Component slippage in the left-side final drive train of a tracked military vehicle was detected after the vehicle had been driven 13,700 km (8500 miles) in combined highway and rough-terrain service. The slipping was traced to the mating surfaces of the final drive gear and the adjacent splined coupling sleeve. Specifications included that the gear and coupling be made from 4140 steel bar oil quenched and tempered to a hardness of 265 to 290 HB (equivalent to 27 to 31 HRC) and that the finish-machined parts be single-stage gas nitrided to produce a total case depth of 0.5 mm (0.020 in.) and a minimum surface hardness equivalent to 58 HRC. Investigation (visual inspection, low-magnification images, 500X images of polished sections etched in 2% nital, spectrographic analysis, and hardness testing) supported the conclusion that the failure occurred by crushing, or cracking, of the case as a result of several factors. Recommendations included reducing the high local stresses at the pitch line to an acceptable level with a design modification. Also suggested was specification of a core hardness of 35 to 40 HRC to provide adequate support for the case and to permit attainment of the specified surface hardness of 58 HRC.

Two fuel injection pump gears that were nitrided in a cyanide bath were submitted by the engine manufacturer for examination of hardness distribution and failure analysis. The gears showed signs of wear after only comparatively brief operation. They were made of normalized unalloyed steel C 45 (Material No. 1.0503) according to DIN 17200 and were normalized. Gear 1 with 1905 h of operation showed at one side pittings on both flanks of the teeth as well as incipient fractures. Gear 2 with 1713 h of operation also showed at one side incipient fractures of the nitride layers at the outer part of the teeth. The nitride layer did not stand up to the high and one-sided compressive stress applied in this case and could not prevent pitting. It could even have accelerated the wear by the incipient break down. Gas nitriding at greater depth under application of a suitable special steel or case hardening would have been better under these circumstances.

Distortion failure occurs when a structure or component is deformed so that it can no longer support the load it was intended to carry. Every structure has a load limit beyond which it is considered unsafe or unreliable. Estimation of load limits is an important aspect of design and is commonly computed by classical design or limit analysis. This article discusses the common aspects of failure by distortion with suitable examples. Analysis of a distortion failure often must be thorough and rigorous to determine the root cause of failure and to specify proper corrective action. The article summarizes the general process of distortion failure analysis. It also discusses three types of distortion failures that provide useful insights into the problems of analyzing unusual mechanisms of distortion. These include elastic distortion, ratcheting, and inelastic cyclic buckling.

Distortion often is observed in the analysis of other types of failures, and consideration of the distortion can be an important part of the analysis. This article first considers that true distortion occurs when it was unexpected and in which the distortion is associated with a functional failure. Then, a more general consideration of distortion in failure analysis is introduced. Several common aspects of failure by distortion are discussed and suitable examples of distortion failures are presented for illustration. The article provides information on methods to compute load limits, errors in the specification of the material, and faulty process and their corrective measures to meet specifications. It discusses the general process of material failure analysis and special types of distortion and deformation failure.

The self-powered flux detectors used in some nuclear reactors are Pt or V-cored co-axial cables with MgO as an insulator and Inconel 600 as the outer sheath material. The detectors are designed to operate in a He atmosphere; to maximize the conduction of heat (generated from the interaction with gamma radiation) and to prevent corrosion. A number of failures have occurred over the years because of a loss of the He cover gas in the assembly. This has resulted in either acid attack on the Inconel 600 sheath in a wet environment or gaseous corrosion in a dry environment. In the latter case, nitriding and embrittlement occurred at temperatures as low as 300 to 400 deg C (determined from an examination of the oxidation of the Zircaloy-2 carrier rod on which the detectors were mounted). Recent results are described and discussed in terms of the oxidation and nitriding kinetics of Zircaloy-2 and Inconel 600, respectively.

A failure analysis was conducted in late 1996 on two rolls that had been used in the production of iron and steel powder. The rolls had elongated over their length such that the roll trunnions had impacted with the furnace wall refractory. The result was distortion and bowing of the roll bodies which necessitated their removal from service. The initial analysis found large quantities of nitrogen had been absorbed by the roll shell. Further research indicated nitrogen pickup accounted for 3% volumetric growth for every 1% by weight nitrogen absorption. This expansion was sufficient to account for the dimensional change observed in the failed rolls. This paper details the failure analysis and resulting research it inspired. It also provides recommendations for cast material choice in highly nitriding atmospheres.

A nozzle in a wastewater vaporizer began leaking after approximately three years of service with acetic and formic acid wastewaters at 105 deg C (225 deg F) and 414 kPa (60 psig). The shell of the vessel was weld fabricated from 6.4 mm (0.25 in.) E-Brite stainless steel plate and measured 1.5 m (58 in.) in diameter and 8.5 m (28 ft) in length. Investigation (visual inspection, chemical analysis, radiography, dye-penetrant inspection, and hydrostatic testing of all E-Brite welds, 4x images, 100x/200x images electrolytically etched with 10% oxalic acid, and V-notch Charpy testing) supported the conclusion that failure of the nozzle weld was the result of intergranular corrosion caused by the pickup of interstitial elements and subsequent precipitation of chromium carbides and nitrides. Carbon pickup was believed to have been caused by inadequate joint cleaning prior to welding. The increase in the weld nitrogen level was a direct result of inadequate argon gas shielding of the molten weld puddle. Two areas of inadequate shielding were identified: improper gas flow rate for a 19 mm (0.75 in.) diam gas lens nozzle, and contamination of the manifold gas system. Recommendations included changes in the cleaning and welding process.

High-temperature corrosion can occur in numerous environments and is affected by various parameters such as temperature, alloy and protective coating compositions, stress, time, and gas composition. This article discusses the primary mechanisms of high-temperature corrosion, namely oxidation, carburization, metal dusting, nitridation, carbonitridation, sulfidation, and chloridation. Several other potential degradation processes, namely hot corrosion, hydrogen interactions, molten salts, aging, molten sand, erosion-corrosion, and environmental cracking, are discussed under boiler tube failures, molten salts for energy storage, and degradation and failures in gas turbines. The article describes the effects of environment on aero gas turbine engines and provides an overview of aging, diffusion, and interdiffusion phenomena. It also discusses the processes involved in high-temperature coatings that improve performance of superalloy.

The first-stage blades in a model 501D5 gas turbine had 16 cooling holes. After 32,000 h of service, the blades exhibited cracking at the cooling holes. The blade material was wrought Udimet 520 alloy, with nominal composition of 57Ni-19Cr-12Co-6Mo-1W-2Al-3Ti-0.05C-0.005B. The cooling holes' surface was not coated. Investigation supported the conclusions that the cracking at the cooling holes was due to grain-boundary oxidation and nitridation at the cooling hole surface, embrittlement and loss of local ductility of the base alloy, temperature gradient from the airfoil surface to the cooling holes, which led to relatively high thermal stresses at the holes located at the thicker sections of the airfoil, and stress concentration of 2.5 at the cooling hole and the presence of relatively high total strain (an inelastic strain of 1.2%) at the cooling hole surface. Recommendations include applying the specially designed methods given in this case study to estimate the metal temperature and stresses in order to predict the life of turbine blades under similar operating conditions.

A pinion gear made of AMS 6470 steel, nitrided all over, lost internal splined teeth due to wear. Spline failure of the power turbine gear caused an engine overspeed and disintegration. Excessive spline wear resulted from a new coupling being mated during overhaul with a worn gear spline. Wear on the spline teeth flanks of the coupling was attributed to severe wear on the mating gear (internal) spline teeth. The assigned cause was an inadequate maintenance procedure which resulted in a wear-damaged component being retained in the power train during engine overhaul. To prevent reoccurrence, specific inspection criteria were issued defining maximum limits for spline wear. A procedure and requirements were specified for installing the coupling and pinion gear at the next overhaul.

An investigation of a damaged crankshaft from a horizontal, six-cylinder, in-line diesel engine of a public bus was conducted after several failure cases were reported by the bus company. All crankshafts were made from forged and nitrided steel. Each crankshaft was sent for grinding, after a life of approximately 300,000 km of service, as requested by the engine manufacturer. After grinding and assembling in the engine, some crankshafts lasted barely 15,000 km before serious fractures took place. Few other crankshafts demonstrated higher lives. Several vital components were damaged as a result of crankshaft failures. It was then decided to send the crankshafts for laboratory investigation to determine the cause of failure. The depth of the nitrided layer near fracture locations in the crankshaft, particularly at the fillet region where cracks were initiated, was determined by scanning electron microscope (SEM) equipped with electron-dispersive X-ray analysis (EDAX). Microhardness gradient through the nitrided layer close to fracture, surface hardness, and macrohardness at the journals were all measured. Fractographic analysis indicated that fatigue was the dominant mechanism of failure of the crankshaft. The partial absence of the nitrided layer in the fillet region, due to over-grinding, caused a decrease in the fatigue strength which, in turn, led to crack initiation and propagation, and eventually premature fracture. Signs of crankshaft misalignment during installation were also suspected as a possible cause of failure. In order to prevent fillet fatigue failure, final grinding should be done carefully and the grinding amount must be controlled to avoid substantial removal of the nitrided layer. Crankshaft alignment during assembly and proper bearing selection should be done carefully.

Breech bolt assemblies from the Gatling guns used on fighter aircraft failed during firing tests. Metallography of the failed components revealed considerable decarburization which resulted in a loss of surface hardness. It was also determined that the maraging steel components were not in the nitrided condition as was required. This resulted in lower wear and fatigue resistance. These components also had a silicon content nearly double of that specified. The high silicon content lowered the notch tensile strength and toughness of the components.

Rolling-contact fatigue (RCF) is a surface damage process due to the repeated application of stresses when the surfaces of two bodies roll on each other. This article briefly describes the various surface cracks caused by manufacturing processing faults or blunt impact loads on ceramic balls surfaces. It discusses the propagation of fatigue cracks involved in rolling contacts. The characteristics of various types of RCF test machines are summarized. The article concludes with a discussion on the various failure modes of silicon nitride in rolling contact. These include the spalling fatigue failure, the delamination failure, and the rolling-contact wear.

An oil scavenge pump was found to have failed when a protective shear neck fractured during the start of a jet engine. Visual inspection revealed that the driven gear in one of the bearing compartments was frozen as was the corresponding drive gear. Spacer wear and thermal discoloration (particularly on the driven gear) were also observed. The gears were made from 32Cr-Mo-V13 steel, hardened and nitrided to 750 to 950 HV. Micrographic inspection of the gear teeth revealed microstructural changes that, in context, appear to be the result of friction heating. The spacers consist of Cu alloy (AMS4845) bushings force fit into AA2024-T3 Al alloy spacing elements. It was found that uncontrolled fit interference between the two components had led to Cu alloy overstress. Thermal cycling under operating conditions yielded the material. The dilation was directed inward to the shaft, however, because the bushing had only a few microns of clearance. The effect caused the oil to squeeze out, resulting in metal-to-metal contact, and ultimately failure.

A lever (machined from a casting made of AISI type 410 stainless steel, then surface hardened by nitriding) that was a component of the main fuel-control linkage of an aircraft engine fractured in flight after a service life of less than 50 h. Investigation (radiographic inspection) supported the conclusions that the lever broke at a cold shut extending through approximately 95% of the cross section. The normally applied load constituted an overload of the remainder of the lever. Recommendations included adding magnetic-particle inspection to the inspection procedures for this cast lever.

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.

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.