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.

Two cases of failure of centrifuge baskets were investigated. The first involved a centrifuge running at approximately 1000 rpm. The basket was constructed from a perforated sheet of stainless steel rolled into a cylinder and joined by a single vee longitudinal weld. Detailed examination showed the weld had not completely penetrated the full depth of the section. The fracture faces showed a gradually progressing fatigue crack developing from a notch, formed by the lack of penetration, at the root of the weld. Microscopic examination of the parent plate showed it was a typical titanium stabilized austenitic steel. It is probable that had the basket been subjected to a periodic inspection by a competent person, this failure would not have occurred. The second case concerned a continuous duty centrifuge operating at 2200 rpm. Fracture had occurred at the circumferential weld attaching the stainless steel skirt to the basket rim and also in the region of the vertical weld which was made when the skirt was formed into a cone. Stress-corrosion cracking of the skirt material, which contained residual stresses due to cold-rolling, had been caused by the presence of sodium chloride.

A fracture mechanics based failure analysis and life prediction of a large centrifugal fan made from low-carbon, medium-strength steel was undertaken following shortcomings in attempts to explain its fatigue life from start stop cycles alone. Measurements of the fracture toughness and flaw size at failure, coupled with quantitative SEM fractography using striation spacing methods, revealed that the cyclic stress amplitudes just prior to failure were much larger than expected, in this particular case. Subsequent improvements in fan design and fabrication have effectively alleviated the problem of slow, high cycle fatigue crack growth, at normal operating stresses in similar fans.

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.

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

A forged 4130 steel cylindrical permanent mold, used for centrifugal casting of gray- and ductile-iron pipe, was examined after pulling of the pipe became increasingly difficult. In operation, the mold rotated at a predetermined speed in a centrifugal casting machine while the molten metal, flowing through a trough, was poured into the mold beginning at the bell end and ending with the spigot end being poured last. After the pipe had cooled, it was pulled out from the bell end of the mold, and the procedure was repeated. Investigation supported the conclusion that failure of the mold surface was the result of localized overheating caused by splashing of molten metal on the bore surface near the spigot end. In addition, the mold-wash compound (a bentonite mixture) near the spigot end was too thin to provide the proper degree of insulation and to prevent molten metal from sticking to the bore surface. Recommendations included reducing the pouring temperatures of the molten metal and spraying a thicker insulating coating onto the mold surface.

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.

The lock ring of a centrifuge drum was fractured after one year's operation. The ring, with a trapezoidal thread on the inside, was made of steel with approximately 0.5%C-1.3%Mn-1.1%Cr and was hardened and tempered to 105 kp/sq mm strength at 11% elongation (d10). It fractured radially in one of four places in which the cross section was weakened by short grooves that served as tool grips for tightening the cover. The fracture propagated from the base of the thread and followed it in a circumferential direction until it was broken through radially at the top across the ring due to a weakening caused by the external reduction of the cross section. The uppermost turn was corroded at the base by pitting favored by differences in ventilation and formation of Evans elements in the narrow gap between thread and counterthread. Metallographic examination showed that the pitting favored intergranular fissures and therefore it can be established that stress corrosion accelerated cracking of the ring Although since the drum was used for the processing of various liquids, the exact corroding medium cannot be stated.

The impeller of a 4 ft. diam extraction fan driven by a 120 hp motor at 1,480 rpm. disrupted suddenly. The majority of the vanes had become detached where they were welded to the plates. At other locations, separation of the vanes was accompanied by tearing of the adjacent plate, failure being initiated at the weld fillets of the inner end of the vanes. An unusual feature was that the blades disclosed regions having a pronounced striated and stepped appearance. The etched microstructure was typical of a low carbon rolled plate having the usual banded appearance. A cross section through the fillet welds and zone showed lamellar tearing, which confirmed that failure had occurred in weld metal adjacent to the fusion face of the fillet to the vane. Results of the investigation indicated that the primary cause of failure of the impeller was the development of fatigue cracks from the unwelded roots of the fillet welds, by which the vanes were attached to the supporting plates. The impeller would have shown increased resistance to fatigue crack initiation if the T joint between the vanes and plates had been of the full penetration type.

A hydroextractor installed new for the drying of sugar massecuite consisted of a metal basket fixed to a vertical spindle. Disruption occurred just after the machine had been run up to speed and was not preceded by any abnormal behavior. The basket assembly consisted of a Ni-Cr-Mo steel shell and two end plates. It was designed to spin at 2200 rpm, using centrifugal force to expel liquids through nearly 3000 drilled holes in the shell wall. Investigators found that the shell separated completely from the bottom plate. The top plate, though it cracked radially, remained attached over most of its circumference. The basket also contained a 22-gauge Monel metal liner that had been perforated by stabbing, raising pronounced burrs that faced each hole. Apart from the local spots of corrosion due to the lining, the inner surface of the basket showed little evidence of general corrosion. What caused the basket to fail was the presence of corrosion-fatigue cracks or fissures radiating from the holes. A secondary cause was that the scantlings of the basket were too light.

This article considers two mechanisms of cavitation failure: those for ductile materials and those for brittle materials. It examines the different stages of cavitation erosion. The article explains various cavitation failures including cavitation in bearings, centrifugal pumps, and gearboxes. It provides information on the cavitation resistance of materials and other prevention parameters. The article describes two American Society for Testing and Materials (ASTM) standards for the evaluation of erosion and cavitation, namely, ASTM Standard G 32 and ASTM Standard G 73. It concludes with a discussion on correlations between laboratory results and service.

This article focuses on the general root causes of failure attributed to the casting process, casting material, and design with examples. The casting processes discussed include gravity die casting, pressure die casting, semisolid casting, squeeze casting, and centrifugal casting. Cast iron, gray cast iron, malleable irons, ductile iron, low-alloy steel castings, austenitic steels, corrosion-resistant castings, and cast aluminum alloys are the materials discussed. The article describes the general types of discontinuities or imperfections for traditional casting with sand molds. It presents the international classification of common casting defects in a tabular form.

Following disruption of the austenitic stainless steel basket of a hydro-extractor used for the separation of crystals of salt (sodium chloride) from glycerin, samples of the broken parts were analyzed. Examination revealed that the fish-plates joining the reinforcing hoops had broken, the shell had split from top to bottom adjacent to the weld, the top and bottom cover plates had become loose, all the rivets having pulled out, and the shaft was also found to be bent. Fracture took place in an irregular manner and was of the shear type towards both ends; it occurred immediately adjacent to the weld or a short distance from it and on alternate sides. Microscopical examination did not reveal any intergranular carbide precipitation, such as is well known to result in the weld-decay mode of failure. It was concluded that the primary cause of failure was stress-corrosion cracking arising from the combined effect of residual stresses and the corrosive effect of the material being centrifuged. If the shell had been stress-relieved after fabrication, the failure likely would not have occurred.

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.

The shafts on two centrifugal pumps failed during use in a petroleum refinery. Light optical microscopy and scanning electron microscopy were used to analyze the damaged materials to determine the cause of failure. The results showed that one shaft, made of duplex stainless steel, failed by fatigue fracture, and the other, made of 316 austenitic stainless steel, experienced a similar fracture, which was promoted by the presence of nonmetallic inclusion particles.

Uncoated high-strength alloy steel cap screws retaining a cast aluminum (356.0) diffuser assembly in a centrifugal refrigerant compressor failed in a brittle manner a short time after the system was placed in operation. Evidence obtained during the failure analysis indicated that the failures were the result of hydrogen embrittlement produced by galvanic corrosion and attendant evolution of hydrogen at the dissimilar junction, which was also the site of the highest tensile stress. Suggested measures for minimizing recurrences included use of lower-strength, galvanically-compatible fasteners and appropriately-applied and treated compatible coatings.

An amusement ride failed when a component in the ride parted, permitting it to fly apart. The ride consisted of a central shaft supporting a spider of three arms, each of which was equipped with an AISI 1040 steel secondary shaft about which a circular platform rotated. The main shaft rotated at about 12 rpm and the platforms at a speed of 20 rpm. The accident occurred when one of the secondary shafts on the amusement ride broke. The point of fracture was adjacent to a weld that attached the shaft to a 16 mm thick plate, which in turn bore the platform support arms. Investigation (visual inspection, 0.4x magnification, and stress analysis) supported the conclusion that a likely cause for the fatigue failure was the combination of residual stresses generated in welding and centrifugal service stresses from operation that were accentuated by areas of stress concentration at the undercut locations. Without the excessive residual stress, the shaft dimensions appeared ample for the service load. Recommendations included applying the fillet weld with more care to avoid undercutting. The residual stresses could be minimized by pre-weld and post-weld heat application.