The A2 tool steel mandrel, part of a rolling tool used for mechanically joining two tubes was fractured after making five rolled joints. A 6.4 mm diam hole was drilled by EDM through the square end of the hardened mandrel due to difficulty was experienced in withdrawing the tool. The fracture progressed into the threaded section and formed a pyramid-shape fragment after it was initiated at approximately 45 deg through the hole in the square end. An irregular zone of untempered martensite with cracks radiating from the surface of the hole (result of melting around hole) was revealed by metallographic examination. A microstructure of fine tempered martensite containing some carbide particles was exhibited by the core material away from the hole. Brittle fracture characteristics with beach marks were exhibited by the fracture surfaces which is characteristic of a torsional fatigue fracture. As a corrective measure, the hole through the square end of the mandrel was incorporated into the design of the tool and was drilled and reamed before heat treatment and specified hardness of the threaded portion and square end of the mandrel was reduced.

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.

The first part of this article focuses on two major forms of machining-related failures, namely machining workpiece (in-process) failures and machined part (in-service) failures. Discussion centers on machining conditions and metallurgical factors contributing to (in-process) workpiece failures, and undesired surface layers and metallurgical factors contributing to (in-service) machined part failures. The second part of the article discusses the effects of microstructure on machining failures and their preventive measures.

Two cases of failure of centrifuge baskets were investigated. The first involved a centrifuge running at approximately 1000 rpm. The basket was constructed from a perforated sheet of stainless steel rolled into a cylinder and joined by a single vee longitudinal weld. Detailed examination showed the weld had not completely penetrated the full depth of the section. The fracture faces showed a gradually progressing fatigue crack developing from a notch, formed by the lack of penetration, at the root of the weld. Microscopic examination of the parent plate showed it was a typical titanium stabilized austenitic steel. It is probable that had the basket been subjected to a periodic inspection by a competent person, this failure would not have occurred. The second case concerned a continuous duty centrifuge operating at 2200 rpm. Fracture had occurred at the circumferential weld attaching the stainless steel skirt to the basket rim and also in the region of the vertical weld which was made when the skirt was formed into a cone. Stress-corrosion cracking of the skirt material, which contained residual stresses due to cold-rolling, had been caused by the presence of sodium chloride.

An investigation was conducted to determine why 16 out of 139 pipe bends cracked during hot induction bending. The pipe conformed to API 5L X65 PSL2 line pipe standards and measured 1016 mm (40 in.) in diam with a wall thickness of 18.5 mm. A metallurgical cross section was removed along a crack on the extrados to document the crack morphology using optical microscopy. In addition to cracking, golden-yellow streaks were visible at the extrados, and the composition was examined using scanning electron microscopy with energy dispersive spectroscopy. Based on the results, investigators concluded the pipe was contaminated with copper at the mill were it was produced.

A hydroextractor installed new for the drying of sugar massecuite consisted of a metal basket fixed to a vertical spindle. Disruption occurred just after the machine had been run up to speed and was not preceded by any abnormal behavior. The basket assembly consisted of a Ni-Cr-Mo steel shell and two end plates. It was designed to spin at 2200 rpm, using centrifugal force to expel liquids through nearly 3000 drilled holes in the shell wall. Investigators found that the shell separated completely from the bottom plate. The top plate, though it cracked radially, remained attached over most of its circumference. The basket also contained a 22-gauge Monel metal liner that had been perforated by stabbing, raising pronounced burrs that faced each hole. Apart from the local spots of corrosion due to the lining, the inner surface of the basket showed little evidence of general corrosion. What caused the basket to fail was the presence of corrosion-fatigue cracks or fissures radiating from the holes. A secondary cause was that the scantlings of the basket were too light.

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.

An intermediate shaft (3 in. diam), part of a camshaft drive on a large diesel engine, broke after two weeks of service. Failure occurred at the end of the taper portion adjacent to the screwed thread. The irregular saw-tooth form of fracture was characteristic of failure from torsional fatigue. A second shaft carried as spare gear was fitted and failure took place in a similar manner in about the same period of time. Examination revealed that the tapered portion of the Fe-0.6C carbon steel shaft had been built up by welding prior to final machining. A detailed check by the engine-builder established that the manufacture of these two shafts had been subcontracted. It was ascertained that the taper portions had been machined to an incorrect angle and then subsequently built-up and remachined to the correct taper. The reduction in fatigue endurance following welding was due to heat-affected zone cracking, residual stresses, the lower fatigue strength of the weld deposited metal, and weld defects.

Two hot water reheat coil valves from a heating/ventilating/air-conditioning system failed in service. The values, a 353 copper alloy 19 mm (3/4 in.) valve and a 360 copper alloy 13 mm (1/2 in.) valve, had been failing at an increasing rate. The failures were confined to the stems and seats. Visual examination revealed severe localized metal loss in the form of deep grooves with smooth and wavy surfaces. Metallographic analysis of the grooved areas revealed uniform metal loss. No evidence of intergranular or selective attack indicating erosion-corrosion was observed, Recommendations included use of a higher-copper brass, cupronickel, or Monel for the valve seats and stems and operation of the valves in either the fully opened or closed position.

An AISI D2 tool steel insert from a forming die used in the manufacture of automotive components failed prematurely during production. Results of various analyses and simulation tests indicated fatigue failure resulting from improper heat treatment. The fatigue fracture originated because of a highly stressed condition produced by a sharp corner combined with low toughness from ineffective tempering. It was recommended that 25 other inserts that belonged to the same die be double tempered.

This article provides an overview of metal additive manufacturing (AM) processes and describes sources of failures in metal AM parts. It focuses on metal AM product failures and potential solutions related to design considerations, metallurgical characteristics, production considerations, and quality assurance. The emphasis is on the design and metallurgical aspects for the two main types of metal AM processes: powder-bed fusion (PBF) and directed-energy deposition (DED). The article also describes the processes involved in binder jet sintering, provides information on the design and fabrication sources of failure, addresses the key factors in production and quality control, and explains failure analysis of AM parts.

A 52000 bearing steel valve guide component operating in the fuel supply system of a transport aircraft broke into two pieces after 26 h of flight. The valve guide fractured through a set of elongated holes that had been electrodischarge machined into the component. Analysis indicated that the part failed by low cycle fatigue. The fracture was brittle in nature and had originated at a severely eroded zone of craters in a hard, deep white layer that was the result of remelting during electrodischarge machining. It was recommended that the remaining parts be inspected using a stereoscopic microscope and/or a borescope.

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.

Two AISI type 316 stainless steel components intended for use in a reducer section for sodium piping in a fast breeder test reactor were found to be severely corroded—the first soon after pickling, and the second after passivation treatments. Metallographic examination revealed that one of the components was in a highly sensitized condition and that the pickling and passivation had resulted in severe intergranular corrosion. The other component was fabricated from thick plate and, after machining, the outer surface represented the transverse section of the original plate. Pickling and passivation resulted in severe pitting because of end-grain effect. Strict control of heat treatment parameters to prevent sensitization and modification of pickling and passivating conditions for machined components were recommended.

This article provides an overview of the effects of various material- and process-related parameters on residual stress, distortion control, cracking, and microstructure/property relationships as they relate to various types of failure. It discusses phase transformations that occur during heat treating and describes the metallurgical sources of stress and distortion during heating and cooling. The article summarizes the effect of materials and the quench-process design on distortion and cracking and details the effect of cooling characteristics on residual stress and distortion. It also provides information on the methods of minimizing distortion and tempering. The article concludes with a discussion on the effect of heat treatment processes on microstructure/property-related failures.

Aluminum alloy BS.1476-HE.15 by virtue of its high strength and low density finds application in the form of bars or sections for cranes, bridges, and other such structures where a reduction in dead weight load and inertia stresses is advantageous. Bars and sections in H.15 alloy are mostly produced by extrusion. Some material processed this way has been prone to exfoliation corrosion. Extended aging for 24 h at a temperature of 185 deg C (365 deg F) virtually suppresses the tendency for exfoliation corrosion to develop. Also, the use of a sprayed coating, either of aluminum or Al-1Zn alloy, was effective in halting and preventing this form of attack. While alarming, the appearance of exfoliation corrosion provides a valuable warning to the engineer or inspector before a severe weakening of the particular sections has occurred.

Proper stress analysis during component design is imperative for accurate life and performance prediction. The total stress on a part is comprised of the applied design stress and any residual stress that may exist due to forming or machining operations. Stress-corrosion cracking may be defined as the spontaneous failure of a metal resulting from the combined effects of a corrosive environment and the effective component of tensile stress acting on the structure. However, because of the orientation dependence in aluminum, it is the residual stress occurring in the most susceptible direction that must be considered of primary importance in material selection for design configuration. A Navy UH-1N helicopter main rotor blade grip manufactured from a 2014-T6 aluminum alloy forging failed because of a design flaw that left a high residual tensile stress along the short transverse plane; this in turn provided the necessary condition for stress corrosion to initiate. A complete failure investigation to ascertain the exact cause of the failure was conducted utilizing stereomicroscopic examination, scanning electron microscopy, metallographic inspection and interpretation, energy-dispersive chemical analysis, physical and mechanical evaluation. Stereomicroscopic examination of the opened crack fracture surface revealed one large fan-shaped region that had propagated radially through the thickness of the material from two distinct origin areas on the internal diam of the grip. Higher magnification inspection near the origin area revealed a flat, wood-like appearance. Scanning electron microscopy divulged the presence of substantial mud cracking and intergranular separation on the fracture surface. Metallographic examination revealed intergranular cracking and substantial leaf separation along the elongated grains parallel to the fracture surface. Chemical composition and hardness requirements were found to be as specified. The blade grip failed due to a stress corrosion crack which initiated on the inner diam and propagated in the short transverse direction through the thickness of the component. The high residual tensile stress in the part resulting from the forging and exposed after machining of the inner diam, combined with the presence of moisture, provided the necessary conditions to facilitate crack initiation and propagation.

A steam-pressurized Yankee dryer shell ruptured during normal operation. Cracking had occurred around much of the circumference at the drive end of the shell, which measured 3.7 m (12 ft) in diameter by 3.4 m (11 ft) long with a head bolted to each end. The crack initiated at a 90 deg corner in contact with the edge of the head. The material was a hardened gray cast iron containing 2.8% Ni and 1.2% Mo. Based on the results of visual, nondestructive, metallographic, and chemical analyses, it was concluded that failure occurred after corrosion fatigue cracking had weakened the shell. An ultrasonic examination of all Yankee dryers of the same type was recommended to look for cracking at the edge of the shell. Modification of the head-to-shell joint was recommended as well.

A mechanical part, which supports the moving part, is termed a mechanical bearing and can be classified into rolling (ball or roller) bearings and sliding bearings. This article discusses the failures of sliding bearings. It first describes the geometry of sliding bearings, next provides an overview of bearing materials, and then presents the various lubrication mechanisms: hydrostatic, hydrodynamic, boundary lubrication, elastohydrodynamic, and squeeze-film lubrication. The article describes the effect of debris and contaminant particles in bearings. The steps involved in failure analysis of sliding bearings are also covered. Finally, the article discusses wear-damage mechanisms from the standpoint of bearing design.