Volume 12 - Fractography Part 4 doc

Fig. 3 Comparison of light microscope (a and b) and SEM (c and d) fractographs of cleavage of faces in a coarse-grain Fe-2.5Si alloy broken at -195 °C (-320 °F). (a) Bright-field illumination. (b) Dark- field illumination. (c) Secondary electron image. (d) Everhart-Thornley backscattered electron image. All 60× Fig. 4 Comparison of light microscope (a and b) and SEM (c and d) fractographs of cleavage facets in a coarse-grain Fe-2.5Si alloy impact specimen broken at -195 °C (-320 °F). (a) Bright- field illumination. (b) Dark-field illumination. (c) Secondary electron image. (d) Everhart-Thornley backscattered electron image. All 60× Fig. 5 Comparison of light microscope (a and b) and SEM (c and d) fractographs of cleavage facets in a coarse-grain Fe-2.5Si alloy impact specimen broken at -195 °C (-320 °F). (a) Bright- field illumination. (b) Dark-filled illumination. (c) Secondary electron image. (d) Secondary e lectron image. (a) and (c) 120×. (b) and (d) 240× Figure 4 shows similar bright-field and dark-field light fractographs of cleavage facets in the Fe-2.5Si alloy as well as secondary electron and Everhart-Thornley backscattered electron images of another area. The images shown in Fig. 4 are similar to those shown in Fig. 3. Figure 5 shows higher-magnification bright-field light fractographs of coarse cleavage facets in the Fe-2.5Si specimen and SEM secondary electron images at the same magnification. All four fractographs are of different areas. Figure 6 shows the interface between the fatigue precrack and test fracture of an X-750 nickel-base superalloy subsize Charpy rising-load test specimen after testing in pure water at 95 °C (200 °F). The interface region is shown by bright-field and dark-field light microscopy (same areas) and by secondary electron and Everhart-Thornley backscattered electron images (different location, same areas). At the magnification used, evidence of fatigue striations in the precracked region is barely visible in the light microscope images compared to the SEM images. The test fracture region is intergranular, but this is not obvious in the light fractographs, Figure 7 shows bright-field and dark-field fractographs of the fatigue-precracked region, in which the striation are more easily observed than in Fig. 6. Figure 8 shows secondary electron SEM fractographs of the striations in the precrack region at the same magnifications as in Fig. 7. Figure 9 shows bright-field and dark-field light fractographs of the intergranular region. The intergranular nature of this zone is more obvious in Fig. 9 than in Fig. 6. Figure 9 also shows corresponding secondary electron and Everhart-Thornley backscattered electron fractographs of the intergranular test fracture. Fig. 6 Comparison of light microscope (a and b) and SEM (c and d) fractographs of the interface between the fatigue-precracked region and the test fracture in an X-750 nickel-base superalloy rising- load test specimen. The test was performed in pure water at 95 °C (200 °F). Note the intergranular nature of the fracture. (a) Bright- field illumination. (b) Dark-field illumination. (c) Secondary electron image. (d) Everhart- Thornley backscattered electron image. All 60× Fig. 7 Light microscope fractographs of the fatigue-precracked region of an alloy X- 750 rising load test specimen. (a) Bright-field image. (b) Dark-field image. (c) Bright-field image. (d) Dark- field image. (a) and (b) 60×. (c) and (d) 240×. Fig. 8 Secondary electron images of the fatigue-precracked region of an alloy X-750 test specimen. (a) 65×. (b) 260× Fig. 9 Comparison of light microscope (a and b) and SEM (c and d) fractographs of the test fracture in an alloy X-750 rising-load test specimen. Test was p erformed in pure water at 95 °C (200 °F). Note the intergranular appearance of the fracture. (a) Bright-field image. (b) Dark-field image. (c) Secondary electron image. (d) Everhart-Thornley backscattered electron image. All 60 × For comparison, Fig. 10 shows a similar X-750 rising-load specimen tested in air in which the test fracture is ductile. Figure 10 shows bright-filed and dark-field light fractographs of the interface between the fatigue precrack and the ductile fracture. A secondary electron fractograph is also included for comparison. Figure 11 shows high-magnification bright-field and dark-field light fractographs and a secondary electron fractograph of the ductile region of the test fracture at the same magnification. Microvoid coalescence (dimples) can be observed in all the fractographs, but the limited depth of field of the light fractographs is obvious. Fig. 10 Comparison of light microscope (a and b) and SEM (c) images of the interface between the fatigue- precrack area (left) and the test fracture region (right) of an alloy X-750 rising- load test specimen broken in air. The test fracture is ductile. (a) Bright-field image. (b) Dark-field image. (c) Secondary electron image. All 68× Fig. 11 Comparison of light microscope (a and b) and SEM (c) images of a ductile fracture in an alloy X- 750 rising-load test specimen broken in air. (a) Bright-field image. (b) Dark- field image. (c) Secondary electron image. All 240× These examples demonstrate that the microscopic aspects of fractures can be assessed with the light microscope. Although the examination is easier and the results are better with SEM, light microscopy results are adequate in many cases. Such examination is easiest to accomplish when the fracture is relatively flat. For rougher, more irregular surfaces, SEM is far superior. As a further note on the use of the light microscope to examine fractures, Fig. 12 shows a cleavage fracture in a low-carbon martensitic steel examined by using three direct and three replica procedures. Figure 12(a) shows an example of the examination of the fracture profile after nickel plating the surface. The flat, angular nature of the fracture surface is apparent. Figures 12(b) and 12(d) show a light microscope direct view the cleavage surface and a light microscope view of a replica. Figures 12(c) and 12(e) shows a direct SEM view of the fracture and a view of a replica of the fracture by SEM, respectively. Lastly, Fig. 12(f) shows a TEM replica of the fracture, but at a much higher magnification. The transmission electron microscope cannot be used at magnifications below about 2500×. Fig. 12 Examples of three direct (a to c) and three replication procedures (d to f) for examination of a cleavage fracture in a low-carbon martensitic steel. (a) Light micr oscope cross section with nickel plating at top. (b) Direct light fractograph. (c) Direct SEM fractrograph. (d) Light fractograph of replica. (e) SEM fractrograph of replica. (f) TEM fractrograph of replica Replicas for Light Microscopy In some situations, primarily in failure analysis, the fracture face cannot be sectioned, generally for legal reasons, so that it can fit within the chamber of the scanning electron microscope. In such cases, the fractographer can use replication procedures with examination by light microscopy, SEM, or TEM. The replication procedures for light microscopy are similar to those traditionally used for TEM fractography (Ref 6, 7, 8). In general, the 0.25-nm (0.01-in) thick cellulose acetate tape used for light microscopy is thicker than that used for TEM. The tape is moistened on one side with acetone, and this side is then pressed onto the fracture surface and held tightly in place, without motion, for 1 to 2 min. When thoroughly dry, the tape is carefully stripped from the fracture surface. An alternate procedure consists of, first, preparing a viscous solution of cellulose acetate tape dissolved in acetone and applying a thin coating to the fracture. Then, a piece of cellulose acetate tape is placed on top of this layer, pressed into the fracture, and held in place for 1 to 2 min. After drying, it is stripped from the fracture. The stripped tape is a negative replica of the fracture and can be viewed as stripped from the fracture, and it can be photographed to record macroscopic fracture features (Fig. 13). This tape is a permanent record of the fracture for future examination even if the fracture is sectioned. Fig. 13 Transparent tape replica of a fracture surface. See the article "Transmission Electron Microscopy" in this Volume for more information on replication techniques. Additional contrast can be obtained by shadowing the replica with either carbon or a heavy metal, such as chromium, molybdenum, gold, or gold-palladium, as is normally done in TEM fractography. Some fractographers also coat the back side of the replica with a reflective metal, such as aluminum, for reflected-light examination, or they tape the replica to a mirror surface. With an inverted microscope, some fractographers place the replica over the stage plate and then place a polished, unetched specimen against the tape to hold it flat and to reflect the light. Others prefer to examine the tape with transmitted light, but not all metallographers have access to a microscope with transmitted-light capability. Figure 14 illustrates low-power examination procedures for examination of replicas by light microscopy. Figure 14(a) shows the replica photographed with oblique illumination from a point source lamp, and Fig. 14(b) shows the same area using transmitted light. Carbon was then vapor deposited onto the replica, and it was photographed again using oblique light from a point source (Fig. 14c). Figure 14(c) exhibits the best overall contrast and sharpest detail. Figure 12(d) illustrates examination of a fracture replica by light microscopy using high magnification. This replica was shadowed with gold-palladium. Fig. 14 Comparison of replica fractographs of a fatigue fracture in an induction- hardened 15B28 steel shaft. Fracture was initiated at the large inclus ion in the center of the views during rotating bending. (a) Oblique illumination from a point source lamp. (b) Same area as (a), photographed using transmitted light. (c) Replica shadowed with a vapor-deposited coating and photographed using oblique illumi nation from a point source lamp. All 30× Fracture Profile Sections Despite the progress made in direct examination of fracture surfaces, examination of sections perpendicular to the fracture, particularly those containing the initiation site, is a very powerful tool of the fractographer and is virtually indispensable to the failure analyst. If the origin of the fracture can be found, the failure analyst must examine the origin site by using metallographic cross sections. This is the only practical method for characterizing the microstructure at the origin and for assessing the role that the microstructure may have had in causing or promoting the fracture. The safest procedure is to cut the sample to one side of the origin, but only after all prior nondestructive examinations have been completed. Cutting must be done in such a manner that damage is not produced. A water-cooled abrasive cutoff machine or a low-speed diamond saw is typically used. For optimum edge retention, it is recommended that the surface be plated, generally with electroless nickel, although some metals cannot be plated in this manner (Ref 9) and require other plating procedures. Figure 15 demonstrates the excellent edge retention that can be obtained with electroless nickel. A number of other edge retention procedures can also be used (Ref 9). Fig. 15 Example of the use of electroless nickel plating to provide edge retention. The micrograph shows wear damage at the surface of a forged alloy steel Medart roll. Etched with 2% nital. 285× Examination of fracture profiles yields considerable information about the fracture mode and mechanism and about the influence of microstructure on crack initiation and propagation. This is accomplished by examining partially fractured (Ref 10, 11, 12, 13, 14, 15) or completely fractured (Ref 16 17, 18, 19, 20, 21, 22, 23, 24, 25, 26) specimens: Quantitative fractography makes extensive use of fracture profiles (additional information is available in the article "Quantitative Fractography" in this Volume). One interesting approach defines a fracture path preference index to describe the probability of a particular microstructural constituent being associated with a particular fracture mode to assess the relationship between fracture characteristics and microstructure (Ref 24). In general, it is easier to assess the relationship between crack path and microstructure by using secondary cracks because both sides of the fracture can be examined. On a completely broken specimen, only one side can be examined; this makes the analysis more difficult. Although most light micrographs are taken with bright-field illumination, the analyst should also try other illumination modes. Fractures should be initially examined on cross sections in the unetched condition and then should be examined after etching. Naturally, a high-quality polish is required, and the effort extended in achieving a high-quality surface is always rewarded with improved results and ease of correct interpretation. Errors in interpretation are made when specimens are not properly prepared. As-polished samples should be examined first with bright-field illumination and then with dark-filled illumination, differential interference contrast (DIC) or oblique light, and polarized light, if the specimen will respond to such illumination. Dark-field illumination is very useful and is highly suited to the examination of cracks and voids. Photography in dark field is more difficult, but not impossible if an automatic exposure device is available. Oblique light and DIC are very useful for revealing topographic (relief) effects. For example, Fig. 16 shows a fatigue crack in an aluminum alloy viewed with bright-field illumination and DIC where the specimen was not etched. Although the second-phase precipitates can be seen in both views, they are more clearly revealed with DIC (compare these views with the bright-field etched micrograph of this specimen shown in Fig. 71). Fig. 16 Comparison of bright-field (a) and Nomarski DIC (b) illumination for examination of a fat igue crack in an as-polished aluminum alloy. See also Fig. 71. Both 600× Another example of the examination of fracture profiles is shown in Fig. 17, which illustrates an impact fracture in an austenitic weld containing phase. This sample is in the as-polished condition and is shown examined with bright-field, DIC, and dark-field illumination. It is clear that full use of the light microscope can provide a better description of the relationship of the crack path to the microstructure. Fig. 17 Comparison of bright-field (a), DIC (b), and dark- field (c), illumination for viewing a partially fractured (by impact) specimen of AISI type 312 weld metal containing substantial phase. All 240× Examination of properly polished specimens without etching often presents a clearer picture of the extent of fracture because etched microstructural detail does not obscure the crack detail. Etching presents other dark linear features, such as grain boundaries, that may be confused with the crack details. Therefore, it is always advisable to examine the specimens unetched first. Also, inclusions and other hard precipitates are more visible in unetched than in etched specimens. After careful examination of the as-polished specimen, the sample should be etched and the examination procedure repeated. Examples of fracture profile sections in the etched condition will be shown later in this article. Taper Sections Taper sections are often used to study fractures (Ref 9, 27, 28, 29, 30, 31). In this method, the specimen is sectioned at a slight angle to the fracture surface. Polishing of this plane produces a magnified view of the structure perpendicular to the fracture edge. The magnification factor is defined by the cosecant of the sectioning angle; an angle of 57° 43' gives a tenfold magnification. Etching Fractures Zapffe (Ref 2) and others (Ref 32, 33, 34, 35, 36, 37) have etched fracture surfaces in order to gain additional information. In general, etching is used to reveal the microstructure associated with the fracture surface (Ref 2, 36) or to produce etch pit attack to reveal the dislocation density and the crystallographic orientation of the fracture surfaces. Figure 18 shows an example of fracture surface etching of a cleavage fracture in a carbon steel sample. Although the etched fractures can be examined by light microscopy, SEM is simpler and produces better results. Fig. 18 Example of the use of etching to produce etch pits (arrows) on a cleavage fracture. (a) As- fractured. 320×. (b) Etched 60 s with nital. 320×. (c) Etched 360 s with nital. 320 ×. (d) Etched 360 s with nital. 1280× Deep-Field Microscopy The deep-field microscope provides greater depth of field for optical examination and photography of fractures (Ref 38, 39, 40). Its theoretical depth of field is 6 mm at 38× and 600 m at 250× (Ref 39). The instrument uses a very thin beam of light to illuminate the specimen. The light beam is at a constant distance from the objective of the microscope and is at the focal plane. During photographic exposure, the specimen is moved at a constant rate up through the light beam. Only the illuminated portions of the specimen are recorded photographically, and all of the illuminated portions are in focus; therefore, the resultant photograph is in focus. The use of the deep-field microscope and the problems encountered in obtaining good fractographs are discussed in Ref 39. References cited in this section 2. C.A. Zapffe and M. Clogg, Jr., Fractography A New Tool for Metallurgical Research, Trans. ASM, Vol 34, 1945, p 77-107 3. C.A. Zapffe et al., Fractography: The Study of Fractures at High Magnification, Iron Age, Vol 161, April 1948, p 76-82 4. C.A. Zapffe and C.O. Worden, Fractographic Registrations of Fatigue, Trans. ASM, Vol 43, 1951, p 958-969 5. C.A. Zapffe et al., Fractography as a Mineralogical Technique, Am. Mineralog., 36 (No. 3 and 4), 1951, p 202-232 6. K. Kornfeld, Celluloid Replicas Aid Study of Metal Fractures, Met. Prog., Vol 77, Jan 1960, p 131-132 7. P.J.E. Forsyth and D.A. Ryder, Some Results of t he Examination of Aluminum Alloy Specimen Fracture Surfaces, Metallurgical, March 1961, p 117-124 8. K.R.L. Thompson and A.J. Sedriks, The Examination of Replicas of Fracture Surfaces by Transmitted Light, J. Austral. Inst. Met., Vol 9, Nov 1964, p 269-271 9. G.F. Vander Voort, Metallography: Principles and Practice, McGraw-Hill, 1984 10. H.C. Rogers, The Tensile Fracture of Ductile Metals, Trans. AIME, Vol 218, June 1960, p 498-506 11. C. Laird and G.C. Smith, Crack Propagation in High Stress Fatigue, Philos. Mag., Vol 7, 1962, p 847-857 12. C. Laird, The Influence of Metallurgical Structure on the Mechanism of Fatigue Crack Propagation, in Fatigue Crack Propagation, STP 415, American Society for Testing and Materials, 1967, p 131-180 13. D.P. Clausing, The Development of Fibrous Fracture in a Mild Steel, Trans. ASM, Vol 60, 1967, p 504-515 14. I.E. French and P.F. Weinrich, The Shear Mode of Ductile Fracture in a Spheroidized Steel, Metall. Trans., Vol 10A, March 1979, p 297-304 15. R.H. Van Stone a nd T.B. Cox, Use of Fractography and Sectioning Techniques to Study Fracture Mechanisms, in Fractographic-Microscopic Cracking Processes, STP 600, American Society for Testing and Materials, 1976, p 5-29 16. R.W. Staehle et al., Mechanism of Stress Corros ion of Austenitic Stainless Steels in Chloride Waters, Corrosion, Vol 15, July 1959, p 51-59 (373t-381t) 17. K.W. Burns and F.B. Pickering, Deformation and Fracture of Ferrite-Pearlite Structures, J. Iron Steel Inst., Vol 202, Nov 1964, p 899-906 18. J.H. Bucher et al., Tensile Fracture of Three Ultra-High-Strength Steels, Trans. AIME, Vol 233, May 1965, p 884-889 19. U. Lindborg, Morphology of Fracture in Pearlite, Trans. ASM, Vol 61, 1968, p 500-504 20. C.T. Liu and J. Gurland, The Fracture Behavior of Spheroidized Carbon Steels, Trans. ASM, Vol 61, 1968, p 156-167 21. D. Eylon and W.R. Kerr, Fractographic and Metallographic Morphology of Fatigue Initiation Sites, in Fractography in Failure Analysis, STP 645, American Society for Testing and Materials, 1978, p 235-248 22. W.R. Kerr et al., On the Correlation of Specific Fracture Surface and Metallographic Features by Precision Sectioning in Titanium Alloys, Metall. Trans., Vol 7A, Sept 1976, p 1477-1480 23. D.E. Passoja and D.J. Amborski, Fracture Profile Analysis by Fourier Transform Methods in Microstructural Science, Vol 6, Elsevier, 1978, p 143-158 24. W.T. Shieh, The Relation of Microstructure and Fracture Properties of Electron Beam Melted, Modified SAE 4620 Steels, Metall. Trans., Vol 5, May 1974, p 1069-1085 25. J.R. Pickens and J. Gurland, Metallographic Characterization of Fracture Surface Profiles on Sectioning Planes, in Proceedings of the 4th International Congress for Stereology, NBS 431, National Bureau of Standards, 1976, p 269-272 [...]... (10 °F) below the 50% shear area drop-weight tear test transition temperature See also Fig 43 , 44 , and 46 Fig 46 Fracture of API grade X-60 line pipe tested 22 °C (40 °F) below the 50% shear area drop-weight tear test transition temperature Temperature given in °F See also Fig 42 , 43 , 44 , and 45 Full-scale testing of line pipe, as shown in Fig 43 , 44 , 45 , and 46 , revealed a good correlation between... within the pipe Fig 43 Fracture of API grade X-60 line pipe tested 5 °C (8 °F) above the 50% shear area drop-weight tear test transition temperature See also Fig 44 , 45 , 46 and text for details Fig 44 Fracture of API grade X-60 line pipe tested 1 °C (2 °F) below the 50% shear area drop-weight tear test transition temperature See also Fig 43 , 45 , and 46 Fig 45 Fracture of API grade X-60 line pipe tested... fractures in Charpy V-notch impact specimens shown at top Both 40 0× Another example of the macroscopic appearance of Charpy V-notch impact specimens is given in Fig 34, which shows four specimens of heat-treated AISI 43 40 tested between -1 96 and 40 °C (-3 21 and 1 04 °F), as well as plots of the test data (Ref 63) The test specimen at -8 0 °C (- 112 °F), which is near the ductile-to-brittle transition temperature,... strength ° C °F M Pa k si M Pa ks i 1 2 0 5 40 0 19 70 2 8 5 169 0 2 3 1 5 60 0 17 30 2 5 1 3 4 5 5 85 0 14 10 4 5 1 0 95 0 5 5 6 5 10 50 6 6 2 0 11 50 94 5 1 3 7 850 12 3 20 63 7 6 7 5 12 50 77 0 1 1 2 670 97 24. 5 66 Fig 31 Macrographs of quenched-and-tempered AISI 41 42 steel tensile specimens showing splitting parallel to the hot-working axis in specimens tempered at 45 5 °C (850 °F) or higher Splitting also... of heat-treated AISI 41 42 alloy 1 8 steel machined from a 28.6-mm (1 -in.) diam bar Specimens 1 and 2 were oil quenched and tempered at 205 and 315 °C (40 0 and 600 °F) and exhibit classic cup-and-cone fractures Specimens 3 to 7, which were tempered at 45 5, 510, 565, and 675 °C (850, 950, 1050, and 125 0 °F), respectively, all exhibit rosette star-type tensile fractures; specimens 4 and 7 exhibit the... Figure 41 shows five ship steel fractures produced by drop-weight testing at temperatures from -3 0 to -7 0 °C (-2 5 to -9 6 °F) The mating halves of each fracture are shown with the fracture starting at the notches at the top of each specimen The specimen broken at -4 5 °C (-5 0 °F) has the best definition of the chevron pattern Testing at higher temperatures produced shear fractures, but testing at -7 0 °C (-9 0... in.) in wall thickness that was made from 8 8 quenched-and-tempered AISI 1030 aluminum-killed fine-grain steel with an impact transition temperature below -4 5 °C (-5 0 °F) At room temperature, ductile rupture occurred when the vessel was pressurized to 59 MPa (8500 psig) (Fig 19) Figure 20 shows an identical vessel pressurized to failure at -4 5 °C (-5 0 °F) at 62 MPa (9000 psig) Greater pressure was required... cup-and-cone tensile fractures of specimens machined from plates (Ref 72) As with the rosette star-type tensile fracture, cup-and-cone fractures have been observed in quenched-and-tempered (205 to 650 °C, or 40 0 to 120 0 °F) alloy steels One study showed that the occurrence of splitting and the tensile fracture appearance varied with test temperature (Ref 72) Tests at 65 °C (150 °F) produced the cup-and-cone... a cup-and-cone fracture or by an alternate slip method producing a double-cup fracture Cup-and-cone fractures are observed in ductile iron-base alloys, brass, and Duralumin; double-cup fractures are seen in face-centered cubic (fcc) metals, such as copper, nickel, aluminum, gold, and silver Three types of tensile fractures have been observed in tests of fcc metals: chisel-point fractures, double-cup... brittle cleavage fracture, for example, the -7 3- °C (-1 0 0- °F) samples of the plates finish rolled at 707 and 538 °C (1305 and 1000 °F) Fig 36 Plot of absorbed-energy Charpy V-notch test data for Fe-1Mn steels finished at different temperatures (indicated on graph) Source: Ref 83 Fig 37 Macrographs showing Charpy V-notch impact specimens from Fe-1Mn steels finish-rolled at four different temperatures and . chisel-point fractures, double-cup fractures, and cup-and-cone fractures (Ref 48 , 49 ). The fracture mode changes from chisel-point to double-cup to cup-and-cone as the precipitate density and alloy content. tensile specimens is provided by tests of 25-mm (1-in.) diam, 1 1 4- mm (4 1 2 in.) long ASTM A490 high-strength bolts (Fig. 26 and 27). Bolt 1 was a full-size bolt with a portion of the shank turned. fatigue-precracked region of an alloy X- 750 rising load test specimen. (a) Bright-field image. (b) Dark-field image. (c) Bright-field image. (d) Dark- field image. (a) and (b) 60×. (c) and (d) 240 ×.