Varian Linatron High-Energy X-ray Applications 2007

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When radiographing curved sections, the concave side should face the source and, when possible, flexible film holders should be formed to the casting contour. While this practice tends to improve film/screen contact, it may increase the image distortion at the edges of the curved film. To control for this, image quality indicators should be placed at the extreme widths of the film, and there should be adequate overlap in adjacent exposures to ensure complete coverage. While an oblique angle may be required in some regions, when possible, the X-ray beam should enter the casting at right angles to the front surface, and the film should be set up perpendicular to the central ray. When the casting thickness varies more than 1/2-inch (12.7 mm) equivalent steel, the multiple-film technique can be employed to cope with the varying section thicknesses. These variations may also create significant scatter, which can be minimized using special procedures (see the discussion on scatter control). Layout and Marking. Radiographic assignments can involve fairly simple techniques. This can be as simple as placing the film cassette under or behind the casting and making the exposure. But, for large, heavy castings such as turbine bodies, the film placement and radiographic projections can be complex and may require careful planning and fixturing. Crane mounted Linatron used to examine large objects. page 42 The radiographer should be furnished with either a marked print of the casting, showing the projections and areas to be covered, or a rough sketch showing the film locations, markers and other relevant information. The marked print or sketch should remain with the casting radiographs. The radiographer should retain a copy to add information about the film and screens used, image quality indicator selection and placement, filtration, exposure and resulting film density, and other pertinent facts about the job. Casting Radiographic Procedures. When compared with lower energy systems, Linatron radiography of castings can often produce significantly more readable film images. The increased latitude and reduced wide angle scatter provide major advantages not found in lower energy radiography. However, high-energy radiographs will have less contrast, and the imaging of fine cracks and filamentary porosity can be difficult. Cracks usually do not progress in a straight line and do not present welldefined edges to the X-ray path. This usually causes images to have penumbral edges, which makes detection difficult. Cracks which lie at angles to the X-ray beam can be seen if their width exceeds the limit imposed by the overall radiographic sensitivity, as determined from the equation; Varian Linatron applications Δx = W/sin ø (see Figure 5-1) where Δx = the crack width in the direction of the radiation W = the crack width ø = the angle of the crack to the x-ray beam
FIGURE 5-1. Relationships between crack width (W), projection angle (ø), and contrast sensitivity (Δx/x). Almost all castings contain discontinuities of various types (inclusions, microshrinkage, etc.) that have no effect on the strength or serviceability of the casting. Repairing these discontinuities is expensive and sometimes can do more harm than good. It is the radiographer’s task to determine the criticality of the various areas and to produce the best possible radiographs. • RADIOGRAPHY OF FLANGES. Whenever flanged areas of a casting are radiographed, the radiograph should cover the flanges, the cylindrical wall, and the corner between them. Film should be placed under the flange and inside the casting, as shown in Figure 5-2. Two or three projections may be required since an exposure that shows shrinkage defects might not show tears. It is important the exposures provide a clear picture of the junction between the flange and the wall. The radiographs must show shrinkage completely if there is shrinkage in the flange and wall areas. Cracks and tears must be found if they are present in the fillet. Flange exposures should be made for the average thickness of the section being radiographed. Multiple film techniques should be used as necessary if multiple thicknesses are present in the radiographed section. page 43 • RADIOGRAPHY OF VALVE BODIES. High-energy Xrays can penetrate both outside walls and provide the necessary sensitivity for demonstrating internal section defect when applied to small and some large valve bodies. This is especially true in the critical valve seat areas. Exposures are made by placing the film behind the valve body and beaming into the object at the best angle to project the critical internal section onto the film. Blocking, filtering, or other scatter reduction techniques should be used to ensure maximum sensitivity. A large D/T ratio is needed to minimize distortion and undesirable enlargement. FIGURE 5-2. Typical flanged casting exposure plan showing 12 exposures at 30˚ increments for three beam orientations (cr = central ray). Varian Linatron applications
Page 1: Varian Linatron High-Energy X-ray A
Page 5: Preface This manual presents theory
Page 8 and 9: FIGURE 1-3. Linatron M System Diagr
Page 10 and 11: Linatron targets are specially desi
Page 12 and 13: FIGURE 2-3. Half-value layer for pr
Page 14 and 15: Beaming and Field Flatness. Electro
Page 16 and 17: FIGURE 3-2. Linatron x-ray intensit
Page 18 and 19: It is most convenient to have expos
Page 20 and 21: Vacuum cassettes are recommended wh
Page 22 and 23: Table 4-3 lists some relative speed
Page 24 and 25: Manufacturers publish density versu
Page 26 and 27: FIGURE 4-5. Plot of density versus
Page 28 and 29: The total unsharpness is reduced to
Page 30 and 31: This wedge block is then radiograph
Page 32 and 33: Increasing Latitude With Multiple F
Page 34 and 35: FIGURE 4-12. Linatron 2 MeV typical
Page 38 and 39: FIGURE 4-16. Linatron typical expos
Page 44 and 45: Radiographic Procedures General Con
Page 48 and 49: Radiography of Welds Radiography is
Page 50 and 51: The width of separation required fo
Page 52 and 53: The propellant in large motors is o
Page 54 and 55: Grain Radiography Coverage Requirem
Page 56 and 57: Bibliography Books Practical Radiog
Page 58 and 59: Normally, the light generated in th
Page 60 and 61: FIGURE 6-3. Real-time automated ins
Page 62 and 63: Real-time image acquisition display
Page 64 and 65: D/T RATIO Ratio of the distance bet
Page 66 and 67: LINEAR ATTENUATION COEFFICIENT Cons
Page 68: Security & Inspection Products 6883

Varian Linatron High-Energy X-ray Applications 2007

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