1、附录 1 外文文献原文及译文 原文： An evaluation of NDT methods for the location and sizing of forging discontinuities In selecting an NDT method for flaw detection in forgings a number of variables must be considered: a) the type of discontinuity to be assessed； b) the method to be used for detection and evaluatio
2、n, and c) the variables associated with the forging itself The variables in item a) will govern the location within the forging and its orientation with respect to a particular surface Item b) could include a considerable array of NDT methods, but for the purpose of this paper only the six most wide
3、ly used are considered 一visual testing (VT), penetrant inspection(PI), magnetic particle inspection(MI), eddy current testing (ET), radiographic inspection (RT) and ultrasonic inspection (UI). In the last item c) the component race include such things as condition, geometry access for inspection. a)
4、 Forging discontinuities The location of the discontinuity will have a significant influence on the selection of the NDT method to be used and they are therefore grouped into three categories, to aid this selection: 1. open to the surface: laps, seam, burst, slugs, cracks and inclusions 2. slightly
5、subsurface: seam, stringers, inclusions and grain structure variations 3. internal: stringers, burst, lamination, grain structure, inclusions and piping A brief review of these terms may be helpful: Lap: folded metal, flattened into the surface but not fusing with it Seam: linear flaws due to oxidiz
6、ed blow holes or ingot splashes, which are elongated by hot working Burst: ruptures caused by failure of plastic deformation by processing at too low a temperature or excessive working of metal Stringers: a bar stock defect, due to non metallic inclusions being squeezed out into long and thin string
7、s Lamination: planar defect aligned parallel to surface, originating in the original ingot from rolled out piping Cracks: transgranular failure, due to localized stresses resulting from non-uniform heating or cooling and non-plastic deformation Inclusions: impurities, such as slag, oxide and sulphid
8、es, often from the original molten stage in forming the billet used for forging Grain structure: depending upon the extent of working, (deformation and recrystllisation) can be as small as 0.5mm or as large as 10mm Piping: a cavity at the centre of the ingot or billet, caused by shrinkage during sol
9、idification Slug: a piece of foreign matter that has been pressed or rolled into the surface of the material b) The NDT Method VT visual testing is the oldest of the NDT methods but still valid and widely used today The system is based upon observation, usually by a human observer, but now increasin
10、gly by digital/video cameras which use pattern recognition to locate dissimilar areas in a surface. The sensitivity will depend upon the method but typically a good observer with simple visual aids can resolve 0.5mm differences aids will include magnifying glasses (up to x10), microscopes(up to x100
11、) and fibred-optic bores copes and endoscopes for viewing internal details in hollow or complex sections. The system is used for surface inspection only with costs in the range $4 to $4000. PT 一 the surface is covered with brightly covered oil (typically red or fluorescent), which will penetrate any
12、 surface openings. After removal of excess, an absorbent, white powder is applied, which draws any trapped oil to the surface. This creates an indication of the presence of the surface opening. This process, like visual inspection, also requires visual acuity, but the indications are enhanced by the
13、 process, since bleed-out spreads the visual image. Costs can range from as little as $4 for a couple of cans, to $8000 for a process line. Both VT and PT are surface inspection systems only arid will therefore detect only those discontinuities that have a definite surface opening Surface cleanlines
14、s is very important, particularly with PT. MT 一 ferromagnetic materials carrying a large flux density； retain the presence internally, with little external evidence other than at the poles. Any discontinuity in the material will disturb this uniform flux and create a small leakage at the site of the
15、 discontinuity. This leakage can be detected by the fact that finely divided； ferromagnetic particles collect at the-site, creating an indication. As with PT, the particles can be colored, to increase contrast, which when viewed under suitable lighting, create a clear visual image of the discontinui
16、ty. However, unlike PT the leakage can pass through thin layers of paint or plating materials, so that the discontinuity does not have to be open to the surface. The system can therefore detect surface AND slight subsurface discontinuities. However this is only possible in ferromagnetic materials, s
17、uch as iron, mild and tool steel, nickel, cobalt and martenstic stainless. It will not operate on Paramagnetic or Diamagnetic materials, such as copper, aluminum and austenitic stainless steel. A small electromagnet can cost as little as $200, but a large bench type machine can cost up to $10 000 an
18、d the cost of electricity can be substantial. ET 一 Direct current flowing in a coil, sets up a longitudinal magnetic, field through the coil, and exhibits a particular resistance to flow. If the current is alternating, then a further effect 一 inductive reactance, adds to this resistance, the total b
19、eing impedance. This impedance also causes a lag between the current and the voltage, called a phase shift. This shift and impedance are characteristics of the coil. If the coil is now placed close to a conducting surface, the reversing magnetic field induces a reversing current in the conducting (e
20、ddy current) which opposes the inducing field. This opposition alters the impedance of the coil and a suitable instrument can detect these changes (both phase angle and/or impedance).For a given ,discontinuity-free surface , a specific alteration will be present which can be zeroed .If the coil now
21、passes over a discontinuity, a change in induction will occur which will be registered by the instrument. However, a change in the conductivity of the material will also effect the induction, as will changes in permeability. Thus, non-uniform heat treatment, segregation and in homogeneities in mater
22、ial composition and structure will also effect the induction and create an indication. Another critical factor is the distance between the coil and the test surface. This lift off can be used in a positive way to determine coating or paint thickness, on conducting materials. But equally, differences
23、 in the coil/specimen gap can result in non-relevant signals. The system can therefore detect surface AND slight subsurface discontinuities. However this is only possible in conducting materials and the proximity of the test coil to the test surface is critical. This means that for any component (ot
24、her than flat plate), special probes are usually designed to follow specific component contours. A small eddy current machine can cost as little as $2000, but a large automated machine can cost up to $20 000 RT 一 Short wavelength, electromagnetic radiation will pass through many materials, depending
25、 upon density and thickness, and then create a range of exposures on either film or a fluoroscopic screen, to present a visual image of the internal composition of the item. Differences in absorption within the material due to such things as gas holes, cracks and bursts will create photographic dens
26、ity differences on the film or detector, which can be interpreted by trained personnel. The source of radiation can be an X-ray tube or a gamma source (such as Iridium or Cobalt) and the images can be generated on either film or as real-time images on fluoroscopic screens. Defect orientation is a vi
27、tal factor in radiography since it is thickness differences, which the process detects. Hence, a lamination type defect, parallel to the film would be almost impossible to detect. On the other hand, a crack perpendicular to the film would almost certainly be detected. It is therefore often the case
28、that a single component would have to be radio graphed from more than one direction, in order to detect most defects. Finally, the radiation used is highly hazardous and therefore any environment in which it is used, must suitably shielded, to prevent exposure of the operator. As well as shielding t
29、he use of X or gamma rays will also require, monitors, alarms, interlocks and personal dosimetry systems, which along with the film itself, adds to the cost. A basic X-ray set up would cost around $10000 and with ancillary equipment and film could cost $3000 per year to run. UT At an interface betwe
30、en materials of differing acoustic impedance, a sound wave will have a proportion reflected and the remainder transmitted. Thus a gas hole or crack in a forging will reflect a sound beam because of their large difference in acoustic impedance with the metal structure containing them. Since ultrasoun
31、d travels in a given material at a known (predictable) velocity, then the distance to a reflector will be a direct function of this time of flight of the pulse of sound. Its location can therefore be estimated .Since the amplitude of the returning signal is also related to the size of the reflector,
32、 then an approximation can be made of the extent of the reflector, in terms of length through-wall thickness and width. The data can be presented as an A scan, on a cathode ray tube (requiring skilled interpretation) or as a B or C scan, where the data are plotted on printers or strip charts as a pe
33、rmanent record. Depths of penetration can be adjusted (by calibration and probe selection) from 10mm to 3 meters in suitable, fine-grained material. However cast, or large grained forged material, could be attenuate signals to the extent that they are untestable. A typical portable flaw detector and
34、 probes would cost around $5000, a fully automated C scan immersion system could cost $2000. c) The variables associated with the forging 1.Surface condition For VT and PT surfaces better than 6.3um Ra would yield the best results. For MT a similar situation exists, where a confusing background coul
35、d result from rough surfaces. ET also requires a smooth a surface for preference, since lift-off effects could be unacceptable. For RT a surface roughness exceeding 1% of material thickness could result in a significant loon of sensitivity. However for UT, a suitably viscous couplant could assist in
36、 sound transmittance, but entry surface noise on the timebase and attenuation would reduce sensitivity. 2.Geometry Flat surfaces are the simplest to inspect, by any method. However, PT is least influenced by geometry, being a liquid process. MT requires that the flux be at 90 to the discontinuity and thus, curved surfaces and hollow sections offer particular problems. VT may require special access equipment and ET will need specially designed probes for curved or irregular surfaces. Since RT relies on absorption differences, variations in thickness due to curvature
