very simple and less costly than X-ray units. Most cobalt-60 sources are cylindrical with dimensions of 3 by 3 to 6 mm and sealed in an appropriate container.
2. Unlike X-rays, gamma-rays from its source are emitted in all directions, therefore, a number of separate castings having cassette containing film, fastened to the back of each casting, are disposed in a circle around the space placed in a central position. This way many castings can be radiographed simultaneously and overnight exposures may be taken without continuous supervision.
3. Gamma rays are used for detecting defects in castings thicker than those inspected by X-rays.
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4. Owing to less scatter, gamma-rays are more satisfactory than X-ray for examining objects of varying thickness, whereas X-rays provide better results for castings of uniform thickness.
5. X-rays are better than gamma rays for detecting small defects in casting sections less than about 50 mm.
6. X-ray method is much more rapid than gamma-ray method, because unlike gamma- ray method, it requires seconds or minutes instead of hours.
3. Magnetic Particle Inspection:
This test is generally used to locate cracks and surface detects in a wide range of products. But in particular, it is employed to detect fatigue cracks at points of local high tensile strengths. The name “Magnuflux” is commonly associated with this process. This method is a relatively simple and easy technique. It is almost free from any restriction as to size, shape, composition and heat treatment of ferromagnetic substance.
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This method involves two steps:
(i) Magnetisation of material, and
(ii) Application of finely divided magnetic particles (either powder or suspended in a liquid).
In homogeneities such as blow holes, cracks and inclusion in a magnetic material produce a distortion in an induced magnetic field. The path of the magnetic flux is distorted because the inhomogeneities have different magnetic properties than the surrounding material. All the magnetic modes of non- destructive testing employ some means of which distortion called leakage flux, can be detected.
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When the flow is a surface flow or lies sufficiently close to the surface, the lines of force set up a pair of poles at the surface, which acts like a small horse shoe magnet to attract and hold some of the magnetic powder. These powder particles form a visible indication of the location and extent of the defect. The surface must be clean, dry and free of rust.
The specimen should be magnetized (for maximum sensitivity) with the direction or magnetic field perpendicular to the defect. A circular field will locate a detect lying parallel to the axis of the specimen but will not locate a transverse flow. Similarly to locate a transverse flow, longitudinal field should be produced (using a current carrying solenoid).
The magnetic powder can be used either in the form of a powder (dry technique) or in a liquid suspension (wet technique). The material used for dry technique consists of finely divided ferromagnetic particles having high permeability and low resistivity. The particles may be coloured to give maxima contrast on the specimen.
In the wet technique finely divided particles of red or black iron oxide are suspended in a light petroleum distillate or in water. Wet method is better for locating minute surface defects; moreover melted powder can be applied both on vertical and underside of horizontal surfaces for detecting cracks.
4. Ultrasonic Testing:
The sound waves whose frequency is above the upper pitch limit of the human ear are called Ultrasonics. These frequencies are generally considered to range from 15 kC/sec. upward.
Ultrasonic testing and inspection is one of the most useful non-destructive methods in metal testing. Rail roads, water or oil pipes, boilers, shafts, die blocks, aircraft parts of forged materials, extruded and forged aluminium parts etc. are tested for cracks, segregation, inclusions or other internal discontinuities. Certain types of welds are also inspected by this method.
Ultrasonic waves are usually generated by the Piezoelectric effect which converts electrical energy to mechanical energy. A. quarter crystal is used for the purpose.
A. The surface of casting to be inspected by ultrasonics is made fairly smooth either by machining or otherwise so that ultrasonic waves can be efficiently transmitted from the probe into the casting and even small defects can be detected properly.
Before transmitting ultrasonic waves, an oil film is provided between the probe and the casting surface; this ensures proper contact between them and better transmission of waves from the probe into the surface of the object to be tested.
B. For carrying out the test/operation, ultrasonic wave is introduced into the metal and the time interval between transmission of the outgoing and receptions of incoming signals are measured with a Cathode Ray Oscilloscope (CRO). The time base of CRO is so adjusted that the full width of the trace represents the section being examined.
C. As the wave is sent from the transmitter probe it strikes the upper surface of the casting and makes a sharp peak or pip (echo) at left hand side of CRO screen. If the casting is sound, this wave will strike the bottom surface of the casting, get reflected and indicated by a pip towards the right hand end of CRO screen.
In case a defect exists in between top and bottom casting surfaces, most of the beam striking this defect will get reflected from the defect, reach the receiver probe and indicate a pip (echo) on the CRO screen before the pip given by the waves striking the far end of the casting and returning. Quantitative assessment of defects can be made on the basis of use of comparison standards containing real or artificial detects such as drilled holes in metal blocks.
Advantages:
(i) It involves low cost and high speed of operation.
(ii) This method is more sensitive than radiography.
(iii) The sensitivity of ultrasonic flow detection is extremely high, being at a maximum when using waves of highest frequency.
(iv) Big castings can be symmetrically scanned for initial detection of major defects.
(v) It is a fast and reliable method of non-destructive inspection.
(vi) The minimum flaw size which can be detected is equal to about 0.1% of the distance from the probe to the detect.
(i) This method of inspection is sensitive to surface roughness.
(ii) The interpretation of oscillograph may not be easy in complex castings.
5. Electrical Method:
The electrical methods consist in measuring the electrical resistance of the material and then to note the variation in the electrical resistance. The variation is co-related to the physical defect. A number of electrical methods have been employed in non-destructive inspection and testing of machinery and wide variety of metallic material and parts for dimensional inaccuracies and physical defects.
A crack detector operates on the principle that if a crack occurs anywhere within the piece it interferes with the flow of electric current through the metal, therefore, increasing its overall resistance. This holds true regardless of the shape of piece.
Operation of the instrument consists of accurately measuring the electrical resistance of some critical machine part between two definitely established contact points, usually chosen at extreme opposite ends and of repeating the measurement at regular intervals. When successive measurements show the increase in resistance at a progressive rate, a fatigue fracture is beginning to propagate and the part should be removed from the surface.
6. Damping Test:
Measurement of damping can give information on the origin of defect such as forming of quenching cracks. For example, an increase in damping was found in steel specimen which had been quenched. It is possible to determine the position of a crack in a cylindrical piece.