Volume 12 - Fractography Part 11 ppsx

581: Area of fracture surface not near inclusion formations shows primarily ductile fatigue striations.. Failure of AISI type 304 tube due to chloride stress-corrosion cracking SCC from

Intentional fracture by overpressurization of ASTM A508, class 2, pressure vessel The intermediate test vessel (ITV) was fabricated as part of the Heavy Section Steel Technology (HSST) program at Oak Ridge National Laboratory, a government-sponsored effort aimed at gaging the toughness of nuclear pressure vessels The ITV measured 1 m (39 in.) in outside diameter with a 150-mm (6-in.) thick wall and was designed for 67-MPa (9.7-ksi) internal pressurization The preflawed vessel contained a semielliptical fatigue-cracked defect measuring 205 mm (8 in.) long and 65 mm (2 in.) deep Failure occurred at the

55 °C (130 °F) test temperature and an internal pressure of 198 MPa (28.7 ksi) nearly three times the design pressure allowed by the ASME code and four times the probable operating pressure Fig 576: Fracture surface Note machined and fatigue precracked flaw at top center Region of fatigue sharpened notch had a dimpled morphology; farther away from the flaw, a cleavage morphology Fig 577: Schematic of fracture surface in Fig 576 Initial failure was by ductile tearing.Flaw grew to approximately 500 mm (20 in.) long and 100 mm (4 in.) deep before onset of brittle propagation (D.A Canonico, C-E Power Systems, Combustion Engineering Inc.)

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Effect of inclusions on fatigue crack propagation (FCP) in ASTM A514F Direction of FCP is toward the top Fig 581: Area of fracture surface not near inclusion formations shows primarily ductile fatigue striations Also note secondary cracking Through-thickness (S-T orientation) FCP specimen at

∆K = 55 MPa m (50 ksi in) SEM, 1020× Fig 582 and 583: Through-thickness (S-L orientation)

fractures near inclusion formations at two levels of ∆K At the low 22-MPa m (20-ksi in) level (Fig 582), fatigue damage is by striation formation in the interinclusion material of an inclusion formation

At the high 55-MPa m (50-ksi in) level (Fig 583), ductile fracture (microvoid coalescence) of the interinclusion material occurs See also Fig 579 and 580 SEM, both at 1020× (A.D Wilson, Lukens Steel Company)

Brittle fracture of tension flange for large box-girder bridge The flange measured 75 cm (30 in.) wide and 55 mm (2 in.) thick and was welded to a trapezoidal box girder It was made of 55-mm (2 -in.) thick ASTM A517H plate Failure occurred catastrophically across the full width of the flange as the concrete deck of the bridge was being placed Fracture was arrested about 100 mm (4 in.) down the web of the girder Ambient temperature: 14 °C (58 °F) Cause of fracture was inadequate toughness in the quenched and tempered 690-MPa (100-ksi) minimum yield strength steel At the time, toughness in A517 was assumed and was not a specification requirement as it is now Also, the then-current specification limited maximum thickness to 50 mm (2 in.) However, thicker plate was allowed for this application Fig 584: Fracture surface of flange Chevrons point to origin at right Fig 585: Close-up

of fracture origin (right) at weld between flange and cross bracing Fig 586 and 587: Photomicrographs of structure of failed flange near the surface (Fig 586) and at the center (Fig 587)

of the 55-mm (2 -in.) thick plate Both at 500× (C.E Hartbower, Consultant)

Effect of inclusions on FCP in ASTM A533B The conventionally melted electric furnace steel was water quenched from 900 °C (1650 °F), tempered at 670 °C (1240 °F), air cooled, stress relieved at 595

°C (1100 °F), and then furnace cooled FCP specimen tested in the through-thickness (S-T, S-L) orientations Direction of fatigue crack propagation is toward the top Fig 588: Fracture surface of S-

L oriented FCP test specimen Fracture surface appearance due to inclusion formation on depressions

or plateaus on the fracture surface Figures 589, 590, and 591 show three kinds of MnS inclusions in type II MnS inclusion formations: highly elongated (Fig 589), flattened or pancaked (Fig 590), and small and round as a result of homogenization (Fig 591) SEM, Fig 589 and 590: 230×, Fig 591: 200× Fig 592: Alumina (Al 2 O 3 ) galaxies on the through-thickness fracture surface of A533B SEM, 200×

∆K in Fig 589, 590, and 592 = 25 MPa m (23 ksi in); in Fig 591, 58 MPa m (53 ksi in) See also Fig 593, 594, 595, 596, 597, and 598 (A.D Wilson, Lukens Steel Company)

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Effects of inclusions on fatigue crack propagation (FCP) in ASTM A533B In conventional electric furnace steel, protrusions perpendicular to the direction of FCP were noted on T-S and L-S fracture surfaces The protrusions result from inclusion formations (type II MnS and Al 2 O 3 ) acting as crack deflectors Figures show T-S orientation features FCP direction is toward the top Fig 595: Protrusions on fracture surface Fig 596: Metallographic cross section reveals MnS inclusions on a protrusion 170× Fig 597: Protrusion SEM, 15× Fig 598: Protrusion with Al 2 O 3 galaxy SEM, 28°

tilt angle, 325× Fig 597 and 598 at ∆K of 40 MPa m (36 ksi in) See also Fig 588, 589, 590, 591,

592, 593, and 594 (A.D Wilson, Lukens Steel Company)

Cavitated intergranular fracture due to hydrogen of ASTM A533B The specimen of pressure vessel steel yield strength, 630 Mpa (91.5 ksi) was exposed for more than 1000 h to temperatures above 550

°C (1020 °F) and gaseous hydrogen at a pressure greater than 17 MPa (2.5 ksi) Under these conditions, hydrogen diffuses into the steel, reacts with thermodynamically less stable carbides, and forms bubbles of methane gas along grain boundaries Mechanical properties plummet and, under impact, failure occurs by rapid coalescence of the methane bubbles, each nucleated at a submicron carbide or inclusion, along prior-austenite grain boundaries These cavitated intergranular fractures are common in steels used for hydrogen service (hydrocracking or coal conversion processes, for example) and resemble both failures in "overheated" steel, where reprecipitation of fine grain boundary sulfides due to an overly high austenitization temperature provides the source of the voids, and high-temperature creep cavitation fractures, where voids form at carbides and grow by vacancy coalescence and matrix creep Fig 599: Fracture surface of Charpy specimen Note secondary cracking due to severe bubble coalescence along grain boundaries SEM, 125× Fig 600: Same as Fig

599, but at higher magnification Note nucleus at bottom of each cavity SEM, 3330× (R.H Dauskardt and R.O Ritchie, University of California)

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Failure of AISI type 304 tube due to chloride stress-corrosion cracking (SCC) from the outside in The oil feed tube was used in a paper mill, where relative humidity was 100% Cracks and evidence of corrosion were found under the coating of the painted side of the tube No damage was noted on the other, unpainted side Torsional stresses in service and chlorides in the paint (at a level of 6600 ppm) provided the conditions necessary for chloride SCC Fig 630: Crack on painted side of tube Angle reveals influence of torsional stresses Fig 631: Stress-corrosion cracks extending from the outside diameter of the tube (at top) toward its inner diameter 10% oxalic acid electrolytic etch; 28× Fig 632: Fractured tube Branching of crack verifies that fracture occurred from the outside diameter (at top)

in 10% oxalic acid electrolytic etch; 28× (Z Flanders, Packer Engineering Associates, Inc.)

Hydrogen-embrittlement failure of AISI type 304 bellows The component was made by rolling sheet into tube, seam welding, and then forming Continuous pressurization with gaseous hydrogen at 172 kPa (25 psi) induced hydrogen embrittlement at roots of bellows convolutes, regions where residual stresses from forming were at a maximum Fig 634 and 635: Bellows assembly and hydrogen-induced circumferential cracks at A and B Fig 636, 637, and 638: Microstructure of fifth convolute cross section, showing twins, transgranular and intergranular fracture modes, and secondary cracking Oxalic acid electrolytic etch, 10×, 50×, and 200×, respectively (R.J Schwinghamer, NASA Marshall Space Flight Center)

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In-service failure of a paper machine suction couch roll due to intergranular stress-corrosion cracking (SCC) The centrifugally cast roll was made of ACI CF-8M (~AISI type 316) and measured 8.8 m (29 ft) long and 1.2 m (49 in.) in diameter with a 65-mm (2.5-in.) wall thickness Cracking initiated near the outside diameter of the roll, 2.7 m (9 ft) from the driven end, and extended roughly circumferentially 320° around the roll An overall sensitized microstructure and no evidence of corrosive attack at the lightly stressed, undriven end of the roll indicated a stress-corrosion mode as opposed to plain intergranular corrosion The specific corrosive agent was not identified Fig 646: Portion of crack surface (after mechanical separation) Cracks initiated at the drilled holes near, but not directly on, the outside diameter of the shell and then propagated from hole to hole and through the wall thickness Fig 647: Close-up of crack face reveals intergranular nature of failure At the

outside diameter (left) and 50 to 60% inward, the grain structure of the cast alloy is columnar The remainder is predominantly equiaxed Fig 648: Evidence that cracking progressed from the outside to the inside diameter Region of roll surface at center is completely separated, but is connected to the rest of the shell at the inside diameter Fig 649: Photomicrograph of structure near the inside diameter of the roll Note intergranular cracking along sensitized, near-equiaxed grain boundaries Ferric chloride etch, 20× (F.W Tatar, Factory Mutual Research Corporation)

Brittle fracture due to severe carburization of a porous metal filter tube The P/M type 316L tube

(5-μm nominal filter grade) was used in a coal-gasification pilot plant to remove entrained ash and char particles from the raw-product gas stream Failure occurred after being in service for 8 h at 540 °C (1000 °F) in contact with a gas stream having the following composition (in vol%): 30% H 2 O, 12.6%

H 2 , 0.43% Ar + O 2 , 35.2% N 2 , 8.4% CO, 11.7% CO 2 , 1.0% CH 4 , and 0.105% H 2 S Particulate matter

in contact with the tube contained 32 to 34 wt% C Carburization proceeded inward from the outside diameter of the tube in the direction of gas flow through the porous wall Source of carbon was the char in the gas stream Fig 655: Section through tube wall, outside diameter at top Outer third of tube thickness is almost completely transformed to mixed (Cr,Fe) 23 C 6 and (Cr,Fe) 7 C 3 carbides Balance of thickness has partially transformed, particularly in the vicinity of pores Carbon content of the bulk material was 0.22 wt%, compared with the 0.03 wt% max specified for type 316L Carbon content at the outside diameter was 1.40 wt% 35× Fig 656: Structure near through-wall crack at outside diameter of failed filter tube Mixed carbides (gray phases) dominate Only a very few small, light-colored islands of untransformed type 316L matrix remain The volume increase associated with the formation of carbides has substantially reduced the amount of porosity (dark areas) 340× Fig 657: Outside diameter of failed filter tube at region containing an axial seam weld (at left) Note that only the surface of the nonporous type 316L filler metal was transformed to carbides 340× (D.R Diercks, Argonne National Laboratory)

Fatigue crack nucleation at slip bands in the plate of a hip screw (compression tube and plate) surgical implant Material and processing same as in Fig 661, 662, and 663 Scanning electron photomicrographs are of the lateral (tension or screw-head-side) surface of plate subjected to fatigue loading Fig 664: Lateral surface of plate Note slip-band activity and intrusions formed by slip bands SEM, 187× Fig 665: Higher-magnification view of area on lateral surface of plate Note microcracking at slip bands and the crack being formed at a grain-boundary triple point SEM, 1160× Fig 666: Fatigue crack formed on lateral surface of plate Growth is along slip planes SEM, 983× Fig 667: Area on lateral plate surface near fracture origin Features include slip-band cracking and crack growth along intersecting slip planes SEM, 1400× (H.R Shetty, Zimmer Inc.)

Fatigue crack nucleation in the plate of compression hip screw Material and processing same as in Fig 661, 662, and 663 This pair of scanning electron photomicrographs shows the medial (compression or bone-side) surface of the failed plate Fig 668: Medial surface near fracture origin Ridges result from grain deformation SEM, 86× Fig 669: Higher-magnification view of representative area in Fig 668 Note slip-band activity and intersecting slip bands See also Fig 664,

665, 666, and 667 SEM, 859× (H.R Shetty, Zimmer Inc.)

Failure of AISI type 316L Jewett nail (hip implant) due to improper installation Fig 680: Fractured implant after removal from body The threaded tip of an impactor-extractor tool is inserted by the surgeon into a hole in the chamfered portion of the implant located where the side plate (left) meets the tri-finned nail (right) During installation, the threaded tip of the tool broke off The jagged edge of the broken tool then gouged the rim of the chamfer Even so, the operation was judged a qualified success The implant failed 4 weeks after installation Fig 681: Close-up of fractured Jewett nail Broken tip of impactor-extractor is in nail portion of implant at right Dashed lines show cutting planes for subsequent microscopic examination Fig 682: Plate end of fractured impact Fracture originates on ridge (R) of chamfer The chamfer-fracture surface interface is indicated by I (see Fig 684, 685, 686, and 687) Fig 683: Close-up of area near fracture origin R in Fig 682 Tensile stress was concentrated

at groove G1, created by fractured edge of impactor-extractor tool Fracture propagated along this groove Also note second and third grooves (G2 and G3) and what appears to be corrosion (areas labeled C) SEM, 50× (R.J Gray, Consultant)