An AISI 4340 Ni-Cr-Mo alloy steel draw-in bolt and the collet from a vertical-spindle milling machine broke during routine cutting of blind recesses after relatively long service life. Based on fracture surface features, it was suspected that the draw-in bolt was the first to fracture, followed by failure of the collet, which shattered one of its arms when it struck the work table. Scanning electron microscopy showed the presence of hairline crack indications along grain facets on the fracture surface of the bolt. This, coupled with stepwise cracking in the material, generally raised suspicion of hydrogen embrittlement. It appeared that fracture in service progressed transgranularly to produce delayed failure under dynamic loading. The pickling process used to remove heat scale was suspected to be the source of hydrogen on the surface of the bolt. The manufacturer was requested to change its cleaning practice from pickling to grit blasting.

Several deburring drums that fractured were filled with abrasive, water, and small parts, such as roller bearing rollers, and rotated on their axis at 36 rpm. Cracks were discovered very early in the service lives of these high-chromium white iron cast structures. All of the fractures were through bolt holes in the mounting flange. The holes had a sharp edge and exhibited uneven wear on the inside diameter. In operation, the mounting bolts were frequently found to be loose and in at least one case broken off. A 25x scanning electron microscopy (SEM) fractograph from near this fracture-initiation area showed fatigue striations. No casting or metallurgical structural defects were found that could explain the failures. This evidence supports the conclusion that cracking was a result of the stress-concentration site at the bolt holes where a fatigue-initiated fracture occurred. Recommendations included that the radii be increased at the sharp corners and that lock-wiring be used to secure against bolt loosening.

The power-type counterbalance spring, formed from hardened-and-tempered carbon steel strip and subsequently subjected to phosphating treatment, fractured at the two locations during fatigue testing. A rust colored dark band at the inside edge of the fracture surface was disclosed during investigation. Etch pits were revealed by the cleaned surface which were never observed on properly phosphated coating. It was interpreted that the spring had been subjected to an abnormal acid attack in pickling or phosphating which had resulted in considerable absorption of hydrogen by the metal and hence embrittlement. The part was concluded to have cracked during phosphating or excessive acid pickling before phosphating.

A 4340 steel shaft, the driving member of a large rotor subject to cyclic loading and frequent overloads, broke after three weeks of operation. The driving shaft contained a shear groove at which the shaft should break if a sudden high overload occurred, thus preventing damage to an expensive gear mechanism. The rotor was subjected to severe chatter, which was an abnormal condition resulting from a series of continuous small overloads occurring at a frequency of around three per second. Investigation (visual inspection, hardness testing, and hot acid etch images) supported the conclusion that the basic failure mechanism was fracture by torsional fatigue, which started at numerous surface shear cracks, both longitudinal and transverse, that developed in the periphery of the root of the shear groove. These shear cracks resulted from high peak loads caused by chatter. The shear groove in the shaft had performed its function, but at a lower overload level than intended. Recommendations included increasing the fatigue strength of the shaft by shot peening the shear groove to minimize chatter.

Welded steel storage vessels used to hold mildly alkaline solution were produced in exactly the same manner from deep-drawn aluminum-killed SAE 1006 low-carbon steel sheet. After the cylindrical shell was drawn, a top low-carbon steel closure was welded to the inside diameter. The containers were then filled with the slightly alkaline solution, pressurized, and allowed to stand under ambient conditions. A small number, less than 1%, were returned because they began to leak in service. Inspection revealed general corrosion and pitting on the inner surfaces. However, other tanks that experienced the same service conditions developed no corrosion. Corrosion was linked to forming defects that provided sites for localized corrosion, and to lack of steam drying after cleaning, which increased susceptibility to general corrosion.

A steel spring used in an automotive application suddenly began to fail in the field, although “nothing had changed” in the fabrication process. Fatigue tests using springs fabricated prior to field failures lasted 500,000 cycles to failure, whereas fatigue tests performed on springs fabricated after field failures lasted only 50,000 cycles to failure. It was discovered that the percent coverage of shot peening prior and subsequent to the increase in failure incidence was much less than 100%, with a shot peening time of 12 min. The residual-stress state of “as fabricated” springs in three conditions were evaluated using XRD: springs manufactured prior to failure incidence increase, 12 min peen; springs manufactured following failure incidence increase, 12 min peen; and 60 min peen. The conclusion was that the failure occurred because low peening time significantly decreased the compressive residual-stress levels in the springs. Recommendation was made to increase the time the spring was shot peened from 12 to 60 min.

A power plant using two steam generators (vertical U-tube and shell heat exchangers, approximately 21 m (68 ft) high with a steam drum diameter of 6 m (20 ft)) experienced a steam generator tube rupture. Each steam generator contained 11,012 Inconel alloy 600 (nickel-base alloy) tubes measuring 19 mm OD, nominal wall thickness of 1.0 mm (0.042 in.), and average length of 18 m (57.75 ft). The original operating temperature of the reactor coolant was 328 deg C (621 deg F). A tube removal effort was conducted following the tube rupture event. Investigation (visual inspection, SEM fractographs, and micrographs) showed evidence of IGSCC initiating at the OD and IGA under ridgelike deposits that were analyzed and found to be slightly alkaline to very alkaline (caustic) in nature. Crack oxide analysis indicated sulfate levels in excess of expected values. The analysis supported the conclusion that that the deposits formed at locations that experienced steam blanketing or dryout at the higher levels of the steam generators. Recommendations included steam generator water-chemistry controls, chemical cleaning, and reduction of the primary reactor coolant system temperature.

The accident at Three Mile Island Unit No. 2 on 28 March 1979 was the worst nuclear accident in US history. By Jan 1990, it was possible to electrochemically machine coupons from the lower head using a specially designed tool. The specimens contained the ER308L stainless steel cladding and the A533 Grade B plate material to a depth of about mid-wall. The microstructures of these specimens were compared to that of specimens cut from the Midland, Michigan reactor vessel, made from the same grade and thickness but never placed in service. These specimens were subjected to known thermal treatments between 800 and 1100 deg C for periods of 1 to 100 min. Microstructural parameters in the control specimens and in those from TMI-2 were quantified. Selective etchants were used to better discriminate desired microstructural features, particularly in the cladding. This report is a progress report on the quantification of changes in both the degree of carbide precipitation and delta ferrite content and shape in the cladding as a function of temperature and time to refine the estimates of the maximum temperatures experienced.

A large number of electropolished copper parts showed evidence of discoloration (tinting) after electropolishing. Because these parts are used in a high-vacuum application, even trace amounts of organic materials would be problematic. Scanning electron microscopy of nondiscolored and discolored areas both showed trace amounts of residue in the form of adherent deposits. EDS, FTIR spectroscopy, XPS, and secondary ion mass spectroscopy (SIMS) analyses indicated that the discoloration to the copper components was due to the development of CuO at localized regions. It was recommended that process changes be made to completely remove residual processing fluids from the part surfaces before electropolishing. The use of more aggressive detergents was suggested, and it was recommended also that a filtering and recirculating system be considered for use in the cleaning and electropolishing tanks.

Rough operation of the roller bearing mounted in an electric motor/gearbox assembly was observed. The bearing components made of low-alloy steel (4620 or 8620) and the cup, cone and rollers were carburized, hardened and tempered. The contact surfaces of these components (cup, cone and roller) were revealed to be uniformly electrolytically etched by visual examination. The action similar to anodic etching was believed to have occurred as a result of stray currents in the electric motor (not properly grounded) and the presence of an electrolyte (moisture) between the cup and roller surfaces of the bearing. As a remedial action, the bearing was insulated for protection from stray currents by grounding of the motor and the moisture was kept out by sealing both bearings in the assembly.

A valve spring made of 4.1 mm diam wire, designed to withstand 10,000,000 stress cycles, fractured after only 2,000,000 cycles. The surface displayed impressions which indicated it had been treated by shot blasting. The spring has broken in two places. Fracture 1 was a torsional fatigue fracture which has started from a lobe-like surface defect and not, as is usual, from a point on the most highly stressed inner surface. Fracture 2, on the other hand, was a bending fatigue fracture with a starting point on both the inner and the outer surface of the spiral. The objective of the shot blasting, to put the surface into a state of even compressive internal stress, which must first be overcome during subsequent bending and torsional loading before the boundary zone comes under tensile stress, was therefore not realized in this case. On the contrary, the shot blasting led to a state of internal stress which favored fracture of the spring.

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.

In the course of a general overhaul, the crankpins and main journals (3 in. diam) of the crankshaft of a four-cylinder oil engine were built up by metal spraying. Four weeks later, the shaft broke through the pin remote from the flywheel (driving) end. The fracture was of the fatigue type. A creeping crack originated in the fillet at the inside surface of the pin and extended parallel to the plane of the web across practically the entire section before complete rupture occurred. The sprayed metal on the fractured pin had very poor adhesion. The surfaces of the main journals had not been grooved but appeared to have been roughened by shot or grit-blasting prior to spraying and the deposit was more firmly adherent to these surfaces than in the case of the pins. It is doubtful, however, whether the adhesion of sprayed metal to a surface prepared even in this manner would always be satisfactory under severe loading conditions, such as those to which a crankpin is subjected in service.

A 321 stainless steel radar coolant-system assembly fabricated by torch brazing with AWS type 3A flux, failed at the brazed joint when subjected to mild handling before installation, after being stored for about two years. It was revealed by visual examination of the failed braze that the filler metal had not covered all mating surfaces. Lack of a metallurgical bond between the brazing alloy and stainless steel and instead mechanical bonding of the filler metal to an oxide layer on the stainless steel surface was revealed by examination of the broken joint at the cup. It was indicated by the thickness of the oxide layer that the steel surface was not protected from oxidation by the flux during torch heating. It was concluded that the failure was caused by lack of a metallurgical bond between the brazing alloy and the stainless steel. Components made of 347 stainless steel (better brazeability) brazed with a larger torch tip (wider heat distribution) and AWS type 3B flux (better filler-metal flow) were recommended for radar coolant-system assembly.

Nodular cast iron crankshafts and their main-bearing inserts were causing premature failures in engines within the first 1600 km (1000 mi) of operation. The failures were indicated by internal noise, operation at low pressure, and total seizing. Concurrent with the incidence of engine field failures was a manufacturing problem: the inability to maintain a similar microfinish on the cope and drag sides of a cast main-bearing journal. Investigation supported the conclusion that the root cause of the failure was carbon flotation due to the crankshafts involved in the failures showing a higher-than-normal carbon content and/or carbon equivalent. Larger and more numerous cope side graphite nodules broke open, causing ferrite caps or burrs. They then became the mechanism of failure by breaking down the oil film and eroding the beating material. A byproduct was heat, which assisted the failure. Recommendations included establishing closer control of chemical composition and foundry casting practices to alleviate the carbon-flotation form of segregation. Additionally, some nonmetallurgical practices in journal-finishing techniques were suggested to ensure optimal surface finish.

Randomly selected dictating-machine drive mechanisms, which contained small ball bearings, were found to exhibit unacceptable fluctuations in drive output during the early stages of production. It was indicated that the bearing raceways were being true brinelled before or during installation of the bearings. The preinstallation practices and the procedures for installing the bearings were carefully studied. It was revealed that during one preinstallation step, the lubricant applied by the bearing manufacturer was removed and the bearing was relubricated with another type of lubricant prior to which the bearings were ultrasonically cleaned in trichloroethylene to ensure extreme cleanness. Equally spaced indentations resembling true brinelling were revealed by careful examination of the bearing raceways. It was concluded that the ultrasonic energy transmitted to the balls brinelled the raceways enough to cause fluctuations in machine output. Solvent-vapor cleaning was employed as a corrective technique for removing bearing lubricant.

Several diesel-engine rocker levers (malleable iron similar to ASTM A 602, grade M7002) failed at low hours in overspeed, over-fuel, highly loaded developmental engine tests. Identical rocker levers had performed acceptably in normal engine tests. The rocker levers were failing through the radius of an adjusting screw arm. The typical fracture face exhibited two distinct modes of crack propagation: the upper portion indicated overload at final fracture, whereas the majority of the fracture suggested a fatigue fracture. Investigation (visual inspection, 1.5x/30x/60x magnification, and nital etched 300x magnification) supported the conclusion that the rocker levers failed in fatigue, with casting defects, or spiking, acting as stress raisers to initiate failures in highly loaded engine tests. Recommendations included shot peening of the levers as an interim measure to reduce the possibility of failure and redesign to increase the cross-sectional area of the levers.

Flow forming technology has emerged as a promising, economical metal forming technology due to its ability to provide high strength, high precision, thin walled tubes with excellent surface finish. This paper presents experimental observations of defects developed during flow forming of high strength SAE 4130 steel tubes. The major defects observed are fish scaling, premature burst, diametral growth, microcracks, and macrocracks. This paper analyzes the defects and arrives at the causative factors contributing to the various failure modes.

Presence of transverse marks which were remnant of grinding was indicated in a failed valve spring made from ground rod. The shot-peening pattern was light at this location. A transverse crack was found to grow from one such mark under the influence of local stress fields until it was reoriented to the plane normal to the major tensile axis by sufficient loading. The shot-peening procedure was altered to create adequate surface compression at all stressed points on the springs.

A type 17-4PH stainless steel tube exhibited brown discoloration after a pickling operation. EDS analysis of the extracted substance revealed relatively high levels of iron and chromium, along with lower amounts of aluminum, silicon, sulfur, chlorine, calcium, manganese, and nickel. The iron, chromium, and nickel are likely in the form of dissolution products from the pickling solution. FTIR analysis revealed the presence of polypropylene and poly(ethylene:propylene). The EDS results showed that the discoloration of the tube was associated with oxidation products of the tube material, as well as adherent organic residue. Analysis by FTIR of the residue revealed detectable levels of two polymeric substances, which were later determined to be construction materials of the pickling tank. It was recommended that more frequent cleaning and/or replacement of the pickling solution be put into place and another type of tank material be considered.