A 17-4 PH steering actuator rod end body broke during normal take-off. Results of failure analysis revealed that the wall thickness of the race was much below the design limits, thus causing the race to rest on the body's swaged edges rather than on the load carrying centerline of the body. This assembly condition generated abnormal high loads on the swaged edges, ultimately resulting in fatigue failure. To prevent a recurrence of similar failure in the future, the dimensions of the race in the spherical bearing were changed, no further failure occurred.

Pipes made of low-carbon Thomas steel had been welded longitudinally employing the carbon-arc process with bare electrode wire made for argon-shielded arc welding. Difficulties were encountered during the cutting of threads because of the presence of hard spots. Microstructural examination showed welding conditions were such that a carburizing atmosphere developed, which led to an increase in carbon content and hardening at certain locations such as terminal bells and lap joints. This explained the processing difficulties during the threading operation.

This article briefly reviews the various metallurgical or environmental factors that cause a weakening of the grain boundaries and, in turn, influence the occurrence of intergranular (IG) fractures. It discusses the mechanisms of IG fractures, including the dimpled IG fracture, the IG brittle fracture, and the IG fatigue fracture. The article describes some typical embrittlement mechanisms that cause the IG fracture of steels.

This article briefly reviews the factors that influence the occurrence of intergranular (IG) fractures. Because the appearance of IG fractures is often very similar, the principal focus is placed on the various metallurgical or environmental factors that cause grain boundaries to become the preferred path of crack growth. The article describes in more detail some typical mechanisms that cause IG fracture. It discusses the causes and effects of IG brittle cracking, dimpled IG fracture, IG fatigue, hydrogen embrittlement, and IG stress-corrosion cracking. The article presents a case history on IG fracture of steam generator tubes, where a lowering of the operating temperature was proposed to reduce failures.

Decarburization of steel may occur as skin decarburization by gases either wet or containing oxygen, and as a deep ongoing destruction of the material by hydrogen under high pressure. Guidelines are given for recognizing decarburization and determining at what point cracks occurred. How decarburization changes workpiece properties and the case of hydrogen decarburization are addressed through examples.

In this article, we report the outcome of an investigation made to uncover the premature fracture of crusher jaws produced in a local foundry. A crusher jaw that had failed while in service was studied through metallographic techniques to determine the cause of the failure. Our investigation revealed that the reason for the fracture was the presence of large carbides at the grain boundaries and in the grain matrix. This led to the formation of microcracks that propagated along the grain boundaries under in-service working forces. It is also believed that the precipitation of carbides at the grain boundaries may have occurred because of improper heat treatment, but not because of a deficiency in composition.

This article introduces some of the general sources of heat treating problems with particular emphasis on problems caused by the actual heat treating process and the significant thermal and transformation stresses within a heat treated part. It addresses the design and material factors that cause a part to fail during heat treatment. The article discusses the problems associated with heating and furnaces, quenching media, quenching stresses, hardenability, tempering, carburizing, carbonitriding, and nitriding as well as potential stainless steel problems and problems associated with nonferrous heat treatments. The processes involved in cold working of certain ferrous and nonferrous alloys are also covered.

Inner rings of spherical roller bearings out of full hardening ball bearing steel 100 CrMn 6 (Fe-1C-1.5Cr-1.1Mn, Material No. 1.3520) failed in service. Due to the cracks, parts from the middle flange broke or the rings failed in radial direction completely. All the cracks and fracture originated from the middle flange. In all of the three rings one flank showed heavy wearing and scouring. The cracks started from the edge of this flank with the cylindrical mantle surface of the middle flange. The cracking resembled fatigue cracking. However, in a fine-grained hardened steel such as this, fracture faces due to stress-cracking and overload fracture look the same. Metallographic examination showed the failure of the rings was a result of repeated heating and rapid cooling of the surface due to the grinding of the bearings on one flank of the middle flange. The stress-cracks (grindcracks) spread in steps which finally led to the breaking off of parts from the middle flange and complete failure of the rings.

A gear manufacturer experienced service problems with various gears and pinions that had worn prematurely or had fractured. All gears and pinions were forged from 1.60Mn-5Cr steel and were case hardened by pack carburizing. Gear Failure: One of the gears showed severe wear on the side of the teeth that came into contact with the opposing gear during engagement. The microstructure at the periphery of a worn tooth at its unworn side consisted of coarse acicular martensite with a large percentage of retained austenite. Pinion Failure: The teeth of the pinion exhibited severe spalling; the microstructure at the surface consisted of coarse acicular martensite with retained austenite. Also, a coarse network of precipitated carbide particles showed that the carburization of the case had appreciably exceeded the most favorable carbon content. This evidence supported the following conclusions: 1) High wear rate on the gears was caused by spalling of the coarse-grain surface layer. The underlying cause of the wear was overheating during the carburization. 2) Pinion failure resulted from overheating combined with excessive case carbon content. Thus, no recommendations were made.

A corrosion resistant chromium nickel steel (X 2 Cr-Ni-Mo 18 10) worm drive used in a chemical plant at 80 deg C and 100 to 200 atm pressure to transport media containing chloride failed during normal operation. Visual inspections showed that the entire surface of the gear was covered with fine branching cracks and was flaking off. Microscopic examination showed that the unetched polished material had disintegrated to an average depth of 1 mm below the surface. A micrograph of the etched surface revealed numerous deformation lines and transgranular cracking. The failure was thus due to stress-corrosion cracking and additional corrosion due to ventilation elements. Because austenitic chromium nickel steels are prone to stress-corrosion cracking, particularly in the presence of chlorine compounds at high temperatures, and because austenitic rust- and acid-resistant steels are prone to smearing and work hardening during machining, it was recommended that these types of steels be machined only with sharp, short tools mounted in rigid structures. In addition, residual stresses should be eliminated by post-process annealing in a protective atmosphere.

This article aims to identify and illustrate the types of overload failures, which are categorized as failures due to insufficient material strength and underdesign, failures due to stress concentration and material defects, and failures due to material alteration. It describes the general aspects of fracture modes and mechanisms. The article briefly reviews some mechanistic aspects of ductile and brittle crack propagation, including discussion on mixed-mode cracking. Factors associated with overload failures are discussed, and, where appropriate, preventive steps for reducing the likelihood of overload fractures are included. The article focuses primarily on the contribution of embrittlement to overload failure. The embrittling phenomena are described and differentiated by their causes, effects, and remedial methods, so that failure characteristics can be directly compared during practical failure investigation. The article describes the effects of mechanical loading on a part in service and provides information on laboratory fracture examination.

Overload failures refer to the ductile or brittle fracture of a material when stresses exceed the load-bearing capacity of a material. This article reviews some mechanistic aspects of ductile and brittle crack propagation, including a discussion on mixed-mode cracking, which may also occur when an overload failure is caused by a combination of ductile and brittle cracking mechanisms. It describes the general aspects of fracture modes and mechanisms. The article discusses some of the material, mechanical, and environmental factors that may be involved in determining the root cause of an overload failure. It also presents examples of thermally and environmentally induced embrittlement effects that can alter the overload fracture behavior of metals.

Several compressor disks in military fighter and trainer aircraft gas turbine engines cracked prematurely in the bolt hole regions. The disks were made of precipitation-hardened AM355 martensitic stainless steel. Experimental and analytical work was performed on specimens from the fifth-stage compressor disk (judged to be the most crack-prone disk in the compressor) to determine the cause of the failures. Failure was attributed to high-strain low-cycle fatigue during service. It was also determined that the cyclic engine usage assumed in the original life calculations had been under estimated, which led to low-cycle fatigue cracking earlier than expected. Fracture mechanics analysis of the disks was carried out to assess their damage tolerance and to predict safe inspection intervals.

A precipitation-hardened stainless steel poppet valve assembly used to shut off the flow of hydrazine fuel to an auxiliary power unit was found to leak. SEM and optical micrographs revealed that the final heat treatment designed for the AM-350 bellows material rendered the AM-355 poppet susceptible to intergranular corrosive attack (IGA) from a decontaminant containing hydroxy-acetic acid. This attack provided pathways for which fluid could leak across the sealing surface in the closed condition. It was concluded that the current design is flight worthy if the poppet valve assembly passes a preflight helium pressure test. However a future design should use the same material for the poppet and bellows so that the final heat treatment will produce an assembly not susceptible to IGA.

The failures of two aircraft components, one from a landing gear and the other from an ejector rack mechanism, were investigated. Both were made from PH 13-8 Mo (UNS S13800) precipitation-hardening stainless steel which had been heat treated to the H1000 and H950 tempers respectively and then chromium plated. The parts were characterized metallographically and mechanically and were found to be compliant. Detailed fractographic examination revealed that the first stage of both failures was similar: subsurface initiation of numerous cracks with a wide range of orientations and cleavage like features. The cracking was followed by fatigue in one case and catastrophic failure in the other. Hydrogen embrittlement was identified as the most likely mechanism of failure.

Two types of chromium-plated hydraulic cylinders failed by cracking on their outer surfaces. In one case, the parts had a history of cracking in the nominally unstressed, as-fabricated condition. In another, cracks were detected after the cylinders were subjected to a pressure impulse test. Both part types were made of 15-5 PH (UNS S15500) precipitation hardening stainless steel. Hydrogen embrittlement cracking was the likely cause of failure for both part types. Cracking of the as-fabricated parts was ultimately prevented by changing the manufacturing procedure to allow for a reheat treatment. For parts that cracked after pressure testing, excessive dimensional changes precluded the inclusion of a reheat treatment as a manufacturing step, and further failure was averted by carefully employing proper machining practices, avoiding abusive machining.

There was a large incidence of surface defects on the crank pins and journals and other areas of crank shafts of a high power automotive engine. The steel used was a Cr-Mo type of nitriding steel. Metallographic observations conclusively proved that the defective areas were entrapment of foreign bodies, resulting from steel making/deoxidizing/teeming stages. The occasionally globular nature of the foreign particles suggested these were formed at the liquid condition of the steel. The ratio of Mn-Si as seen on electron probe microanalysis also suggested the globules high in Mn content might have resulted in deoxidizing stage. Particularly the absence of Fe in some areas in the inclusion was indicative of precipitation deoxidation by ferromanganese/ferrosilicon. The defects apparently did not have time to coalesce and rise up to the top.

A process vessel heating coil, consisting of several 3 ft diam turns, was supplied with steam at 400 psi and a temperature of 343 deg C (650 deg F). At bi-weekly intervals well water was introduced to effect rapid cooling of the contents. After about eight months, leakage developed from a circumferential crack on the underside of the uppermost turn. Shorter cracks were found at a similar location on the bottom turn, and further leakage occurred at pinhole perforations adjacent to the crack in the top turn and near to a butt-weld in the coil. Microscopic examination revealed that the cracks were predominantly of the intergranular variety. In addition, transgranular cracks were present. Material was an austenitic stainless steel of the type specified but the absence of columbium and titanium in significant amounts showed that it was not stabilized against intergranular carbide precipitation. The transgranular cracks indicated that failure was due partly to stress-corrosion. It was concluded that the chlorides provided the main corrodent for both the stress and intercrystalline-corrosion cracking.

Alloy 430 stainless steel tube-to-header welds failed in a heat recovery steam generator (HRSG) within one year of commissioning. The HRSG was in a combined cycle, gas-fired, combustion turbine electric power plant. Alloy 430, a 17% Cr ferritic stainless steel, was selected because of its resistance to chloride and sulfuric acid dewpoint corrosion under conditions potentially present in the HRSG low-pressure feedwater economizer. Intergranular corrosion and cracking were found in the weld metal and heat-affected zones. The hardness in these regions was up to 35 HRC, and the weld had received a postweld heat treatment (PWHT). Metallographic examination revealed that the corroded areas contained undertempered martensite. Fully tempered weld areas with a hardness of 93 HRB were not attacked. No evidence of corrosion fatigue was found. Uneven temperature control during PWHT was the most likely cause of failure.

A cold-formed Grade TP 304 stainless steel swaged region of a reheater tube in service for about 8000 hours cracked because of sulfur-induced stress-corrosion cracking (SCC). Cracking initiated from the external surface and a high sulfur content was detected in the outer diameter and crack deposits. Comparison of the microstructure and hardness of the swaged region and unswaged Grade TP 304 stainless steel tube metal indicated that the swaged section was not annealed to reduce the effects of cold working. The high hardness created during swaging increased the stainless steel's susceptibility to sulfur-induced SCC. Because SCC requires water to be present, cracking most likely occurred during downtime or startups. To prevent future failures, the boiler should be kept dry during downtime to avoid formation of sulfur acids, and the swaged sections of the tubes should be heat treated after swaging to reduce or eliminate strain hardening of the metal.