A stainless steel screw securing an orthopedic implant fractured and was analyzed to determine the cause. Investigators used optical and scanning electron microscopy to examine the fracture surfaces and the microstructure of the austenitic stainless steel from which the screw was made. The results of the study indicated that the screw failed due to fatigue fracture stemming from surface cracks generated by stress concentration likely caused by grooves left by improper machining.

The exhaust valve of a truck engine failed after 488 h of a 1000 h laboratory endurance test. The valve was made of 21-2 valve steel in the solution treated and aged condition and was faced with Stellite 12 alloy. The failure occurred by fracture of the underhead portion of the valve. Analysis (visual inspection, electron probe x-ray microanalysis, hardness testing, 4.5x fractograph) supported the conclusions that failure of the valve stem occurred by fatigue as a result of a combination of a nonuniform bending load, which caused a mild stress-concentration condition, and a high operating temperature in a corrosive environment. When the microstructure near the stem surface was examined, it was apparent that carbide spheroidization had occurred. Also, there was a coarsening of the carbide network within the austenite grains. The microstructure indicated that the underhead region of the valve was heated to about 930 deg C (1700 deg F) during operation. The cause of fatigue fracture, therefore, was a combination of non-uniform bending loads and overheating. No recommendations were made.

The cold start advance solenoid sleeve was found leaking through the wall during troubleshooting complain of a diesel engine that failed to start in cold weather. The component was revealed to be a tubular product with a “bulb” section at one end and threads on the other. The manufacturing method used to create the bulb shape was hydroforming, using a 300 series stainless steel tube in the full-hard condition. The leak was attributed to a crack in the sleeve in the radius between the bulb area and the cylindrical portion of the sleeve. Fatigue cracks initiated at multiple sites near the OD of the sleeve were revealed by scanning electron microscopy of the broken-open crack. It was revealed by analysis that during the hydroforming process, heavy biaxial strains were imparted to the sleeve wall. It was interpreted that when combined with the heavy strains inherently present in the full-hard 300 series stainless steel, the hydroforming strains in the radius caused the microcracking. The root cause for this failure was identified to be omission of an intermediate stress relief or annealing treatment prior to hydroforming to the final shape.

Several type 304 stainless steel fire truck water tanks developed through-wall leaks after being in service for approximately two years. One representative tank underwent a comprehensive laboratory analysis, which included metallographic examinations and chemical analyses. The examinations revealed a classic case of microbiologically influenced corrosion (MIC), which preferentially attacked the heat affected zones of the tank welds, resulting in the leaks.

An unacceptable degree of porosity was identified in several closure welds on stainless steel containers for plutonium-bearing materials. The pores developed in the weld tie-in region due to gas trapped by the weld pool during the closure process. This paper describes the efforts to trace the root cause of the porosity to the geometric conditions of the weld joint and establish corrective actions to minimize such porosity.

A weld in a fuel-line tube broke after 159 h of engine testing. The 6.4-mm (0.25-in.) OD x 0.7-mm (0.028-in.) wall thickness tube and the end adapters were all of type 347 stainless steel. The butt joints between tube and end adapters were made by automated gas tungsten arc (orbital arc) welding. It was found that the tube had failed in the HAZ. Examination of a plastic replica of the fracture surface in a transmission electron microscope established that the crack origin was at the outer surface of the tube. The crack growth was by fatigue; closely spaced fatigue striations were found near the origin, and more widely spaced striations near the inner surface. The quality of the weld and the chemical composition of the tube both conformed to the specifications. However, the fuel-line assembly had vibrated excessively in service. The fuel-line fracture was caused by fatigue induced by severe vibration in service. Additional tube clamps were provided to damp the critical vibrational stresses. No further fuel-line fractures were encountered.

The cause of the fatigue failure in the retaining ring of the compressor region of an aero-engine turbine was found to be the presence of a high concentration of nonmetallic inclusions. The results of chemical analysis were used to estimate the phases present. The most frequently observed inclusions were spinel solid solutions of the type MO middot; N2O3, where M = Fe, Mn, or Mg and N = Cr or Al. The detrimental inclusions were corundum, calcium aluminates, cristobalite, and silicates. The most detrimental phases were traced on the surfaces of the specimens fractured using impact loading; the comparison is being made with the polished surfaces and the tensile specimen fracture surfaces. The inclusions in the failed retaining ring were compared with the ones in a similar component obtained from a used engine. In the case of the latter, a large number of fine and elongated (Mn, Cr, Fe)S inclusions were present along with spinels. The nondeformable, rigid oxide particles are considered more undesirable than the sulfides as far as fatigue life of the component is concerned. It has been reported that the presence of sulfides may eliminate the stresses due to oxides.

A welded elbow assembly (AISI type 321 stainless steel, with components joined with ER347 stainless steel filler metal by gas tungsten arc welding) was part of a hydraulic-pump pressure line for a jet aircraft. The other end of the tube was attached to a flexible metal hose, which provided no support and offered no resistance to vibration. The line was leaking hydraulic fluid at the nut end of the elbow. Investigation supported the conclusion that failure was by fatigue cracking initiated from a notch at the root of the weld and was propagated by cyclic loading of the tubing as the result of vibration and inadequate support of the hose assembly. Recommendations included changing the joint design from a cylindrical lap joint to a square-groove butt joint. Also, an additional support was recommended for the hose assembly to minimize vibration at the elbow.

An aircraft fuel-nozzle-support assembly exhibited cracks along the periphery of a fusion weld that attached a support arm to a fairing in a joint that approximated a T-shape in cross section. The base metal was type 321 stainless steel. Examination showed a good-quality weld penetrating to the support arm beneath, but revealed notch configurations at the inner mating surfaces at each edge of the fairing, the result of welding a poor fit-up of the support arm to the fairing. Fractures that originated at the cracks were examined by stereomicroscope and were found to contain fatigue marks that indicated crack propagation from multiple origins at the inner surface of the weld edge. Fatigue cracking was initiated at stress concentrations created by the notches at the inner surfaces between the support arm and the fairing, enhanced by poor fit-up in preparation for welding.

Two freshwater tanks (0.81 mm (0.032 in) thick, type 321 stainless steel) were removed from aircraft service because of leakage due to pitting and rusting on the bottoms of the tanks. One tank had been in service for 321 h, the other for 10 h. There had been departures from the specified procedure for chemical cleaning of the tanks in preparation for potable water storage. The sodium hypochlorite sterilizing solution used was three times the prescribed strength, and the process exposed the bottom of the tanks to hypochlorite solution that had collected near the outlet. Investigation (visual inspection, 95x unetched images, chemical testing with a 5% salt spray, chemical testing with sodium hypochlorite at three strength levels, samples were also pickled in an aqueous solution containing 15 vol% concentrated nitric acid (HNO3) and 3 vol% concentrated hydrofluoric acid (HF) and were then immersed in the three sodium hypochlorite solutions for several days) supported the conclusion that failure of the stainless steel tanks by chloride-induced pitting resulted from using an overly strong hypochlorite solution for sterilization and neglecting to rinse the tanks promptly afterward. Recommendations included revising directions for sterilization and rinsing.

A steam-condensate line (type 316 stainless steel tubing) began leaking after five to six years in service. The line carried steam condensate at 120 deg C (250 deg F) with a two hour heat-up/cool-down cycle. No chemical treatment had been given to either the condensate or the boiler water. To check for chlorides, the inside of the tubing was rinsed with distilled water, and the rinse water was collected in a clean beaker. A few drops of silver nitrate solution were added to the rinse water, which clouded slightly because of the formation of insoluble silver chloride. This and additional investigation (visual inspection, and 250x micrograph etched with aqua regia) supported the conclusion that the tubing failed by chloride SCC. Chlorides in the steam condensate also caused corrosion of the inner surface of the tubing. Stress was produced when the tubing was bent during installation. Recommendations included providing water treatment to remove chlorides from the system. Continuous flow should be maintained throughout the entire tubing system to prevent concentration of chlorides. No chloride-containing water should be permitted to remain in the system during shutdown periods, and bending of tubing during installation should be avoided to reduce residual stress.

A type 321 stainless steel bellows expansion joint on a 17-cm (6 in.) OD inlet line (347 stainless) in a gas-turbine test facility cracked during operation. The line carried high-purity nitrogen gas at 1034 kPa (150 psi) with a flow rate of 5.4 to 8.2 kg/s (12 to 18 lb/s). Cracking occurred in welded joints and in unwelded portions of the bellows. The bellows were made by forming the convolution halves from stainless steel sheet, then welding the convolutions together. Evidence from visual examination, liquid penetrant inspection chemical analysis, hardness tests, and metallographic examination of sections etched with Vilella's reagent supports the conclusions that failure of the bellows occurred by intergranular fatigue cracking. Secondary degrading effects on the piping existed as well. Recommendations included the acceptability of Type 321 stainless steel (provided open-cycle testing does not result in surface oxidation and crevices) Although type 347 stainless steel would be better, and Inconel 600 would be an even better choice. Welds would also need modified processing for reheating and annealing. Prevention of oil leakage into the system would minimize carburization of the piping and bellows.

Argonne National Laboratory has conducted analyses of failed components from nuclear power-generating stations since 1974. The considerations involved in working with and analyzing radioactive components are reviewed here, and the decontamination of these components is discussed. Analyses of four failed components from nuclear plants are then described to illustrate the kinds of failures seen in service. The failures discussed are (1) intergranular stress-corrosion cracking of core spray injection piping in a boiling water reactor, (2) failure of canopy seal welds in adapter tube assemblies in the control rod drive head of a pressurized water reactor, (3) thermal fatigue of a recirculation pump shaft in a boiling water reactor, and (4) failure of pump seal wear rings by nickel leaching in a boiling water reactor.

Cracks on the outer surface near a hanger lug were revealed by visual inspection of a type 316 stainless steel main steam line of a major utility boiler system. Cracking was found to have initiated at the outside of the pipe wall or immediately beneath the surface. The microstructure of the failed pipe was found to consist of a matrix precipitate array (M23C6) and large s-phase particles in the grain boundaries. A portable grinding tool was used to prepare the surface and followed by swab etching. All material upstream of the boiler stop valve was revealed to have oriented the cracking normally or nearly so to the main hoop stress direction. Residual-stress measurements were made using a hole-drilling technique and strain gage rosettes. Large tensile axial residual stresses were measured at nearly every location investigated with a large residual hoop stress was found for locations before the stop valve. It was concluded using thermal stress analysis done using numerical methods and software identified as CREPLACYL that one or more severe thermal downshocks might cause the damage pattern that was found. The root cause of the failure was identified to be thermal fatigue, with associated creep relaxation.

The secondary cooling water system pressure boundary of Savannah River Site reactors includes expansion joints utilizing a thin-wall bellows. While successfully used for over thirty years, an occasional replacement has been required because of the development of small, circumferential fatigue cracks in a bellows convolute. One such crack was recently shown to have initiated from a weld heat-affected zone liquation microcrack. The crack, initially open to the outer surface of the rolled and seam welded cylindrical bellows section, was closed when cold forming of the convolutes placed the outer surface in residual compression. However, the bellows was placed in tension when installed, and the tensile stresses reopened the microcrack. This five to eight grain diameter microcrack was extended by ductile fatigue processes. Initial extension was by relatively rapid propagation through the large-grained weld metal, followed by slower extension through the fine-grained base metal. A significant through-wall crack was not developed until the crack extended into the base metal on both sides of the weld. Leakage of cooling water was subsequently detected and the bellows removed and a replacement installed.

One inch diam Type 304 stainless steel piping was designed to carry containment atmosphere samples to an analyzer to monitor hydrogen and oxygen levels during operational and the design basis accident conditions that are postulated to occur in a boiling water reactor. Only one of six lines in the system had thru-wall cracks. Shallow incipient cracks were detected at the lowest elevations of one other line. The balance of the system had no signs of SCC attack. Chlorides and corrosion deposits in varying amounts were found throughout the system. The failure mechanism was transgranular, chloride, stress-corrosion cracking. Replacement decisions were based on the presence of SCC attack or heavy corrosion deposits indicative of extended exposure time to chloride-contaminated water. The existing uncracked pipe, about 75 percent of the piping in the system, was retained despite the presence of low level surface chlorides. Controls were implemented to insure that temperatures are kept below 150 deg F, or, walls of the pipe are moisture-free or the cumulative wetted period will never exceed 30 h.

The top tube of a horizontal superheater bank in the reheat furnace of a steam generator ruptured after seven years in service. The rupture was found to have occurred in the ferritic steel tubing (2.25Cr-1Mo steel (ASME SA-213, grade T-22)) near the joint where it was welded to austenitic stainless steel tubing (type 321 stainless steel (ASME SA-213, grade TP321H)). The surface temperature of the tube was found to be higher than operating temperature in use earlier. The ferritic steel portion of the tube was found to be longitudinally split and heavily corroded in the region of the rupture. A red and white deposit was found on the sides and bottom of the tube in the rupture area. The deposit was produced by attack of the steel by the alkali acid sulfate and had thinned the tube wall. It was concluded that rupture of the tube had occurred due to thinning of the wall by coal-ash corrosion. The thinned tubes were reinforced by pad welding. Type 304 stainless steel shields were welded to the stainless steel portions of the top reheater tubes and were held in place about the chromium-molybdenum steel portions of the tubes by steel bands.

An intergranular stress-corrosion cracking failure of 304 stainless steel pipe in 2000 ppm B as H3BO3 + H2O at 100 deg C was investigated. Constant extension rate testing produced an intergranular type failure in material in air. Chemical analysis was performed on both the base metal and weld material, in addition to fractography, EPR testing and optical microscopy in discerning the mode of failure. Various effects of Cl-, O2 and MnS are discussed. Results indicated that the cause of failure was the severe sensitization coupled with probable contamination by S and possibly by Cl ions.

A 150 mm (6 in.) schedule 80S type 304 stainless steel pipe (11 mm, or 0.432 in., wall thickness), which had served as an equalizer line in the primary loop of a pressurized-water reactor, was found to contain several circumferential cracks 50 to 100 mm (2 to 4 in.) long. Two of these cracks, which had penetrated the pipe wall, were responsible for leaks detected in a hydrostatic test performed during a general inspection after seven years of service. Investigation (visual inspection, visual and ultrasonic weld examination, water analysis, and chemical analysis) supported the conclusion that the failure was caused by SCC due to stress, sensitization, and environment. Recommendations included replacing all pipe sections and installing them using low-heat-input, multiple-pass welding procedures.

The accident at Three Mile Island Unit No. 2 on 28 March 1979 was the worst nuclear accident in US history. By Jan 1990, it was possible to electrochemically machine coupons from the lower head using a specially designed tool. The specimens contained the ER308L stainless steel cladding and the A533 Grade B plate material to a depth of about mid-wall. The microstructures of these specimens were compared to that of specimens cut from the Midland, Michigan reactor vessel, made from the same grade and thickness but never placed in service. These specimens were subjected to known thermal treatments between 800 and 1100 deg C for periods of 1 to 100 min. Microstructural parameters in the control specimens and in those from TMI-2 were quantified. Selective etchants were used to better discriminate desired microstructural features, particularly in the cladding. This report is a progress report on the quantification of changes in both the degree of carbide precipitation and delta ferrite content and shape in the cladding as a function of temperature and time to refine the estimates of the maximum temperatures experienced.