A cast silicon bronze (UNS C86700) impeller that had been severely corroded was submitted for failure analysis. The failed part was used to pump potable water, but service life and chlorine content of the water were unknown. The impeller displayed a Cu-rich red phase on its surfaces and showed a pattern very similar to dezincification. Further investigation to determine the cause of damage using light microscopy and SEM-EDS techniques revealed that the microstructure consisted of multiple phases and that a Si-rich phase was being preferentially attacked, leading to increased porosity. After a thorough examination, it was concluded that the part had failed due to dealloying via desiliconification.

The origins of the casting are unknown. It is included here as a classic case of intergranular corrosion. The part (apparently a pump outlet) was named the “rubber casting” because of the severity of the intergranular attack. Every grain boundary has been attacked to the extent that the casting could be twisted and stretched as through made of rubber. The chemistry of the casting was acceptable for CN-7M. The reason the part failed is a continuous film of carbide with a continuous crack running parallel to the carbides. This sensitized structure produces an area depleted in protective chromium, making it susceptible to corrosion. Two solutions to this problem are available. The simplest is to ensure correct heat treatment to dissolve grain-boundary carbide film and return the protective chromium to the depleted zone. Alternatively, a low-carbon (0.03% maximum C, for example, CF-3) grade can be specified. Procedures are given in a reference for screening castings that may be susceptible to intergranular corrosion due to processing errors.

The drive shaft on a centrifugal pump fractured after five months of operation. The fracture occurred at a packing gland inside a sleeve against which the packing material sealed. The shaft and sleeve were of duplex stainless steel. In contrast to a previous conclusion that heating had caused property deterioration resulting in embrittlement and fracture, it was concluded that the shaft must have fractured (most probably by fatigue cracking originating at the change of section) and that heating had then taken place from friction between the rotating input shaft and the remaining part attached to the pump. High temperature was thus a result, not the cause, of the failure.

Two intermediate impeller drive gears (made of AMS 6263 steel, gas carburized, hardened, and tempered) exhibited evidence of pitting and abnormal wear after production tests in test-stand engines. The gears were examined for hardness, case depth, and microstructure of case and core. It was found that gear 1 had a lower hardness than specified while the case hardness of gear 2 was found to be within limits. Both the pitting and the wear pattern were revealed to be more severe on gear 1 than on gear 2. Surface-contact fatigue (pitting) of gear 1 (cause of lower carbon content of the carburized case and hence lower hardness) was found to be the reason for failure. It was recommended that the depth of the carburized case on impeller drive gears be increased from 0.4 to 0.6 mm to 0.6 to 0.9 mm to improve load-carrying potential and wear resistance. A minimum case-hardness requirement was set at 81 HRA.

Deterioration of the vanes and a wearing away of the area surrounding the mainshaft-bearing housing of the pump bowl for a submersible water pump used in a well field were noticed during a maintenance inspection. The bowl was sand cast from gray iron and had been in service approximately 45 months. Visual examination of the vanes and the area surrounding the mainshaft-bearing housing revealed a dark corrosion product that was soft, porous, and of low mechanical strength. There were areas with severe erosion. Macrographs of sections through the pump shell and a vane showed darker areas representing graphitic residue and corrosion products that were not removed by erosion. Exposure of the pump bowl to the well water resulted in graphitic corrosion, which generated a soft, porous graphitic residue impregnated with insoluble corrosion products. Failure of the pump bowl resulted from the continuous erosion of the residue by action of the water within the pump.

Two water pumps were taken out of service because of reduced output. Visual inspection revealed considerable material loss in both impellers, which were 25.4 cm (10 in.) in diam x 1.3 cm (0.5 in.) wide and made from a cast bronze alloy. Several similar water pumps operating under nearly identical conditions, drawing water from an open tank through a standpipe, had no observable failures. Etched micrographs 100x of samples taken from the impellers showed clean, pockmarked, severely eroded surfaces, characteristic of cavitation damage. Investigation also revealed that considerable quantities of air were being drawn into the system when water in the supply tank dropped below a certain level. It was concluded that cavitation erosion (due to the uptake of air) caused metal removal and microstructural damage in the impellers. Recommendations included adding a water-level control to the piping system and excluding air from the pump inlet.

A 1-in. diam pump spindle fractured within the length covered by the boss of the impeller which was attached to the spindle by means of an axial screw. The pump had been in use in a chemical plant handling mixtures of organic liquids and dilute sulfuric acid having a pH value of 2 to 4 at temperatures of 80 to 90 deg C (176 to 194 deg F). The fracture was unusual in that it was of a fibrous nature, the fibers-which were orientated radially-were readily detachable. The surface of the spindle adjacent to the fracture had an etched appearance and the mode of cracking in this region suggested that failure resulted from an intergranular attack. Subsequent microscope examination confirmed the generally intergranular mode of failure. A macro-etched section near the fracture revealed a radial arrangement of columnar crystals, indicating that the spindle was a cast and not a wrought product as had been presumed. Spectroscope examination showed this particular composition (Fe-23Cr-18Ni-1.8Mo-1.2Si) did not conform to a standard specification and is apparently a proprietary alloy. It was evident that the particular mode of failure was related to the inherent structure of the material.

After operating for six months, a pump impeller (of nickel-containing cast iron) showed considerable corrosion. Cross sections showed substantial penetration of the wall thickness without loss of material. The observed supercooled structure implied low strength but would not affect corrosion resistance. Etching of the core structure showed a selective form of cast iron corrosion (spongiosis or graphitic corrosion) which lowered the strength of the cast iron enough that a knife could scrape off a black powder (10.85% C, 1.8% S, 1.45% P). Analysis showed that some of the “sulfate” found in the scrubbing water was actually sulfide (including hydrogen sulfide) and was the main cause of corrosion.

Two damaged impellers made of austenitic cast iron came from a rotary pump used for pumping brine mixed with drifting sand. On one of the impellers, pieces were broken out of the back wall in four places at the junction to the blades. The fracture edges followed the shape of the blade. Numerous cavitation pits were seen on the inner side of the front wall visible through the breaks in the back wall. The back wall of the as yet intact second impeller which did not show such deep cavitation pits was cracked in places along the line of the blades. The microstructure consisted of lamellar graphite and carbides in an austenitic matrix and was considered normal for the specified material GGL Ni-Cu-Cr 15 6 2. It was concluded that the cause of the damage was porosity at the junction between back wall and blades arising during the casting process. Cavitation did not contribute to fracture but also could have led to damage in the long term in the case of a sound casting. It is therefore advisable in the manufacture of new impellers to take care not only to avoid porosity but also to use alloy GGL Ni-Cu-Cr 15 6 3, which has a higher chromium content and is more resistant to cavitation.

During a routine start-up exercise of a standby service water pump, a threaded coupling that joined sections of a 41.5 ft (12.7 m) long pump shaft experienced fracture. The pump was taken out of service and examined to determine the cause of fracture. It was apparent early in the examination that the fracture involved hydrogen stress cracking. However, the nature of the corrosive attack suggested an interaction between the threaded coupling and biological organisms living in the freshwater environment of the pump shaft. The organisms had colonized on the coupling, changing the local environment and creating conditions favorable to hydrogen stress cracking. This paper describes the analysis of the fracture of the coupling and provides an example of how biologically induced corrosion can result in unexpected fracture of a relatively basic machine part.

A stuffing box (sand cast from ASTM A 536, grade 60-45-10, ductile iron) began leaking water after two weeks of service. The machine was operating at 326 rpm with a discharge water pressure of 21.4 MPa (3100 psi). Investigation (visual inspection, mechanical analysis, and nital etched 100x magnification) supported the conclusion that the crack initiated at the inner edge of a lubrication hole and had propagated toward both the threaded and flange ends of the casting. An appreciable residual-stress concentration must have been present and caused propagation of the crack. The residual stress might have been caused when a fitting was tightly screwed into the lubrication hole, and it might have been concentrated by notches at the inner end of the hole created when the drill broke through the sidewall to the stuffing box.

The broken end of a shaft from a centrifugal pump had a smooth fracture surface characteristic of failure from fatigue. Failure occurred in the plane of the keyway end and followed a slightly helical path, indicating that combined bending and torsional stresses were responsible. The material was a Cr-Mo-Ni alloy steel of the En 19 type in the hardened and tempered condition and of satisfactory quality. The assembly also included a copper sleeve attached by a circumferential braze behind the plane of fracture. The cracks were examined for the presence of copper, thinking that penetration by molten copper may have played a role, but no evidence was seen. An absence of chromium plating at the region of the heat-affected zone was also observed but could not be explained. Unfortunately, the end portion of the shaft was not available for examination.

One 49-in. impeller of a two-stage centrifugal air compressor disrupted without warning, causing extensive damage to the casings, the second impeller, and the driving gear box. Prior to the mishap, the machine had run normally, with no indications of abnormal vibration, temperature, or pressure. Initial failure had taken place in the floating dished inlet plate (eye plate) of the first-stage impeller. Failure occurred predominantly by tearing along the lines of rivet holes for the longer blades, these extended for practically the full radial width of the dished plate. Examination of the fractured surfaces showed that failure had been preceded by fatigue cracking. The material from which the dish plate was forged was a Ni-Cr-Mo steel in the oil hardened and tempered condition. Fractographic examination of the surface of the cracks showed striation markings indicative of the progress of fatigue cracks. Failure of the one impeller and the cracking of the others were attributed to “low-cycle high-strain fatigue” due to fluctuating circumferential (hoop) stresses.

An air compressor was installed at a chemical plant in which nitric acid was produced by burning ammonia with air. It was a 5000 hp, 5-stage centrifugal machine running at 6000 rpm, compressing air to 5 atm. Failure of the first stage impeller occurred due to a segment from the back plate becoming detached. On the remaining portion, cracks were visible running between the holes for rivets by which the vanes were attached. Metallographic examination of selected sections from the backplate revealed the material to be in the hardened and tempered condition, and the cracking to have initiated on the internal surface of the plate at the crevice between the plate and the vane. It was evident that the impeller failed by stress-corrosion cracking, which initiated in the crevice between the vanes and back plate and propagated through the plate along the line of the rivets where working stresses would be greatest. The compressor intake was situated in the vicinity of nitric acid pumps which had a history of leakage troubles, and which had evidently given rise to the nitrates found on the impeller.

Post-service destructive evaluation was performed on two commercially pure zirconium pump impellers. One impeller failed after short service in an aqueous hydrochloric acid environment. Its exposed surfaces are bright and shiny, covered with pockmarks, and peppered with pitting. Uniform corrosion is evident and two deep linear defects are present on impeller blade tips. In contrast, the undamaged impeller surfaces are covered with a dark oxide film. This and many other impellers in seemingly identical service conditions survive long lives with little or no apparent damage. No material or manufacturing defects were found to explain the different service performance of the two impellers. Microstructure, microhardness and material chemistry are consistent with the specified material. Examination reveals the damage mechanism to be corrosion-enhanced cavitation erosion, the most severe form of erosion corrosion. Cavitation damage to the protective oxide film caused the zirconium to lose its normally outstanding corrosion resistance. The root cause of the impeller failure is most likely the introduction of excessive air into the pump due to low liquid level, a bad seal or inadequate head. Corrosion pitting, crevice corrosion, and solidification cracks (casting defect) also contributed to the failure.

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.

An investigation was conducted to determine the cause of numerous cracks and other defects on the surface of a cast ASTM A743 grade CA-15 stainless steel main boiler feed pump impeller. The surface was examined using a stereomicroscope, and macrofractography was conducted on several cross sections removed from the impeller body. Areas that appeared to have the most severe surface damage were sectioned, fractured open, and examined using SEM. The chemistry of the impeller and an apparent repair weld were also analyzed. The examination indicated that the cracks were shrinkage voids from the original casting process. Surface repair welds had been used to fill in or cover over larger shrinkage cavities. It was recommended that more stringent visual and nondestructive examination criteria be established for the castings.

An investigation of the impeller and deposit samples from a centrifugal compressor revealed that an aluminum IR-12 refrigerant reaction had occurred, causing extensive damage to the second-stage impeller and contaminating the internal compressor components. The spherical surface morphology of the impeller fragments suggested that the aluminum had melted and resolidified. The deposits were similar in composition and were identified by XRD as consisting primarily of aluminum trifluoride. In addition, EDS analysis detected major amounts of chlorine and iron. Results of a combustion test indicated that the compressor deposit was comprised of a 9. 8 wt% carbon and that the condenser deposit contained 8.7 wt% carbon. It was concluded that the primary cause of failure was the rubbing of the impeller against the casting and that a self-sustaining Freon fire had occurred in the failed compressor

Radial cracking occurred adjacent to 11 vanes in a 19-vane impeller operating in a chemical plant environment. The impeller vanes were fillet welded to both the disk and the cover Cracks were next to the fillet welds and near the cover outer diameter They generally did not extend to the outer diameter. The entire impeller surface was tested by the dry magnetic particle method. Visual and microstructural examinations revealed intergranular cracking. Energy-dispersive spectroscopy of corrosion products contained in the cracks disclosed the presence of chlorine and sulfur The failure was attributed to stress-corrosion cracking caused by a corrosive atmosphere.

Several large-diameter type 304L stainless steel impeller/propeller blades in a circulating water pump failed after approximately 8 months of operation. The impeller was a single casting that had been modified with a fillet weld buildup at the blade root. Visual examination indicated that the fracture originated near the blade-to-hub attachment in the area of the weld buildup. Specimens from four failed castings and from an impeller that had developed cracks prior to design modification were subjected to a complete analysis. A number of finite-element-method computer models were also constructed. It was determined that the blades failed by fatigue that had been accelerated by stress-corrosion cracking. The mechanism of failure was flow-induced vibration, in which the vortex-shedding frequencies of the blades were attuned to the natural frequency of the blade/hub configuration. A number of solutions involving material selection and impeller redesign were recommended.