Volume 12 - Fractography Part 9 ppsx

In-service rotary bending fatigue fracture of automotive axle shaft direct-on bearing design due to improper heat treatment.. Fatigue fracture in a spring of AISI 1070 steel wire with a

Figure 211 is a polished and etched cross section of a partially flame-hardened crane gear of AISI 1045 steel in which fatigue cracks formed at the roots of many teeth after one year of service The cracks have been sharply delineated by etching Note that hardening did not extend even to the root fillet of any tooth Figure 212 is a polished and etched cross section of a portion of a gear that was subjected to full-contour flame hardening; note the better tooth form, with fully radiused tooth roots Both etched

Fractographs of case structure on shaft side of axle failure in Fig 220 and 221 Fig 222: Structure at fracture origin SEM, 20× Fig 223 (Same as Fig 222, but at higher magnification Note primarily intergranular fracture SEM, 1000× Fig 224: Same as Fig 223, but at higher magnification SEM,

Fractographs of core structure on shaft side of axle failure in Fig 220 and 221 Fig 225: Case-core transition zone SEM, 50× Fig 226 (Core structure at transition zone (see Fig 225) showing primarily cleavage fracture SEM, 1000× Fig 227: Same as Fig 226, but at higher magnification SEM, 2000× (General Motors Research Laboratories)

Classic bending overload fracture of automotive axle shaft (direct-on bearing design) Material and heat treatment: modified AISI 1050, induction hardened and tempered to 60 HRC at the bearing diameter The part was broken in the laboratory Fig 228: Fracture surface with chevrons in case- hardened zone pointing to single origin at top Fine- and coarse-grained structures in case and core regions, respectively, indicate that heat treatment was properly performed Fig 229: Side view of fractured axle shaft in Fig 228 (Z Flanders, Packer Engineering Associates, Inc.)

In-service rotary bending fatigue fracture of automotive axle shaft (direct-on bearing design) due to improper heat treatment The modified AISI 1050 part was induction hardened and tempered to 60 HRC min at the bearing diameter However, the hardened zone did not extend into the flange fillet radius Fig 230: Fracture surface Note ratchet marks at several locations around circumference indicating multiple fatigue origins Fig 231: Side view of axle shaft in Fig 230 (Z Flanders, Packer

Fatigue failure of crane wheel by spalling of a section of its selectively hardened tread Failure of the quenched and tempered medium-carbon steel forging occurred after 2 years of operation on a crane serving an open-hearth furnace Fig 239: Spalled section of tread Fatigue crack originated at a forging defect (arrow) 3× Fig 240: Close-up of failure origin 0.75× Fig 241: Oblique lighting reveals texture of fracture surface (compare with Fig 240) Note beach marks associated with fatigue crack

Fatigue fracture of 90-mm (3.6-in.) diam medium-carbon steel axle housing (217 to 229 HB) The part was subjected to unidirectional bending stresses normal for the application Fig 243: Fracture surface showing four major fatigue crack origins (arrows) Cracks progressed up both sides of the tube and joined at the small final rupture area at top Note increasing coarseness of fracture surface as cracks grew Fig 244: Higher-magnification view of origin area in Fig 243 Note beach marks A small

Torsional overload fracture of AISI 1060 drive shaft for power boiler stoker grate Service temperature: 370 °C (700 °F) Boiler had a history of fractured shafts This particular grate had jammed twice before, and each time a new key had to be machined due to twisting of the shaft Mechanical properties at surface of shaft: yield strength, 348 MPa (50.5 ksi); tensile strength, 556 MPa (80.6 ksi); elongation, 31% Core properties: yield, 300 MPa (43.5 ksi); tensile, 620 MPa (89.9 ksi); elongation, 25% Fig 249 and 250: One half of broken shaft and its fracture surface Fig 251 and 252: Mating half of shaft (two of three keyways are visible) and its fracture surface (note twisting and smearing) All at ~0.5× (Z Flanders, Packer Engineering Associates, Inc.)

Ferrite at prior-austenite grain boundaries and spheroidized pearlite characterize microstructure of drive shaft in Fig 249, 250, 251, and 252 All etched in 2% nital Fig 253 and 254: Structure at surface

of shaft (radial orientation) 100× and 400× Fig 255 and 256: Transverse microstructure 100× and

Fatigue fracture in a spring of AISI 1070 steel wire with a hardness of 54 HRC Fig 272: Side view of one fracture surface, deep etched to show a long seam (at arrow) 10× Fig 273: Frontal view of the mating fracture surface, showing the fatigue-crack origin (at right), which is at a second seam 10×

Cause of brittle behavior in a small percentage of flat spring parts made of annealed AISI 1070 was traced to a nitrided/nitrogenized layer, 0.25 mm (0.010 in.) thick, produced during heat treating The heat-treating operation included a 5-min stay in a shaker hearth furnace having a dissociated ammonia atmosphere However, some of the parts stuck to the furnace hearth, which increased their residence time to as long as 30 min or more The extended heating time, plus the presence of a small amount of undissociated ammonia, led to formation of the embrittling layer Fig 274 and 275: Grain- boundary damage at surface of AISI 1070 spring part SEM, 1000× and 2000× Fig 276 and 277: Nitrides (or carbonitrides) in grain boundaries SEM, 1000× and 5000× (J.H Maker, Associated Spring, Barnes Group Inc.)

Power spring made of tempered AISI 1074 strip that fractured during coiling Note significant surface damage These defect were probably caused by mechanical action or lubrication breakdown during strip production while the material was in the annealed condition Fig 279: SEM, 25× Fig 280: Same

as Fig 279, but at higher magnification SEM, 100× (J.H Maker, Associated Spring, Barnes Group Inc.)

Low-cycle fatigue fracture of AISI 1074 strip Failure initiated at defects that were probably formed during hot rolling of the band Binocular examination of defects revealed dark (oxidized) areas Metallographic sections contained decarburized regions intimately associated with the defects Fig 281; SEM, 50× Fig 282: Same as Fig 281, but at higher magnification SEM, 250× (J.H Maker, Associated Spring, Barnes Group, Inc.)

Fracture of power spring made of tempered AISI 1074 Initially, a "scab" or some other type of mechanical damage was suspected as the cause of failure because of the surface appearance of the steel However, SEM examination revealed that fracture was due to a large nonmetallic inclusion (probably alumina, type B) located within 6 to 13 μm (0.25 to 0.50 mils) of the surface Fig 283: Surface of fractured power spring and portion of fracture area showing origin Note surface damage near "C" and prominent feature, "D," at edge near fracture origin SEM, 50× Fig 284: Close-up of feature at fracture edge in Fig 283 reveals the subsurface inclusion at "D" and its effects on surface appearance ("C") SEM, 400× Fig 285: Longitudinal microsection near origin SEM, 200× (J.H Maker, Associated Spring, Barnes Group Inc.)

Hydrogen embrittlement fracture of AISI 1074 part The part was formed in the annealed condition, quenched and tempered to 48 HRC, and then electroplated A surface imperfection in the steel was initially thought to have played a key role in the fracture However, SEM examination revealed an intergranular fracture mode indicative of hydrogen embrittlement due, in this case, to insufficient postplate baking Fig 286: Fractured part after stripping of electroplated coating Note surface imperfection (boxed) near edge SEM, 30× Fig 287: Close-up of boxed area in Fig 286 SEM, 150× Fig 288: High-magnification view of fracture surface reveals that part actually failed intergranularly due to hydrogen embrittlement Note secondary cracking SEM, 1500× (J.H Maker, Associated Spring, Barnes Group Inc.)

Failure of tempered AISI 1074 power spring due to galling Power springs should be lubricated between coils and should not be wound too tight When these requirements are not met, the resultant abrasion between overlapping coils can produce galling and, occasionally, frictionally induced, untempered martensite, Maximum stress also rises sharply under these conditions In this case, galling

is the only visible cause of failure Fig 289: SEM 80× Fig 290: SEM, 400× (J.H Maker, Associated Spring, Barnes Group Inc.)

Figure 292, 293, and 294 show surfaces of "transverse fissures" that occurred in the heads of AREA 59-kg (131-lb) railroad rails made of AISI 1075 steel All three fissures were caused by the presence of hydrogen-induced flakes (or "fisheyes"), which appear as dark gray areas within the bright zones These bright zones are fatigue cracks that were initiated by the flakes Beach marks are visible in the fatigue zones, particularly near the initiations of final fast fracture Figure 295 shows the surface of a longitudinal fracture in an AREA 59-kg (131-lb) rail of AISI 1075 steel that had been hydrogen embrittled That white arrowhead points to an unusually large nonmetallic stringer that acted as a site for hydrogen segregation All shown at actual size

Premature fatigue failures of springs due to hydrogen embrittlement Cause was traced to a combination of inadequate stress relieving and use of an acid cleaning process Material: ASTM A228 (0.70 to 1.0% C) Fig 296: Three hydrogen/residual tensile stress induced cracks on wire surface SEM, 30× Fig 297: Boxed area in Fig 296 SEM, 150× Fig 298: Surface of one wire fracture Hydrogen crack is at upper left and is 0.35 mm (0.014 in.) deep At lower right is region of fatigue SEM, 100× Fig 299: High-magnification view of intergranular hydrogen crack on fracture surface of ASTM A228 wire in Fig 298 (compare with Fig 300) SEM, 2000× Fig 300: Morphology of fatigue crack on fracture surface of ASTM A228 wire in Fig 298 (compare with Fig 299) SEM, 2000× (J.H Maker, Associated Spring, Barnes Group Inc.)

Fatigue failure of spring made of oil-tempered carbon valve spring wire (ASTM A230) Failure occurred after 10,000 cycles of engine operation Fracture surface morphology is atypical One possible cause of failure: a ribbonlike subsurface inclusion Fig 301: SEM, 40× Fig 302: Higher- magnification view of boxed area in Fig 301 SEM, 200× See also Fig 303 and 304 (J.H Maker, Associated Spring, Barnes Group Inc.)

Higher-magnification views of the fractured ASTM A230 spring wire shown in Fig 301 and 302 Fig 303: Secondary electron SEM image, 500× Fig 304: Backscattered electron SEM image, 300× See the article "Scanning Electron Microscopy" in this volume for additional information on secondary and backscattered electron images (J.H Maker, Associated Spring, Barnes Group Inc.)

Detail fracture (see Fig 308) in head of railroad rail that had been removed from revenue track and installed in test track to determine crack growth rate under heavy train traffic Material: high-carbon rail steel (0.69 to 0.82% C.) Fig 309: Featureless region inside contour 1 outlines crack at time of removal from service Contours 1 through 9 and the unlabeled contours that follow identify crack propagation steps during testing Contours separate inclined ridges that result from the unique traffic pattern on the test track The detail fracture grew in fatigue to 80% of rail head area before breaking under the test train 2.4× Fig 310: Longitudinal section along diagonal line in Fig 309 Contours 1 through 9 are identified Note ridge structure of crack surface that results from combined stage I and stage II fatigue crack propagation The less prominent, horizontal ridges in Fig 309 result from combined stage I/stage III propagation 13× (J.M Morris, U.S Department of Transportation)

Fatigue crack propagation in compact tension test specimen of rail steel (0.69 to 0.82% C) Fig 311: Fracture surface of typical specimen from rail head Crack plane lies in the cross section First two circles at top identify precracked region Fatigue crack propagation extends from second to sixth circle (from top to bottom) Bottom one third of surface is region of fast fracture 1.66× Fig 312 and 313: Fractographs of fatigue propagation region in Fig 311 illustrate typical fatigue striations Note wavelike crests and variations of wavelength near inclusion sites TEM replicas, 4900× and 4000× Fig

314 and 315: Two additional fractographs of fatigue crack propagation region in Fig 311 Sharp crest outlines and lack of wavelength change near inclusion sites (arrows) identify these features as lamellar pearlite structure (see Fig 316) TEM replicas, both at 4000× (J.M Morris, U.S Department of Transportation)

Fractographs of fast fracture region in rail steel (0.69 to 0.82% C) compact tension specimen similar to that in Fig 311 Fig 317: Typical transgranular cleavage At right-center is a cleavage region that follows steps along pearlite lamellae TEM replica, 5000× Fig 318: Comparison of transgranular cleavage (left) with intergranular cleavage (right) TEM replica, 5000× (J.M Morris, U.S Department

of Transportation)

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Fracture of AISI 4146 axle due to improper induction hardening that caused burning (incipient melting) A surface hardness of 50 to 58 HRC was specified Actual hardness: approximately 15 HRC Fig 367: Fracture surface of shaft Fig 368: Fractography of intergranular portion of fracture surface SEM, 15× Fig 369: Higher-magnification fractograph of intergranular portion of fracture surface (see Fig 368) reveals partial melting of grain boundaries Also note very large grain boundaries SEM, 90× Fig 370: Microstructure of intergranular region of fracture surface 2% nital, 165× Fig 371: High-magnification view of microstructure at burned grain boundary Note decarburization, intergranular oxidation, and presence of spheroidal oxides 2% nital, 920× (Z Flanders, Packer Engineering Associates, Inc.)