A pair of steam generators operating at a pressurized water reactor site were found to be leaking near a closure weld. The generators were the vertical U-tube type, constructed from ASTM A302 grade B steel. The shell material exhibited high hardness values prior to confirmatory heat treatment, indicating high residual stresses in the area of the weld. All cracks were transgranular and were associated with pits on the inside surfaces of the vessels. It was concluded that the cracking was caused by a low-cycle corrosion fatigue phenomenon, with cracks initiating at areas of localized corrosion and propagating by fatigue. The cause of the pitting/cracking was related to the unit's copper species in solution.

A failure analysis was conducted to determine the cause of recurring failure of flexible bellows in an exhaust hose assembly. The bellows were made of type 321 stainless steel. Visual examination showed that cracks followed a path along the seam weld in the bellows. Most of the cracks followed a multidirectional/circular pattern, occasionally chipping off the convolutions, an indication of high-resonance fatigue-type cracking. Scanning electron fractography showed fatigue striations throughout the fracture surface. The microstructure consisted of relatively large grains and an abnormal degree of titanium-base stringers. Wall thickness was about 0.15 mm (0.006 in.) underside. It was concluded that the high vane pass frequency excited the natural vibration of the bellows to a higher resonance and cracked the bellows after a relatively short service period. The assembly was redesigned, and no further cracking occurred.

The 14-cm diam main hoist shaft of a mobile shovel was found to have multiple crack indications when ultrasonically inspected in the field. A crack around the entire circumference at the change in section was revealed by magnetic-particle inspection of the shaft. The crack was found to coincide with the junction of the fillet and the smaller diam at this change in section. A slight step in the continuity of the fillet and some machining marks were noted at this junction. A fine crack extending 2.5 mm from the surface and originating at the machining marks was revealed by microscopic examination. The shaft was identified by chemical analysis to be 1040 steel (hardness 170 HRB) which was concluded to have insufficient fatigue strength. The step at the base of the fillet was revealed as the point of initiation of the fatigue crack. Shaft material was changed to 4140 steel oil-quenched and tempered to a hardness of 302 to 352 HRB and all machining discontinuities were removed.

Fractures and a crack occurred in a length of excavator boom rope. Failure took place at regions where local corrosion was evident. Microscopic examination of longitudinal sections disclosed that the majority of the cracks were broad, these being typical of corrosion-fatigue fissures. In addition, cracking took the form more typical of a fatigue crack and appeared to have originated at a stress-raiser introduced by a corrosion pit on the surface of the wire. The tendency for corrosion-fatigue cracking or the formation of pits from which fatigue cracks can develop can he reduced, if not prevented, in wire ropes by regular attention to lubrication.

Several compressor diaphragms from five gas turbines cracked after a short time in service. The vanes were constructed of type 403 stainless steel, and welding was performed using type 309L austenitic stainless steel filler metal. The fractures originated in the weld heat-affected zones of inner and outer shrouds. A complete metallurgical analysis was conducted to determine the cause of failure. It was concluded that the diaphragms had failed by fatigue. Analysis suggests that the welds contained high residual stresses and had not been properly stress relieved. Improper welding techniques may have also contributed to the failures. Use of proper welding techniques, including appropriate prewelding and postwelding heat treatments, was recommended.

Two examples concerning fabricated mild steel rotor spiders which failed due to lack of torsional rigidity, probably supplemented by the presence of high internal stress, are described. The machine concerned in the first case was a 3,000 hp three-phase slip-ring motor. In the second case the machine was a 200 kW alternator, direct-driven by a diesel engine running at 750 rpm. Both the foregoing failures reveal the same basic weakness, i.e., insufficient rigidity when subjected to variations or reversals of torque. In the first case, the bars welded to the arms were inadequately supported in a lateral direction, so that excessive stresses of a fluctuating nature were set up in the welds as a result of the frequent load changes that arose in service. This weakness was eliminated when designing the replacement spider. In the second example, failure also arose as a result of deficient torsional rigidity with the consequent development of excessive stresses in the welds at the junctions of the bars with the sleeve, the torque being of a fluctuating character due to the impulses imparted by the engine.

Two diesel engine crankshafts of similar dimensions, the journal diam being approximately 7 in., failed due to cracking originating in the fillet at the junction between the crankpin and the web nearest to the flywheel. The cracks were discovered before rupture occurred. Several small cracks originated in the fillet, ran together and developed as two main crack fronts that ultimately merged into one, a typical example of a fatigue failure. Electromagnetic crack detection revealed the presence of a number of discontinuities which were located at a position that would correspond to the vertical axis of the original ingot. The crankshaft had not been stress-relieved after a welding operation had been carried out. The only satisfactory course to follow when dealing with a highly stressed part in which defects of the type in question are revealed during machining is to scrap the forging.

A crankshaft flange from a marine diesel engine illustrated a less-common case of fretting-fatigue cracking. The crankshaft was from a main engine of a sea-going passenger/vehicle ferry. The afterface of the flange was bolted to the flange of a shaft driving the gearbox. Cracks observed were sharp, transgranular, and not associated with any decarburization or other microstructural anomalies in the steel. Cracking of this main engine crankshaft flange was very likely a consequence of fatigue cracking initiated at fretting damage. The cause of the fretting was from loosening of the bolts.

A hollow, splined alloy steel aircraft shaft (machined from an AMS 6415 steel forging – approximately the same composition as 4340 steel – then quenched and tempered to a hardness of 44.5 to 49 HRC) cracked in service after more than 10,000 h of flight time. The inner surface of the hollow shaft was exposed to hydraulic oil at temperatures of 0 to 80 deg C (30 to 180 deg F). Analysis (visual inspection, 15-30x low magnification examination, 4x light fractograph, chemical analysis, hardness testing) supported the conclusions that the shaft cracked in a region subjected to severe static radial, cyclic torsional, and cyclic bending loads. Cracking originated at corrosion pits on the smoothly finished surface and propagated as multiple small corrosion-fatigue cracks from separate nuclei. The originally noncorrosive environment (hydraulic oil) became corrosive in service because of the introduction of water into the oil. Recommendations included taking additional precautions in operation and maintenance to prevent the use of oil containing any water through filling spouts or air vents. Also, polishing to remove pitting corrosion (but staying within specified dimensional tolerances) was recommended as a standard maintenance procedure for shafts with long service lives.

Two cracks were discovered in a deck plate of an aircraft during overhaul and repair after 659 h of service. The cracks were on opposite sides of the deck plate in the flange joggles. The plate had been formed from 7178-T6 aluminum alloy sheet. Analysis (visual inspection, 0.2x/2x/2.3x electron microscope fractographs, hardness testing, and electrical conductivity testing) supported the conclusions that the failure was caused by fatigue cracks originating on the inside curved surface of the flanges. The cracks had initiated in surface defects caused by either corrosion pitting or forming notches, acting in combination with lateral forces evidenced by the moderate distortion of the fastener holes. Recommendations included eliminating the surface defects by revised cleaning and/or forming procedures. Revised design and installation should also alleviate the lateral forces.

A commercial aircraft wheel half, machined from an aluminum alloy 2014 forging that had been heat treated to the T6 temper, was removed from service because a crack was discovered in the area of the grease-dam radius during a routine inspection. Neither the total number of landings nor the roll mileage was reported, but about 300 days had elapsed between the date of manufacture and the date the wheel was removed from service. The analysis (visual inspection, macrographs, micrographs, electron microprobe) supported the conclusions that the wheel half failed by fatigue. The fatigue crack originated at a material imperfection and progressed in more than one plane because changes in the direction of wheel rotation altered the direction of the applied stresses. Recommendations included rewriting the inspection specifications to require sound forgings.

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.

The master connecting rod of a reciprocating aircraft engine revealed cracks during routine inspection. The rods were forged from 4337 (AMS 6412) steel and heat treated to a specified hardness of 36 to 40 HRC. H-shaped cracks in the wall between the knuckle-pin flanges were revealed by visual examination. The cracks were originated as circumferential cracks and then propagated transversely into the bearing-bore wall. No inclusions in the master rod were detected by magnetic-particle and x-ray inspection. Three large inclusions lying approximately parallel to the grain direction and fatigue beach marks around two of the inclusions were revealed by macroscopic examination of the fracture surface. Large nonmetallic inclusions that consisted of heavy concentrations of aluminum oxide (Al2O3) were revealed by microscopic examination of a section through the fracture origin. The forging vendors were notified about the excess size of the nonmetallic inclusions in the master connecting rods and a nondestructive-testing procedure for detection of large nonmetallic inclusions was established.

The 1040 steel crankshaft in a reciprocating engine cracked within one year of operation. The journals of the main and crankpin bearings were inspected by the magnetic-particle method. Three to six indications of 1.5 to 9.5 mm long discontinuities were observed in at least four of the main-bearing journals. A crack along the fillet, almost entirely through the web, was observed in one of the main-bearing journals. Numerous coarse segregates, identified as sulfide inclusions, were identified by macroetching the surface during metallographic examination of a section taken through the main-bearing journal at the primary crack. Fatigue cracking with low-stress high-cycle characteristics was disclosed during macroscopic examination of the crack surface. Sulfide inclusions, which acted as stress raisers, were found to be present in the region where cracking originated. As a corrective measure, ultrasonic inspection was used in addition to magnetic-particle inspection to detect discontinuities.

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.

High-horsepower electric motors were utilized to drive large compressors (made of 4340 steel shafts and gear-type couplings) required in a manufacturing process. The load was transmitted by two keys 180 deg apart. Six of the eight compressor shafts were found cracked in a keyway and one of them fractured after a few months of operation. Visual examination of fractured shaft revealed that the cracks originated from one of the keyways and propagated circumferentially around the shaft. The shaft and coupling slippage was indicated by the upset keys and this type of fracture. The shaft surface both near and in the keyways indicated fretting which greatly reduced the fatigue limit of the shaft metal and initiated fatigue cracks. Fatigue marks were observed on the fractured key. Repetitive impact loading was responsible for propagation of the cracks. The high cyclic bending stresses were caused by misalignment between the electric motor and compressor and were transmitted to the shaft through the geared coupling. Flexible-disk couplings capable of transmitting the required horsepower were installed on the shafts as a corrective measure.

A system of carbon steel headers, handling superheated water of 188 deg C (370 deg F) at 2 MPa (300 psi) for automobile-tire curing presses, developed a number of leaks within about four months after two to three years of leak-free service. All the leaks were in shielded metal arc butt welds joining 200 mm (8 in.) diam 90 deg elbows and pipe to 200 mm (8 in.) diam welding-neck flanges. A flange-elbow-flange assembly and a flange-pipe assembly that had leaked were removed for examination. Investigation (visual inspection, hardness testing, chemical analysis, magnetic-particle testing, radiographic inspection, and 2% nital etched 1.7x views) showed varying IDs on the assemblies and supported the conclusions that the failures of the butt welds were the result of fatigue cracks caused by cyclic thermal stresses that initiated at stress-concentrating notches at the toes of the interior fillet welds on the surfaces of the flanges. Recommendations included using ultrasonic testing to identify the appropriate joints and then replacing them. Special attention to accuracy of fit-up in the replacement joints was also recommended to achieve smooth, notch-free contours on the interior surfaces.

Three examples of corrosion-fatigue cracking from the toes of substantial fillet welds applied to seal-leaking riveted seams in steam accumulators are described. In the first case, this practice resulted in a disastrous explosion; in the second, which involved two identical vessels at the same location, cracking in course of development was discovered during internal inspection. Microscope examination of several specimens cut to intersect a crack showed it to be typical of corrosion-fatigue; it was in the form of a broad fissure, contained oxide deposits, and the termination was blunt-ended. The two cases not only serve to illustrate the danger of applying fillet welds to seal the lap edges of riveted seams, but point to the inadvisability of employing riveted construction for vessels intended for service under conditions involving frequent pressure and thermal fluctuations, as it is extremely difficult to maintain the tightness of riveted seams under these conditions. Such vessels are now almost exclusively of all-welded construction

Cracks formed on cylinder inserts from a water-cooled locomotive diesel engine, on the water side in the neck between the cylindrical part and the collar. Cracks were revealed by magnetic-particle inspection. As a rule, several parallel cracks had appeared, some of which were very fine. The part played by corrosion in the formation of the cracks was demonstrated with the help of metallographic techniques. The surface regions of the cracks widened into funnel form, which is a result of the corrosive influence of the cooling water. Actual corrosion pits could not be found indicating that the vibrational stresses had a greater share in the damage than the corrosive influence. Cracks appeared initially only in those engines in which no corrosion inhibitor had been added to the cooling water. The cracking was caused by corrosion fatigue. The combined presence of a corrosive medium and cyclical operating stress was needed to cause cracks. No cracks appeared when corrosion inhibitor was added to the cooling water.

Two tubular AISI 1025 steel posts (improved design) in a carrier vehicle failed by cracking at the radius of the flange after five weeks of service. The posts were two of four that supported the chassis of the vehicle high above the wheels. The original design involved a flat flange of low-carbon low-alloy steel that was welded to an AISI 1025 steel tube, and the improved design included placing the welded joint of the flange farther away from the flange fillet. Investigation (visual inspection and chemical analysis) supported the conclusion that the failures in the flanges of improved design were attributed to fatigue cracks initiating at the aluminum oxide inclusions in the flange fillet. Recommendations included retaining the improved design of the flange with the weld approximately 50 mm (2 in.) from the fillet, but changing the metal to a forging of AISI 4140 steel, oil quenched and tempered to a hardness of 241 to 285 HRB. Preheating to 370 deg C (700 deg F) before and during welding with AISI 4130 steel wire was specified. It was also recommended that the weld be subjected to magnetic-particle inspection and then stress relieved at 595 deg C (1100 deg F), followed by final machining.