Volume 17 - Nondestructive Evaluation and Quality Control Part 15 ppsx

Fig. 8 Ultrasonic spectrum analyzer output showing change in transmitted intensity with density of green compact. Source: Ref 11 Figure 9 shows that the velocity of ultrasonic waves in green compacts is about half the velocity in sintered compacts and that it is essentially invariant with density (Ref 13). It has also been shown that the velocity of ultrasound in green parts is highly anisotropic and that the experimental reproducibility is very poor (Fig. 10). It has been proposed that the anisotropy in velocity is due to the orientation of porosity (Ref 15). Fig. 9 Effect of density on ultrasonic velocity in green and sintered cylindrical Ancorsteel 1000- B specimens. Source: Ref 12 Fig. 10 Anisotropy of ultrasound in green transverse rupture strength bars. Source: Ref 14 The variation in the velocity of ultrasound with applied pressure during the compaction of ceramic powders has been measured in situ by fixing transducers to the ends of the punches (Ref 16). Unlike the case of finished green P/M compacts, a clear relationship was found between longitudinal wave velocity and compacting pressure (Fig. 11), probably because the constraint of the punches and die forced the individual particles together, providing an efficient acoustic coupling between particles. Fig. 11 Ultrasonic wave velocity in ceramic powders, measured during compaction. Source: Ref 16 Ultrasound Transmission in Sintered Parts. Early work relating the physical properties of cast iron to the velocity of sound waves suggested the potential for evaluating P/M steels in the same way (Ref 17). As expected, both the velocity of sound in P/M parts and their resonant frequencies have been related to density, yield strength, and tensile strength. Plain carbon steel P/M specimens were used in one series of tests and the correlation was found to be close enough for the test to be used as a quick check for the degree of sintering in production P/M parts (Ref 12). Other work has demonstrated the relationship between sound velocity and tensile strength in porous parts (Fig. 12). The same types of relationships have also been documented in powder forgings Ref 19. Fig. 12 Correlation of ultrasonic velocity with tensile strength of sintered steel. Source: Ref 18 Sintered parts have been found to transmit ultrasound according to the relationships shown in Fig. 13. The highest wave velocities occurred in the pressing direction. An additional distinction was found between the velocities in the longitudinal and lateral axes of an oblong specimen, and these results were shown to be reproducible between different powder lots and specimen groups. The anisotropy of velocity diminished at higher densities and disappeared above 6.85 g/cm 3 . Fig. 13 Anisotropy of ultrasound velocity in sintered transverse rupture strength bars. Source: Ref 14 Ultrasonic Imaging: C-Scan. The C-Scan is a form of ultrasonic testing in which the testpiece is traversed by the ultrasound transducer in a computer-controlled scan protocol (Fig. 14). The transmitted intensity is recorded and analyzed by computer, and a gray-mapped image is output. Fig. 14 Schematic of a C-Scan scanning protocol for an adhesive-bonded structure. Source: Ref 20 In one trial, seeded oxide inclusions were detected in porous sintered steels using a C-Scan (Ref 21). The inclusions consisted of admixed particles of chromium oxide and alumina at concentrations of 65 to 120 particles per square centimeter. Inclusions as small as 50 μm in diameter were detected. Additional information on the C-Scan can be found in the articles "Ultrasonic Inspection" and "Adhesive-Bonded Joints" in this Volume. Ultrasonic Imaging: Scanning Acoustic Microscopy (SAM). Ultrasonic waves can be focused on a point using a transducer and lens assembly, as shown in Fig. 15 and described in the article "Acoustic Microscopy" in this Volume. In this way, the volume of the specimen being examined is highly limited, so that reflections from defects can be closely located at a given depth and position in the specimen. In SAM, the specimen is moved by stepper motors in a raster pattern, and an image of the entire structure can be built up. Scanning acoustic microscopy has been shown to be capable of resolving small surface and subsurface cracks, inclusions, and porosity in sintered, fully dense ceramics (Ref 22). Fig. 15 Schematic of the image-forming process in scanning acoustic reflecting microscopy. Source: Ref 22 Ultrasonic Imaging: Scanning Laser Acoustic Microscopy (SLAM). When a continuous plane wave impinges on a sample that is roughly flat in shape, it propagates through and is emitted from the sample with relatively little scattering, retaining its planar nature. When the plane wave is emitted from the sample, it contains information on variations in properties that were encountered in the interior of the sample, which takes the form of variations in intensity with position in the plane. A scanning laser acoustic microscope detects these variations as distortions in a plastic sheet that is placed in the path of the plane wave. The information is gathered by a laser that scans a reflective coating on one side of the sheet, as shown in Fig. 16 and explained in the article "Acoustic Microscopy" in this Volume. Fig. 16 General configuration used in scanning laser acoustic microscopy. Source: Ref 23 Therefore, although ultrasonic testing is not appropriate for evaluating green P/M parts, it is applicable to the assessment of sintered components. Optimum results dictate careful selection and placement of the transducers because the orientation of the defects influences the ability to detect them. Small defects close to the specimen surface can be masked by surface echoes. Although enhanced image analysis techniques appear beneficial, it is unlikely that the more sophisticated techniques, such as C-Scan and SLAM, will be cost effective for most ferrous P/M parts in the near future. Resonance Testing. When a structural part is tapped lightly, it responds by vibrating at its natural frequency until the sound is damped. Both the damping characteristics and the natural frequency change with damage to the structure. Changes in the natural frequency can be detected with a spectrum analyzer, as shown in Fig. 17. Fig. 17 Schematic of resonance test configuration. Source: Ref 24 [...]... techniques for the nondestructive evaluation of both green and sintered P/M parts In addition to detecting cracks in green parts, as well as part- to -part density variation, studies have shown that changes in resistivity due to poor carbon pickup during sintering were also detectable (Ref 31) Resistivity testing has also been used later in the processing sequence to screen heat-treated parts for incomplete... rotating-type ultrasonic flaw detection system Parameter Specifications Dimension of material, mm (in.) 1 5-3 2 (0.5 9-1 .26) Testing method Normal-beam method and angle-beam method Testing frequency, MHz 10 and 5 Number of rotations of probe, rev/min 1000 Signal transmit Noncontact rotation transmit Marker One each for near-surface flaw and inside flaw Source: Ref 1 Fig 5 Schematic of a typical rotating-type... 1970, p 201 19 E.R Leheup and J.R Moon, Yield and Fracture Phenomena in Powder Forged Fe-0.2C and Their Prediction by NDT Methods, Powder Metall., Vol 23 (No 4), 1980, p 177 20 K Subramanian and J.L Rose, C-Scan Testing for Complex Parts, Adv Mater Process inc Met Prog., Vol 131 (No 2), 1987, p 40 21 A Hecht and E Neumann, Detection of Small Inclusions in P/M Alloys by Means of Nondestructive Ultrasonic... located on the inside area and the near-surface area (Fig 6) Flaws inside the material are detected with the normal-beam method at each face of the material In this method, the untested zone remains in the near-surface area Therefore, surface and near-surface area flaws are detected with the angle-beam method at each face of the material That is, six normal-beam probes and six angle-beam probes are located... 1980, p 177 K Subramanian and J.L Rose, C-Scan Testing for Complex Parts, Adv Mater Process inc Met Prog., Vol 131 (No 2), 1987, p 40 A Hecht and E Neumann, Detection of Small Inclusions in P/M Alloys by Means of Nondestructive Ultrasonic Testing, in Horizons of Powder Metallurgy, 1986 International Powder Metallurgy Conference Proceedings, Part II, P 783 G.Y Baaklini and P.B Abel, Flaw Imaging and Ultrasonic... Ketterer and N McQuiddy, "Resistivity Measurements on P/M Parts: Case Histories," Paper presented at the Prevention and Detection of Cracks in Ferrous P/M Parts Seminar, Metal Powder Industries Federation, 1988 E.R Leheup and J.R Moon, Electrical Conductivity and Strength Changes in Green Compacts of Iron Powder When Heated in Range 5 0-4 00 °C in Air, Powder Metall., Vol 23 (No 4), 1980, p 217 E.R Leheup and. .. indicated by magnetic particle testing The prior test method consisted of sintering, sectioning, and magnetic particle inspection, a 2-h process This part was also the subject of a series of experiments demonstrating that the resistivity test method had high reproducibility and was not operator sensitive Fig 24 Resistivity-measurement device for examining P/M parts Courtesy of R.A Ketterer and N.F McQuiddy,... Vision and Pattern Recognition of Magnetic Particle Indications, Mater Eval., Vol 42 (No 11), Nov 1984, p 150 6 39 I Hawkes and C Spehrley, Point Density Measurement and Flaw Detection in P/M Green Compacts, Mod Develop Powder Metall., Vol 5, 1970, p 395 Nondestructive Inspection of Powder Metallurgy Parts R.C O'Brien and W.B James, Hoeganaes Corporation References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 J.W... Automatic Transmission Clutch Plate The part, shown in Fig 27, was pressed, sintered, and sized The resistivity test was then used to screen for part- to -part density variations to levels below 6.8 g/cm3, which was shown to be a minimum density level for achieving the radial crush strength specification for the part Again, a limiting resistivity value was determined for the part; resistivity values below 27.5... Metal Parts, Mater Eval., April 1979, p 76 A Gallo and V Sergi, Orientation of Porosity of P/M Materials Evaluated by Ultrasonic Method, in 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 Horizons in Powder Metallurgy, 1986 International Powder Metallurgy Conference Proceedings, Part II, p 763 M.P Jones and G.V Blessing, Ultrasonic Evaluation of Spray-Dried Alumina Powder During and After . promising techniques for the nondestructive evaluation of both green and sintered P/M parts. In addition to detecting cracks in green parts, as well as part- to -part density variation, studies. in a computer-controlled scan protocol (Fig. 14). The transmitted intensity is recorded and analyzed by computer, and a gray-mapped image is output. Fig. 14 Schematic of a C-Scan scanning. 31). Example 1: Automotive Air Conditioner Compressor Part. The resistivity-measuring equipment and hand-held probe are shown in Fig. 24. The part, shown in Fig. 25, was tested for green cracking