Stainless steel is frequently used for bone fracture fixation in spite of its sensitivity to pitting and cracking in chloride containing environments (such as organic fluids) and its susceptibility to fatigue and corrosion fatigue. A 316L stainless steel plate implant used for fixation of a femoral fracture failed after only 16 days of service and before bone callus formation had occurred. The steel used for the implant met the requirements of ASTM Standard F138 but did contain a silica-alumina inclusion that served as the initiation point for a fatigue/corrosion fatigue fracture. The fracture originated as a consequence of stress intensification at the edge of a screw hole located just above the bone fracture; several fatigue cracks were also observed on the opposite side of the screw hole edge. The crack propagated in a brittle-like fashion after a limited number of cycles under unilateral bending. The bending loads were presumably a consequence of leg oscillation during assisted perambulation.

Two aircraft-engine tailpipes of 19-9 DL stainless steel (AISI type 651) developed cracks along longitudinal gas tungsten arc butt welds after being in service for more than 1000 h. Binocular-microscope examination of the cracks in both tailpipes revealed granular, brittle-appearing surfaces confined to the HAZs of the welds. Microscopic examination of sections transverse to the weld cracks showed severe intergranular corrosion in the HAZ. The fractures appeared to be caused by loss of corrosion resistance due to sensitization, that could have been induced by the temperatures attained during gas tungsten arc welding. Tests demonstrated the presence of sensitization in the HAZ of the gas tungsten arc weld. The aircraft engine tailpipe failures were due to intergranular corrosion in service of the sensitized structure of the HAZs produced during gas tungsten arc welding. All gas tungsten arc welded tailpipes should be postweld annealed by re-solution treatment to redissolve all particles of carbide in the HAZ. Also, it was suggested that resistance seam welding be used, because there would be no corrosion problem with the faster cooling rate characteristic of this technique.

The governor on an aircraft engine failed and upon disassembly of the unit, it was discovered that the retainer for the flyweight pivot pins was broken. The channel-shaped retainer was made of 0.8 mm (0.030 in.) thick 1018 or 1020 steel. The part was plated with copper, which acted as a stop-off during carburizing of the offset, circular thrust-bearing surface surrounding the 16-mm (0.637-in.) diam hole. The bearing surface was case hardened to a depth of 0.05 to 0.1 mm (0.002 to 0.005 in.), then austempered to obtain a minimum hardness of 600 Knoop (1-kg, or 2.2-lb, load). Considerable vibration was created in the installation because of the design of the mechanical device used to transmit power to the governor. The pins were permitted to slide axially a small distance. Analysis (visual inspection, microscopic examination, and ductility measurements) supported the conclusion that failure of the retainer was the result of fatigue caused by vibration in the flyweight assembly. Impact of the pivot pins on the retainer also contributed to failure. Recommendations included redesign of the flyweight assembly, and replacement of the channel-shaped retainer with a spring-clip type of pin retainer.

Inspections and microstructural analysis revealed intergranular corrosion of 6061-T6 aluminum alloy aircraft fuel line beneath ferrules. The cause of the corrosion was traced to the fuel line marking process, which involved electrolytic labeling. Although subsequent rinsing of the fuel lines washed off most of the electrolyte, some was trapped between the 6061-T6 tubing and the ferrule. This condition made intergranular corrosion of the fuel lines inevitable. The attack caused grains to become dislodged, giving the appearance of pitting. Corrosion penetrated approximately 0.13 mm (0.005 in.) into the tubing. Experiments indicated that the corrosion products were inactive. It was recommended that another marking process be used that does not involve corrosive materials. The prevention of electrolyte from being trapped between the tubing and ferrules by using a MIL-S-8802 sealant was recommended.

A forged aluminum alloy 2014-T6 catapult-hook attachment fitting (anodized by the chromic acid process to protect it from corrosion) from a naval aircraft broke in service. Spectrographic analysis, visual examination, microscopic examination, and tensile analysis showed minute cracks on the inside surface of a bearing hole, and small areas of pitting corrosion were visible on the exterior surface of the fitting. The analysis also revealed a small number of rosettes, suggestive of eutectic melting, in an otherwise normal structure. These examinations and analyses support the conclusion that the presence of chromic acid stain on the fracture surface proved that the forging had cracked before anodizing. This suggest that the crack initiated during straightening, either after machining or after heat treatment. The structure and composition of the alloy appear to have been acceptable. Ductility was acceptable so rosettes found in the microstructure are believed to have been nondamaging. Had they contributed to the failure, the ductility would have been very low. The recommendations included inspection for cracks and revising the manufacturing process to include a fluorescent liquid-penetrant inspection before anodizing, because chromic acid destroys the penetrant. This inspection would reduce the possibility of cracked parts being used in service.

The master connecting rod of a reciprocating aircraft engine revealed cracks during routine inspection. The rods were forged from 4337 (AMS 6412) steel and heat treated to a specified hardness of 36 to 40 HRC. H-shaped cracks in the wall between the knuckle-pin flanges were revealed by visual examination. The cracks were originated as circumferential cracks and then propagated transversely into the bearing-bore wall. No inclusions in the master rod were detected by magnetic-particle and x-ray inspection. Three large inclusions lying approximately parallel to the grain direction and fatigue beach marks around two of the inclusions were revealed by macroscopic examination of the fracture surface. Large nonmetallic inclusions that consisted of heavy concentrations of aluminum oxide (Al2O3) were revealed by microscopic examination of a section through the fracture origin. The forging vendors were notified about the excess size of the nonmetallic inclusions in the master connecting rods and a nondestructive-testing procedure for detection of large nonmetallic inclusions was established.

A broken aircraft crankshaft and a severely damaged main brass bearing were examined to determine whether engine failure was initiated in the bearing or in the crankshaft. The steel crankshaft failure was a classical fatigue fracture. The bearing had been subjected to extremely high temperatures, as indicated by melting in the brass components and the extreme distortion in the rollers. Microscopic examination on the crankshaft material showed it to be a good quality steel. On the other hand, the chromium plate was thick, porous, and cracked in many places, including the point of the main fatigue crack. It was concluded that the over-all failure was initiated in the crankshaft, and the failure of the bearing resulted from that failure.

An elbow of 70 mm OD and 10 mm wall thickness made from St 35.29, and exposed to 315 atmospheres internal pressure in an oil hydraulic shear installation, cracked lengthwise after a short operating period. Because the stress was not sufficient to explain the fracture of this elbow under this pressure, an investigation was conducted to establish whether material or processing errors had occurred. Microscopic examination showed that a ferritic-pearlitic structure in select locations was very fine-grained. Other signs of fast cooling as compared to normally formed structure of the core zone were noted. It was also possible that the pipe was resting on a cold plate during bending or that it came in touch with a cold tool. This apparently caused the strains at the transition to the cross-sectional part that had been cooled more slowly. The location of the crack at just this point gave rise to the conclusion that it was formed either by the sole or contributive effect of these stresses.

The bottom flange of a vertical pipe coupled to an isolating valve in a steam supply line to a turbine failed. Steam pressure was 1,500 psi and the temperature 416 deg C (780 deg F). Multiple cracking occurred in the bore of the flange. A quarter-segment was cut out and examined. The cracks were located in the part of the flange that formed a continuation of the pipe bore. The majority of them originated at the end of the flange bore and extended axially along the pipe and radially across the flange face. Magnetic crack detection revealed a further number of cracks in the weld deposit. While the fracture in the weld metal was of the ductile type exhibiting a fine fibrous appearance, that in the flange material was of the cleavage type. Microscopic examination revealed that the cracks were blunt-ended fissures of the type characteristic of corrosion-fatigue. It was concluded that cracking was due to corrosion-fatigue, which arose from the combined effect of a fluctuating tensile stress in the presence of a mildly corrosive environment.

An 850 ft. long steam main working at a pressure of 120 psi ruptured. Two lengths of pipe were submitted for examination, one containing the rupture and the other from an unaffected part removed to facilitate repair. The rupture surfaces were smooth, suggesting that the failure had taken place either at a weld in the pipe or at a discontinuity in the material. Microscopic examination through the joints at the ends of the rupture confirmed that the pipe had been made from strip and the edges lap-welded. The second case concerned an 8 in. diam pipe in which a longitudinal defect was discovered running the entire length. Examination of a section through the pipe containing the defect, showed this to be a scarf-welded pipe, only about half the section of the scarf having been satisfactorily welded together. It was concluded that both pipes had been defective at the time of manufacture and that service conditions had served to extend the defects.

In an electric power station, seven turbine blades out of 112 broke or cracked within 8 to 14 months after commencement of operation. The blades in question were all located on the last running wheel in the low pressure section of a 35,000 kW high pressure condensing turbine. They were milled blades without binding wires and cover band. They did not fracture at the fastening, i.e. the location of highest bending stress, but in a central region which was 165 to 225 mm away from the gripped end. The blades were fabricated from a stainless heat-treatable chromium steel containing 0.2C and 13.9Cr. Microstructural examination showed the blades were destroyed by flexural vibrations which evidently reached their maximum amplitude at the location of fracture. Erosion of the inlet edge, possibly in connection with vibration-induced corrosion cracking, contributed to fracture.

A 75 mm OD x 7.4 mm wall thickness carbon steel boiler tube ruptured. A substantial degree of corrosion on the water-side surface leaving a rough area in the immediate vicinity of the rupture was revealed by visual examination. Decarburization and extensive discontinuous intergranular cracking was revealed by microscopic examination of a cross section through the tube wall at the fracture. It was concluded that the rupture occurred because of hydrogen damage involving the formation of methane by the reaction of dissolved hydrogen with carbon in the steel. Hydrogen was produced by the chemical reaction that corroded the internal tube surface. Steel embrittled by hydrogen can be restored only if grain boundary cracking or decarburization had not occurred but since the material embrittled in this manner, its replacement was recommended.

Severe pitting corrosion of a carbon steel tube was observed in the air preheater of a power plant, which runs on rice straw firing. Approximately 1450 tubes were removed from Stage 3 of the preheater (air inlet and flue gas outlet) due to corrosion and local bursting. Samples from Stage 2 (where corrosion was low) and Stage 3 (severe corrosion) were taken and subjected to visual inspection, SEM, x-ray diffraction, microhardness measurement, and chemical and microstructural analysis. It was determined that extended non-operation of the plant resulted in the settlement of corrosive species on the tubes in Stage 3. The complete failure of the tube occurred due to diffusion of these elements into the base metal and precipitation of potassium and chlorine compounds along the grain boundaries, with subsequent dislodging of grains. The nonmetallic inclusions acted as nucleating sites for local pitting bursting. Nonuniform heat transfer in Stage 3 operation accelerated the selective corrosion of front-end tubes. The relatively high heat transfer in this stage resulted in condensation of some corrosive gases and consequent corrosion. Continuous operation of the plant with some precautions during assembly of the tubes reduced the corrosion problem.

The horizontal heat-exchanger tubes made of copper alloy C70600, in one of two hydraulic-oil coolers in an electric power plant, leaked after 18 months of service. River water was used as the coolant in the heat-exchanger tubes. Several nodules on the inner surface and holes through the tube wall, which appeared to have formed by pitting under the nodules, were revealed by visual examination. Steep sidewalls, which indicated a high rate of attack, were revealed by microscopic examination of a section through the pit which had penetrated the tube wall. The major constituent of reddish deposit on the inner surfaces of the tubes was revealed to be iron oxide and slight manganese dioxide. Effluent from steel mills upstream was indicated by the presence of these and other constituents to be the source of most of the solids found in the tubes. It was concluded that the tubing failed by crevice corrosion. The tubing in the cooler was replaced, and cooling-water supply was changed from river to city water, which contained no dirt to deposit on the tube surfaces. An alternate solution of installing replacement tubes in the vertical position to make deposition of solids from river water less likely was suggested.

Several articulated bellows of 10 in. ID developed leakage from the convolutions after a service life of some 18 months. One of the units received from examination showed cracking at the crown of a convolution and at the attachment weld to the pipe. Sectioning of the bellows revealed many others cracks on the internal surface which did not penetrate to the outside. Microscopical examination showed multiple intergranular, tree-like cracking typical of stress-corrosion cracking. Concentration of sodium hydroxide occurred in the bellows unit and the stress-corrosion cracking which developed was of the form known as caustic cracking. It was recommended that water for de-superheater use should be taken after the deaerator and prior to the addition of salts which may deposit or concentrate in the desuperheater.

A 150 cm ID boiler drum made form ASTM A515, grade 70, steel failed during final hydrotesting at a pressure of approximately 26 MPa. Brittle fractures were revealed in between two SA-106C nozzles and remainder was found to involve tearing. Short, flat segments of fracture area, indicative of pre-existing cracks, were revealed by examination of the fracture surface at the drain grooves arc gouged at the nozzle sites. A thin layer of material with a dendritic structure was observed at the groove surface. The dendritic layer was revealed by qualitative microprobe analysis to contain over 1% C, higher than the carbon content of the base metal. The cracks in the drain groove surface could have occurred after arc gouging, during subsequent stress-relieving, or during the hydrostatic test. Flame cutting is not recommended for the type of steel used in the boiler drum because it can lead to local embrittlement and stress raisers, potentially initiating major failures.

Caustic cracking is the term used to describe one of the forms in which stress-corrosion cracking manifests itself in carbon steels. In the present study, persistent leakage occurred after ten weeks of service from tube expansions in the steam and mud drum of a two-drum D type boiler, which failed to respond to repeated expansion. The leakage was traced to circumferential cracking in the portion of Fe-0.11C-0.46Mn-0.018S-0.011P tubes within the expanded region. Microscopic examination indicated that all cracks started from the outer surface of the tubes in the expanded portion. The form of cracking which was mostly intergranular. Examination at higher magnification disclosed that a selective attack had taken place on the carbide constituents of the pearlite grains. An alkaline deposit on the fireside surface of the tube resulted from the evaporation of boiler water which had found its way past the tube expansions. This indicated that this operation had not resulted in a satisfactorily tight joint.

One tube in a watertube boiler developed leakage from a perforation. The external surface was covered with a dark deposit indicative of local fusion. Perforation resulted from the development of a crack from the internal surface. Microscopic examination revealed extensive intergranular penetration by molten copper. Particles of copper were seen in scale deposits on the bore of the tube. The tube in general showed a ferritic structure with partially spheroidized carbide. The fact that fusion of the copper had occurred indicated temperatures of 1100 deg C (2012 deg F) had been experienced locally, and the structural condition suggested that the tube in general had been heated at a lower temperature of the order of 600 deg C (1112 deg F) for some appreciable time. In this instance, overheating of the tube in the absence of the copper deposits may not have led to failure.

An unusual type of defect was discovered during hydraulic testing of a water-tube boiler after repairs to the superheater tubes following erosion from soot-blowers. When the pressure reached 700 psi, slight leakage was found to be taking place from one of the superheater tubes in a region where there appeared to be a split, approximately 8 in. long. What was thought to be a split was actually a pronounced fold. Microscopic examination showed that a corrosion-fatigue fissure had developed from one of the inside corners of the fold, presumably as a result of the fluctuating bending stresses to which this portion of the tube would be subjected because of the discontinuity in the tube wall. It was from this fissure that the leakage occurred. It was evident that the defect developed during the manufacture of the tube, probably in the course of a drawing or rolling operation without an internal plug. The diam of this portion of the tube was reduced by local collapse and folding of the section rather than by longitudinal extension of the tube itself.

Three blades from 45,000 kW, 3,000 rpm turbine were received for examination, comprising the root of blade 28, blade 89 showing a crack in one of the root teeth, and blade 106 which was free from defects. Microscopic examination of the blade material showed it to be a ferritic stainless steel of the type commonly used for turbine blades. A number of non-metallic inclusions were present which had been drawn into threads in rolling; these appeared to consist largely of duplex silicates. The failure of blade 28 was the result of the development of a creeping crack. Magnetic crack examination of blade 89 revealed a crack in a tooth in an identical position to the start of the crack in blade 28 but on the opposite, i.e., steam inlet, side of the blade. Similar examination of blade 106 did not reveal any cracks. Cracking was associated with unsatisfactory bedding of the blade teeth on the faces of the wheel grooves. It was concluded that the blade failures were due primarily to over-loading of the individual blade teeth due to incorrect fitting in the wheel. Vibration was an important contributory factor, as it resulted in the imposition of fluctuating stresses on the overloaded teeth. Non-metallic inclusions in the blade material playing a minor part.