Several surgical tool failures were analyzed to understand why they occur and how to prevent them. The study included drills, catheters, and needles subjected to the rigors of biomedical applications such as corrosive environments, high stresses, sterilization, and improper cleaning procedures. Given the extreme conditions to which surgical tools can be exposed, and the potential for misuse, failures are inevitable and systematic methods for analyzing them are necessary to keep them in check.

Rotor blades in the compressor section of a J79 engine had failed. Optical, stereoscopic, microhardness testing, and SEM examinations were conducted to determine the cause. The blades were made of STS403 and were used uncoated. They were damaged over an extensive area, from the 15th through the 17th compressor stages, as were stator vanes and casing sections. The fractured surface of the 17th blade showed multiple origins along with secondary cracking and extensive propagation that preceded separation. The metallographic analysis of the microstructure suggested work hardening. Based on the results, the cause of the fractured blade was high-amplitude fatigue due to severe stall. After normal engine usage of five months, the blade fractured sending fragments throughout the combustion and turbine sections.

A heat transport pump in a heavy water reactor failed (exhibiting excessive vibration) during a restart following a brief interruption in coolant flow due to a faulty valve. The pump had developed a large crack across the entire length of a bearing journal. An investigation to establish the root cause of the failure included chemical and metallurgical analysis, scanning electron fractography, mechanical property testing, finite element analysis of the shrink fitted journal, and a design review of the assembly fits. The journal failure was attributed to corrosion fatigue. Corrective actions to make the journals less susceptible to future failures were implemented and the process by which they were developed is described.

Type 410 stainless steel bolts were used to hold together galvanized gray cast iron splice case halves. Before installation, the bolts were treated with molybdenum disulfide (MoS 2 ) antiseize compound. Several failures of splice case bolts were discovered in flooded manholes after they were in service for three to four months. Laboratory experiments were conducted to determine if the failure mode was hydrogen-stress cracking, if sulfides accelerate the failure, if heat treatment can improve the resistance against this failure mode, and if the type 305 austenitic stainless steel would serve as a replacement material. Based on test results, the solution to the hydrogen-stress cracking problem consisted of changing the bolt from type 410 to 305 stainless steel, eliminating use of MoS2, and limiting the torque to 60 N·m (540 in.·lb).

A type 431 stainless steel mushroom-head closure fractured in service at a hydrogen pressure of 3000 atm. Fracture occurred at room temperature after miscellaneous chemical service that included exposures to hydrogen at temperatures from ambient to 350 deg C (662 deg F). Investigation (visual inspection and 2400x/6600x TEM analysis) supported the conclusion that failure was caused by hydrogen embrittlement, not SCC as might have been suspected. No recommendations were made.

Ball bearings made of type 440C stainless steel hardened to 60 HRC and suspected as the source of intermittent noise in an office machine were examined. A number of spots on the inner-ring raceway were revealed by scanning electron microscopy. The metal in the area around the spot was evidenced to have been melted and welded to the inner-ring raceway. It was revealed by randomly spaced welded areas on the raceways that the welding was the result of short electrical discharges between the bearing raceways and the balls. The use of an electrically nonconductive lubricant in the bearings was suspected to have caused the electric discharge by accumulation and discharge of static charge. The electrical resistance between the rotor and the motor frame lubricated with electrically conductive grease and the grease used in the current case was measured and compared to confirm the fact the currently used grease was nonconductive. It was concluded that the pits were formed by momentary welding between the ball and ring surfaces. The lubricant was replaced by electrically conductive grease as a corrective measure.

Stainless steel bolts broke after short-term exposure in boiler feed-pump applications. Specifications required that the bolts be made of a 12% Cr high-strength steel with a composition conforming to that of AISI type 410 stainless steel. Several bolts from three different installations were examined. It was found that fracture of the bolts was by intergranular stress corrosion. A metallic copper-containing antiseizure compound on the bolts in a corrosive medium set up an electro-chemical cell that produced trenchlike fissures or pits for fracture initiation. Because the bolts were not subjected to cyclic loading, fatigue or corrosion fatigue was not possible. To prevent reoccurrence, bolts were required to conform to the specified chemical composition. The hardness range for the bolts was changed from 35 to 45 HRC to 18 to 24 HRC. Petroleum jelly was used as an antiseizure lubricant in place of the copper-containing compound. As a result of these changes, bolt life was increased to more than three years.

Two nuts were used to secure the water-supply pipes to the threaded connections on hot-water and cold-water taps. The nut used on the cold-water tap fractured about one week after installation. Examination of the fracture surfaces of the coldwater nut did not reveal any obvious defects to account for the fracture, but there were indications of excessive porosity in the nut. The fracture had occurred through the root of the first thread that was adjacent to the flange of the tap. It was found that the nut from the cold-water tap failed by SCC. Apparently, sufficient stress was developed in the nut to promote this type of failure by normal installation because there was no evidence of excessive tightening of the nut. Corrosion testing of the nuts indicated that the fractured nut was highly susceptible to intergranular corrosion because of either a deficiency in magnesium content or excessive impurities, such as lead, tin, or cadmium. This composition problem with zinc alloys was recognized many years ago, and particular attention has been directed toward ensuring that high-purity zinc is used. This corrective measure reportedly resulted in virtual elimination of this type of defect.

The radial-contact ball bearings (type 440C stainless steel and hardened) supporting a computer microdrum were removed for examination as they became noisy. Two sizes of bearings were used for the microdrum and a spring washer that applied a 50 lb axial load on the smaller bearing was installed in contact with the inner ring for accurate positioning of the microdrum. The particles contained in residue achieved after cleaning of the grease on bearings with a petroleum solvent were attracted by a magnet and detected under a SEM (SEM) to be flaked off particles from the outer raceway surface. Smearing, true-brinelling marks, and evidence of flaking caused by the shifting of the contact area (toward one side) under axial load, was revealed by SEM investigation of one side of the outer-ring raceway. The true-brinelling marks on the raceways were found to be caused by excessive loading when the bearing was not rotating or during installation. It was concluded that the bearings had failed in rolling-contact fatigue. The noise was eliminated and the preload was reduced to 30 lb by using a different spring washer as a corrective measure.

AISI type 410 stainless steel tube bundles in a heat exchanger experienced leakage during hydrostatic testing even before being in service. The inside surfaces of the tubes was observed to have been pitted. Chloride-ion pitting was revealed by the undercutting in the cross section of a pit and further confirmed by x-ray spectrometry. It was concluded that the failure was caused by pitting due to chlorides in the water used to flush the tubes before service. The use of brackish water to flush or test stainless steel equipment was recommended to avoid pitting.

During installation, a clamp-strap assembly, specified to be type 410 stainless steel-austenitized at 955 to 1010 deg C (1750 to 1850 deg F), oil quenched, and tempered at 565 deg C (1050 deg F) for 2 h to achieve a hardness of 30 to 35 HRC, and used for securing the caging mechanism on a star-tracking telescope, fractured transversely across two rivet holes closest to one edge of the pin retainer in a completely brittle manner. Comparison with a non-failed strap using microscopic examination, spectrographic analysis, and slow-bend tests showed that both fit the 410 stainless steel specs, but hardness and grain size were different. Reheat treatment of full-width specimens showed that coarse grain size (ASTM 2 to 3) was responsible for the brittle fracture, and excessively high temperature during austenitizing caused the large grain size in the failed strap. The fact that the hardness of the strap that failed was lower than the specified hardness of 30 to 35 HRC had no effect on the failure because that of the non-failed strap was even lower. Recommendation was that the strap should be heat treated as specified to maintain the required ductility and grain size.

The support bearing of a hydrofoil vessel failed after only 220 h of operation. The bearing consisted of an outer ring made of chromium-plated AISI type 416 stainless steel and an inner ring with a spherical outer surface made of AISI type 440C stainless steel, with a plastic material, bonded to the outer ring, between the two. The inner ring was found to have failed in four places. The two metallic rings were allowed to come in contact with each other by the disappearance of the plastic material. It was revealed by examination of the fracture surfaces of the inner ring that the failure was caused by fatigue initiated in corrosion pits (caused by seawater). The fracture was found to be transgranular. It was recommended that the inner and outer rings should both be made from the more corrosion resistant 17-4 PH (AISI type 630) stainless steel.

The pointed ends of several stainless steel forceps split or completely fractured where split portions broke off. All the forceps were delivered in the same lot. The pointed ends of the forceps are used for probing and gripping very small objects and must be true, sound, and sharp. Analysis supported the conclusion that the failures to be the result of seams in the steel that were not joined during hot working. Recommendations included that closer inspection of the product take place at all stages of manufacturing. Inspection at the mill will minimize discrepancies at the source, and the inspection of the finished product will help detect obscure seams.

A spoon that was twisted and broken by a person claiming to possess parapsychic powers was submitted for failure analysis. Exemplar tests were conducted on material taken from the bowl region of the same spoon. In addition, tests on other unbroken samples of spoons were evaluated in order to establish both macroscopic and microscopic comparative behavior. These controlled tests produced known failure mechanisms and their respective fracture morphology in this material. A direct comparison could then be made with the unknown failure. The paper identifies a method of analysis that should be applied when analyzing failures of unknown origin.

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.

When a steam turbine was put out of service, cracks were noticed on many of the blades in the low pressure section round the stabilization bolts and perpendicular to the blade axis. The blades were made from chrome alloy steel X20-Cr13 (Material No. 1.402). When the bolts were brazed into the blades inadmissible localized overheating of the steel must have occurred, which resulted in transformation stresses and hence reduced deformability. The cracks arose as a consequence of careless brazing. Whether the cracks should be considered as stress cracks over their entire extent or partially as fatigue cracks produced by vibration in the operation of the turbine as a result of steplike growing of microcracks could not be deduced from the fracture surfaces. Microfractography showed that the cracks developed in stages.

A spindle made of hardenable 13% chromium steel X40 Cr13 (Material No. 1.4034) that was fastened to a superheated steam push rod made of high temperature structural steel 13Cr-Mo44 (Material No. 1.7335) by means of a convex fillet weld, fractured at the first operation of the rod directly next to the weld bead. Investigation showed that the fracture of the superheated steam push rod spindle was caused by hardening and hardening crack formation in the weld seams and adjoining areas. It would have been preferable to avoid welding near the cross sectional transitions altogether in consideration of the crack sensitivity of high hardenability steels. If for some reason this was not possible, then all precautions should have been taken that are applicable to the particular steel, such as preheating, slow cooling and stress relief tempering after welding. The selection of an austenitic additive material should have been considered because it could have equalized stresses due to its high elongation. Most probably, however, a material of lower hardenability should have been selected for the spindle if high operating properties were of paramount importance.

Failures of 10Cr-Mo9-10 and X 20Cr-Mo-V12-1 superheated pipes during service in steam power generation plants are described. Through micrographic and fractographic analysis, creep and overheating were identified as the cause of failure. The Larson-Miller parameter is computed, as a function of oxidation thickness, temperature and time, confirming the creep failure diagnostic.

A T-bolt was part of the coupling for a bleed air duct of a jet engine on a transport plane. Specifications required that the 4.8 mm diam component be made of AISI type 431 stainless steel and heat treated to 44 HRC. The operating temperature of the duct is 425 to 540 deg C (800 to 1000 deg F), but that of the bolt is lower. The T-bolt broke after three years of service. The expected service life was equal to that of the aircraft. It was found that the bolt broke as a result of SCC. Thermal stresses were induced into the bolt by intermittent operation of the jet engine. Mechanical stresses were induced by tightening of the clamp around the duct, which in effect acted to straighten the bolt. The action of these stresses on the carbides that precipitated in the grain boundaries resulted in fracture of the bolt. Due to the operating temperatures of the duct near the bolt, the material was changed to A-286, which is less susceptible to carbide precipitation. The bolt is strengthened by shot peening and rolling the threads after heat treatment. Avoiding temperatures in the sensitizing range is desirable, but difficult to ensure because of the application.

Three wing flap hinge bearings were received by the laboratory for analysis. The bearings were fabricated from chromium-plated type 440C martensitic stainless steel. The intergranular fracture pattern seen in the electron fractographs, coupled with the corrosion pits observed on the inner diam of the bearings, strongly suggested that failure initiated by pitting and progressed by SCC or hydrogen embrittlement from the plating operation. It was recommended that the extent of the flap hinge bearing cracking problem be determined by using nondestructive inspection because it is possible to crack hardened type 440C during the chromium plating process. An inspection for pitting on the bearing inner diam was also recommended. It was suggested that electroless nickel be used as a coating for the entire bearing. A review of the chromium plating and baking sequence was recommended also to ensure that a source of hydrogen is not introduced during the plating operation.