Volume 12 - Fractography Part 8 doc

ASM Handbook Metals Handbook ASM Handbook Metals Handbook Materials Characterization Introduction: Atlas of Fractographs Distribution of Content Type of illustration 50 23 12 8 Photographs 7 Table 2 Causes of fractures illustrated in the Atlas of Fractographs for various ferrous and nonferrous alloys Fracture source Material Parts Test specimens Total Dimple rupture Cleavage Fatigue (a) Decohesive rupture (b) 4 . . . 13 4 8 5 9 52 3 5 . . . 4 7 2 2 3 . . . 5 4 8 . . . 4 7 Total 238 326 564 72 46 217 80 149 ASM Handbook Metals Handbook Failure Analysis and Prevention Table 3 Causes of fractures or failures illustrated in the Atlas of Fractographs for various engineered materials Fracture source Material Parts Test specimens Total Ductile Brittle Fatigue Mixed mode 1 . . . 1 . . . 8 3 8 Total 15 39 54 5 19 6 3 21 Pure Irons: Atlas of Fractographs Pure Irons Fig. 1 Grain- boundary cavitation in iron. This is the mechanism by which metals typically fail when subjected to elevated temperatures and low strain rates. Composition, in parts per million: 70 C, 60 S, 54 O, 11 N, 40 P. Rod, 13 mm (0. 5 in.) in diameter, was made by vacuum induction melting, chill casting, and swaging. Heat treatment: recrystallize for 30 min at 850 °C (1560 °F), austenitize for 1 h at 1100 °C (2010 °F), air cool. Sample was tensile tested at 700 °C (1290 °F) and an initial strain rate of 1.1× 10 -6 /s. Test was interrupted other 12 h, and the sample was then broken by impact at -100 °C (- 150 °F). Fracture surface reveals cavitated grain boundaries. The intergranular cavities nucleated on second-phase (iron sulfide) partic les, examples of which are shown at A, B, and C. SEM, 4500× (E.P. George and D.P. Pope, University of Pennsylvania) Fig. 2 Sl ip lines in iron. Composition, in parts per million: 160 C, 40 S, 13 O, 6 N, 30 P. Rod, 13 mm (0.5 in.) in diameter, was made by vacuum induction melting, chill casting, and swaging. Heat treatment: recrystallize for 30 min at 850 °C (1560 °F), austenitize for 1 h at 1100 °C (2010 °F), air cool. Sample was tensile tested to failure at 700 °C (1290 °F) and an initial strain rate of 4.4 × 10 -5 /s. Slip lines, smoothened by diffusional flow, are visible on grain boundaries of the elevated-temperature fracture s urface. Failure was intergranular and resulted from the nucleation, growth, and eventual coalescence of grain-boundary cavities (see Fig. 1 ). In this case, however, cavity outlines were masked by the slip steps created on grain boundaries due to severe plastic deformation within the grains. The wavy nature of the slip lines is a characteristic of body- centered cubic iron. SEM, 2180× (E.P. George and D.P. Pope, University of Pennsylvania) Fig. 3 Stereo pair of scanning electron microscope views of the fracture surface of a Charpy impact test bar of high-purity iron. The specimen was broken after being cooled to equilibrium in liquid nitrogen (-196 °C, or - 321 °F). Flat cleavage has taken place on a variety of sharply divergent crystal planes. There are fine river patterns evident on nearly all of the facets. The cleavage steps that delineate the "river" systems are, however, very minute in height, providing only very small departures from a single crystallographic plane of crack growth. Characteristi c of iron when fractured at low temperature is the formation of "tongues," one of which is located at the lower end of the very bright facet. There even appears to be a river pattern on the left- hand portion of the "tongue." A second, smaller tongue projects from the facet shown obliquely at top left. 1100× Fig. 4 Surface of a room-temperature tensile-test fracture in a specimen taken from an ingot prepared by adding Fe 2 O 3 to pure iron in a vacuum melt equilibrated at 1550 °C (2820 °F) in a silica crucible. The ingot contained 0.07% O in the form of FeO. The fracture surface contains dimples that initiated at globular FeO inclusions averaging 5.2 μm in diameter. SEM, 1800× Fig. 5 Stereo pair of scanning electron microscope fractographs of the surface of a tensile-test f racture obtained at room temperature. The alloy was a low-carbon iron to which an appreciable amount of Fe 2 O 3 had been added to form an aggregate of FeO inclusions. The dimples that are characteristics of ductile rupture are evident here, and many of these dimples contain one or more globular oxide inclusions that are readily apparent. There appear to be two sizes of these oxide inclusions some being about 6 μ m in diameter and other about 3 μ m in diameter. Unlike many inclusions displayed in other fractographs, which have relatively smooth, unbroken contours several of the particles shown here possess sizable surface defects. Some of these defects may be expos ed internal shrinkage cavities. The surfaces of the dimples show contours that vaguely resemble fatigue- striation marks. The differences in topographic contours of the dimples displayed in this stereo pair of fractographs can be appreciated by viewing the fractographs stereographically, which provides a three- dimensional effect. It then becomes apparent that the dimples are chimneylike cavities with nearly vertical walls in many instances and with bottoms at great depth that appear black and without detail. The FeO inclusions appear to cling to the cavity walls, many at a point part way to the bottom of the "chimney." Most of the separating walls between adjacent chimneys are extremely thin, which makes it surprising that these walls did not rupture at a point closer to the bottom of the chimney. 1200× Fig. 6 Fig. 7 Fig. 8 Sequence of SEM fractographs, at increasing magnifications (80×, 950×, and 5000×, respectively), that show a fracture in an iron alloy containing 0.14% S and 0.04% O. The fracture was obtained by bending at room temperature. Several spheroidal oxide inclusions are visible, most of them having diameters in the range of 1 to 3 μm. The rectangle in Fig. 6 indicates the area that is shown at higher magnification in Fig. 7, and the rectangle in Fig. 7 indicates the area that is shown at still higher magnification in Fig. 8. The 6-μm-diam oxysulfide particle in Fig. 8 shows a shrinkage cavity plus a white spot from an electron beam impingement in fluorescent x-ray analysis. It is quite evident that, during the process of microvoid coalescence, the iron matrix has become detached from the globular inclusions at the metal-to-oxide and metal-to-sulfide interfaces, leaving these inclusions unaffected by the applied stresses and severe deformation taking place around them. Fig. 9 Fracture produced at room temperature by bendi ng iron containing 0.02% C, 0.14% S, and 0.04% O cast in a 7 × 7 × 20 cm (2.75 × 2.75 × 8 in.) ingot mold. A carbon- FeO reaction caused blowholes such as shown at top. Note the fine lamellar structure at bottom. See also Fig. 10. SEM, 75× Fig. 10 Higher-magnification view of the blowhole shown at the top in Fig. 9 , showing the interior of the blowhole that resulted from the carbon- FeO reaction. The pendants are droplets of a liquid oxysulfide that spread over the surface of the blowhole during freezing of the ingot. SEM, 1400× Fig. 11 Fracture by room-temperature bending in casting of similar composition to that of casting in Fig. 9 , but containing 1.1% Mn. The numerous inclusions contained within the dimples are particles of manganese oxide and manganese sulfide trapped between growing dendrite branches. SEM, 1500× Fig. 12 Higher-magnification view of the same fractured specimen shown in Fig. 11. X- ray fluorescent analysis of the inclusions indicates that some of them are Mn(Fe)O and that others are Mn(Fe)S, but the exact amount of contained iron was not determined. SEM, 4000× Fig. 13 Low- carbon iron containing a high percentage of oxygen, fractured in fatigue at room temperature. A large oxide inclusion has been nearly completely disengaged from its original pocket. Fatigue striations detour around, or extend into, the pocket. Crack propagation was from bottom to top. SEM, 2400× Fig. 14 Intergranular fracture that was generated in a specimen of oxygen- embrittled Armco iron by a Charpy impact test at room temperature. The grain facets appear sharp and clean. Note the secondary cracks, which follow grain boundaries. See also Fig. 15 and 16 for views of other regions of this fracture. SEM, 55× Fig. 15 View of another region of the surface of the impact fracture shown in Fig. 14 , showing facets that resulted from a combinat ion of intergranular rupture and transcrystalline cleavage. Note the array of small river patterns at the bottom edge of the large facet at center. See also Fig. 16. SEM, 655× Fig. 16 View of a third region of the surface of the impact fracture shown in Fig. 14 and 15 . Note the almost perfect grain-boundary surfaces and the sharp edges and points at which the separated-grain facets meet. The secondary cracks are equally clean separations. SEM, 670× [...]... Fig 83 Fig 84 Typical fracture surface morphologies for an annealed ferritic ductile iron Composition: 3.6% C, 2.2% Si, 0.3% Mn, 0.7% Ni, 0.2% Mo (same as in Fig 76, 77, 78, 79, 80 , 81 , and 82 ) Fig 83 : Dimpled rupture (ductile fracture) at room temperature SEM, 80 0× Fig 84 : Quasi-cleavage (brittle fracture) at low temperature SEM, 1600× (R.C Voigt and L.M Eldoky, University of Kansas) Fig 85 Fig 86 ... Voigt and L.M Eldoky, University of Kansas) Fig 87 Fracture surface of ferritic-pearlite ductile iron in Fig 85 and 86 The low-temperature fracture occurred via a brittle, quasi-cleavage mode SEM, 715× (R.C Voigt and L.M Eldoky, University of Kansas) Fig 88 Ductile-to-brittle transition in an annealed ferritic ductile iron (same alloy as in Fig 83 and 84 ) Above demarcation line is region of dimpled... Surface of a fatigue-test fracture in an experimental crankshaft of induction-hardened 8 0-6 0-0 3 ductile iron with a hardness of 197 to 225 HB Fatigue-crack origin is at arrow A Porosity at arrow B was unrelated to fracture initiation 2.5× Fig 42 Surface of a fatigue-test fracture in an experimental crankshaft of ductile-iron with a hardness of 241 to 255 HB Note the multiple fatigue-crack origins at... also Fig 24 TEM p-c replica, 5000× Fig 24 Same specimen as in Fig 23 after 5.5 cycles of 2.5° bending At center is the same area as in Fig 23 (reversed left to right) showing slip-band cracks that have grown at slip steps TEM p-c replica, 2000× Fig 25 Cleavage fracture in Armco iron broken at dry-ice temperature (-7 8. 5 °C, or -1 09.3 °F) The light band shows where cleavage followed a twin-matrix interface... thickness of the twin TEM p-c replica, 3000× Fig 26 Cleavage fracture in Armco iron broken at dry-ice temperature (-7 8. 5 °C, or -1 09.3 °F), showing facets of which most have the same orientation Facets that depart from the general orientation appear lighter or darker than the majority TEM direct carbon replica, 3000× Fig 27 Cleavage fracture in Armco iron broken at -4 5 °C (-4 9 °F) Instead of cleavage... produced here by microscopic plastic flow TEM p-c replica, 2000× Fig 28 Cleavage fracture in Armco iron broken at -1 96 °C (-3 21 °F), showing river patterns, tongues, and (from bottom right to top left) a grain boundary TEM p-c replica, 3000× Gray Irons: Atlas of Fractographs Gray Irons Fig 29 Fatigue fracture of sand-cast gray iron bread-crumb grinder The ASTM A159 part was machined and hot dip galvanized... ferritic ductile irons undergoes a gradual ductile-to-brittle transition Fig 62: Fracture surface of ferritic ductile iron tested at -4 0 °C (-4 0 °F) Note river patterns and plateaus characteristic of brittle fractures SEM, 150× Fig 63 and 64: High-magnification views compare internodular areas of ferritic ductile irons tested at room temperature and at -4 0 °C (-4 0 °F), respectively SEM, ~1900× (F.J Worzala,... due to rebounding SEM, 2000× Fig 1 08: Crack path of an imminent near-surface spall SEM, 1000× (H.W Leavenworth, Jr., U.S Bureau of Mines) Low-Carbon Steels: Atlas of Fractographs Low-Carbon Steels Fig 109 Fig 110 Fractures obtained in tension in low-carbon steel specimens that were tested at elevated temperature The specimens were taken from a series of 225-kg (500-lb) laboratory heats melted to yield... 610 and 670 strain units, respectively SEM, both at 120 0× Fig 78 and 79: After 700 and 760 strain units, respectively SEM, both at 600× (R.C Voigt and L.M Eldoky, University of Kansas) Fig 80 Fig 81 Effects of plastic deformation on internodule bridges in ferritic ductile iron Material and sample preparation same as in Fig 76, 77, 78, and 79 Fig 80 : Surface plastic deformation and microcracking ahead... Dimly visible in the central dimple is a globular particle of FeO; the particle is shown more clearly in Fig 19 SEM, 2400× Fig 19 Same fracture-surface area as that shown in Fig 18, but processed with a different exposure to bring out the shape and size of the globular particles of FeO in the central dimple The small dark spot at the lower right on the FeO particle is a shrinkage area SEM, 2400× Fig 20 . higher magnification in Fig. 8. The 6- m-diam oxysulfide particle in Fig. 8 shows a shrinkage cavity plus a white spot from an electron beam impingement in fluorescent x-ray analysis. It is quite. right) showing slip-band cracks that have grown at slip steps. TEM p-c replica, 2000× Fig. 25 Cleavage fracture in Armco iron broken at dry-ice temperature (-7 8. 5 °C, or - 109.3 °F). The light. Fig. 41 Surface of a fatigue-test fracture in an experimental crankshaft of induction-hardened 8 0-6 0- 03 ductile iron with a hardness of 197 to 225 HB. Fatigue- crack origin is at arrow A.