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.

A type 316L stainless steel angled plate failed. The fatigue fracture was found to have occurred at a plate hole. Symmetric cyclic bending forces were revealed by the fatigue damage at the fracture edge at the top surface of the plate. Fatigue striations and slip bands produced on the surface during cyclic loading were observed. The material was showed by the deformation structure to be in the cold-worked condition and was termed to not be the cause of the implant failure.

During the internal fixation, the type 316LR stainless steel cortical bone screw failed. Extensive spiral deformation was revealed by the fracture surface. Dimple structure characteristic of a ductile failure mode was observed with dimples oriented uniformly in the deformation direction. A zone of heavily deformed grains at the fracture edge was revealed by longitudinal metallographic examination. The shearing fractures of a commercially pure titanium screw and a cast cobalt-chromium-molybdenum alloy were discussed for purpose of comparison.

The plate used to treat a pseudarthrosis in the proximal femur was investigated for reasons of non-progress of healing. Fatigue cracks were revealed on the top surface of the small section of the plate at the fifth screw hole. The plate was found to be heavily loaded by comparison of intensity of these structures, compared to results of systematic crack-initiation experiments. It was revealed by fatigue bending tests that the fatigue life of plates with asymmetrically arranged holes is at least as long as for plates with holes situated in the center. Fatigue began at the large section only after a fatigue crack begins to propagate into the small plate section. A large secondary crack which had developed parallel to the main crack in the center of the surface was revealed. The fifth hole was situated at the transition between the supporting bone and the defect and hence stress concentration was revealed to be high.

Fretting and fretting corrosion at the contact area between the screw hole of a type 316LR stainless steel bone plate and the corresponding screw head was studied. The attack on the 316LR stainless steel was only shallow. Mechanical grinding and polishing structures were exhibited by a large portion of the contact area. Fine corrosion pits in the periphery were observed and intense mechanical material transfer that can take place during fretting was revealed. Smearing of material layers over each other during wear was observed and attack by pitting corrosion was interpreted to be possible.

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.

Type 316LR stainless steel screws that failed by fatigue were studied. It was found that fatigue fracture can occur on different thread levels, depending on the loading situation. The initiation of secondary fatigue cracks was occasionally found parallel to the fracture plane. The screws were used with a relatively rigid plate to treat a fracture complication in the upper end of the femur. The fatigue failures were explained by signs of unstable fixation revealed by radiographs.

Cerclage wire, which was used with two screws and washers for a tension band in a corrective internal fixation, was found broken at several points and corroded after nine months in service. The material was examined using energy-dispersive x-ray analysis and determined not to be in compliance with standards (type 304 stainless steel without molybdenum). The screws and washers were found to be made of remelted implant-quality type 316L stainless steel and were intact. Signs of sensitization, characterized by chromium carbide precipitates at the grain boundaries, were revealed by the microstructure. Intercrystalline corrosion with pitted grains was indicated by SEM fractography. Improper heat treatment of the steel was interpreted to have led to intercrystalline corrosion and implant separation.

Heavy pitting corrosion on type 304 stainless steel bone screw was studied. A screw head that exhibited heavy pitting corrosion attack was observed. Deep tunnels that penetrated the screw head and followed the inclusion lines were revealed. The screw was inserted in a plate made of type 316LR stainless steel and some mechanical fretting and very few corrosion pits were revealed. Type 304 stainless steel was deemed not to be satisfactory as an implant material.

A narrow bone plate made of type 316 stainless steel and used to stabilize an open midshaft femur fracture failed. A crack at a plate hole next to the fracture site had been revealed by a radiograph taken 13 weeks after the operation. The plate was revealed to be slightly bent in the horizontal plane, and the fracture gap was considerably open. The screws and plates supplied by different manufacturers were revealed to be different with respect to microcleanliness (primary inclusion content) of the materials and only one of them was found to be according to specifications. The local crack formation was influenced by the presence of larger inclusions. The screw failed was revealed to have failed through a fatigue mechanism by the presence of striations in the scanning electron micrograph. The crack in the plate was revealed to have originated at the upper, outer corner of the plate by the beach marks which indicated the action of asymmetric bending and rotational forces.

Failures of four different 300-series austenitic stainless steel biomedical fixation implants were examined. The device fractures were observed optically, and their surfaces were examined by scanning electron microscopy. Fractography identified fatigue to be the failure mode for all four of the implants. In every instance, the fatigue cracks initiated from the attachment screw holes at the reduced cross sections of the implants. Two fixation implant designs were analyzed using finite-element modeling. This analysis confirmed the presence of severe stress concentrations adjacent to the attachment screw holes, the fatigue crack initiation sites. Conclusions were reached regarding the design of these types of implant fixation devices, particularly the location of the attachment screw holes. The use of austenitic stainless steel for these biomedical implant devices is also addressed. Recommendations to improve the fixation implant design are suggested, and the potential benefits of the substitution of titanium or a titanium alloy for the stainless steel are discussed.

A compression hip screw is a device designed to hold fractures in the area of the femur in alignment and under compression. A side plate, which is an integral part of the device, is attached by screws to the femur, and it holds the compression screw in position. The device analyzed had broken across the eighth hole (of nine holes) from the end of the plate. The detailed metallurgical failure analysis of the device, including metallography and fractography, is reported here. It was found that the device had adequate metallurgical integrity for the application for which it was intended. It is believed that failure was caused by the lack of a screw in the ninth hole. Evidence is also presented which indicates that the device was bent prior to insertion, and the local plastic deformation may have caused structural changes leading to premature crack initiation.

This paper summarizes several cases of metallurgical failure analysis of surgical implants conducted at the Laboratory of Failure Analysis of IPT, in Brazil. Investigation revealed that most of the samples were not in accordance with ISO standards and presented evidence of corrosion assisted fracture. Additionally, some components were found to contain fabrication/processing defects that contributed to premature failure. The implant of nonbiocompatible materials results in immeasurable damage to patients as well as losses for the public investment. It is proposed that local sanitary regulation agencies create mechanisms to avoid commercialization of surgical implants that are not in accordance with standards and adopt the practice of retrieval analysis of failed implants. This would protect the public health by identifying and preventing the main causes of failure in surgical implants.

A tubular heat exchanger in a refinery reformer unit leaked after one month of service. The exchanger contained 167 type 304 stainless steel U-bent integral-finned tubes. Cracks in the tube wall were revealed during examination. Hardness of the tube was found to be 30 HRC at the inside surface and up to 40 HRC at the base of the fin midway between the roots which indicated that the fins were cold formed and not subsequently annealed thus susceptible to SCC because of a high residual stress level. It was revealed by metallographic examination that the fracture was predominantly by transgranular branched cracking and had originated from the inside surface. It was concluded that the tubes failed in SCC caused by chlorides in the presence of high residual stresses. The finned tubes were ordered in the annealed condition as a corrective measure.

Type 316L stainless steel pipes carrying brine at 120 deg C (250 deg F) and at a pH of about 7, failed by perforation at or near circumferential butt-weld seams. The failure was examined optically and radiographically in the field. Specimens were removed and examined metallographically and with a SEM in the laboratory. The examinations revealed a combination of failure mechanisms. The pitting failure of the welds was attributed to localized attack of an activated surface, in which anodic pits corroded rapidly. Additionally, SCC driven by residual welding stresses occurred in the base metal adjacent to the welds. Use of highly stressed austenitic stainless steels in high-chloride environments having a temperature above 65 deg C (150 deg F) should be discouraged. Solution annealing or shot peening to reduce residual stresses may be advisable. If heat treatment is not feasible after welding, the substitution of a more corrosion-resistant alloy, such as Incoloy 800 or 825, may be necessary.

Three samples from a ruptured 316 stainless steel tube were examined. The tube, 114 mm OD, wall thickness 8.00 mm, with 13 mm thick 321 stainless steel fins welded to the outer surface of the tube, was part of a heater through which sour gas, containing methane plus H2S and CO, passed at 1150 psig. The sour gas was heated to 600 deg F by burners playing on the outside of the tube burning “sweet” gas plus air. The inner and outer surfaces of all samples showed evidence of corrosive attack. Electron probe microanalysis showed the corrosion products contained sulfur with iron, together with nickel to a lesser extent. Local thinning, cavitation, and ductile deformation markings associated with the unmatched sample taken from the center of the fire showed the tube ruptured as a result of overheating. Overheating while the temperature recorder was off the chart caused severe loss of tube strength, resulting in ductile rupture. The minimum overheating temperature could be deduced at around 1200 deg F due to the presence of a eutectic observed metallographically within the surface corrosion products.

The wires used in a wet precipitator for cleaning the gases coming off a basic oxygen furnace failed. The system consisted of six precipitators, three separate dual units, each composed of four zones. Each zone contained rows of wires (cold drawn AISI 1008 carbon steel) suspended between parallel collector plates. It was determined that the 1008 wires failed because of corrosion fatigue. It was decided to replace all of the wires in the two zones with the highest rates of failure with cold-drawn type 304 austenitic stainless steel wire. These expensive wires, however, failed after a week by transgranular SCC. Annealed type 430 ferritic stainless steel was subsequently suggested to prevent further failures.

The corner of a welded sheet construction made from austenitic corrosion-resistant chromium-nickel steel showed corrosive attack of the outer sheet. This attack was most severe at the points subjected to the greatest heat during welding. Particularly large amounts of weld metal had been applied. Microscopic examination showed grain disintegration was promoted by the thickness of the weld bead and the amount of heat required to produce it. If nonstabilized austenitic sheet is to be used in the future, one of the particularly low-carbon steels, X2 CrNi 18 9 or X2 CrNiMo 18 10, is recommended.

Stainless steel liners (AISI type 321) used in bellows-type expansion joints in a duct assembly installed in a low-pressure nitrogen gas system failed in service. The duct assembly consisted of two expansion joints connected by a 32 cm (12 in.) OD pipe of ASTM A106 grade B steel. Elbows made of ASTM A234 grade B steel were attached to each end of the assembly, 180 deg apart. A 1.3 mm (0.050 in.) thick liner with an OD of 29 cm (11 in.) was welded inside each joint. The upstream ends were stable, but the downstream ends of the liners remained free, allowing the components to move with the expansion and contraction of the bellows. Investigation (visual inspection, hardness testing, and 30x fractographs) supported the conclusion that the liners failed in fatigue initiated at the intersection of the longitudinal weld forming the liner and the circumferential weld by which it attached to the bellows assembly. Recommendations included increasing the thickness of the liners from 1.3 to 1.9 mm (0.050 to 0.075 in.) in order to damp some of the stress-producing vibrations.

After about two years in service, a 303 stainless steel valve in contact with a carbonated soft drink in a vending machine occasionally dispensed a discolored drink with a sulfide odor. According to the laboratory at the bottling plant, the soft drink in question was strongly acidic, containing citric and phosphoric acids and having a pH of 2.4 to 2.5. Investigation (visual inspection, chemical analysis, immersion testing in the soft drink, and 100x unetched micrographs) supported the conclusion that the failure was caused by the size and distribution of sulfide stringers in the alloy used in the valve. Manganese sulfide stringers in the valve were exposed at end-grain surfaces in contact with the beverage. The stringers, which were anodic to the surrounding metal, were subject to corrosion, producing a hydrogen sulfide concentration in the immediately adjacent liquid. Recommendations included changing the valve material to type 304 stainless steel.