Threads of a bone screw (Co-Cr-Mo alloy, type ASTM F75) had broken off, and other threads had cracked. 15x sectioning showed porosity, and 155x magnification showed gas holes, segregation, and dissolved oxides. This supports the conclusion that manufacturing defects caused the failure.

A cast housing, part of a multi-shaft yoking mechanism, failed during assembly and installation of the equipment in which it was to be used. The housing, or yoke body, was cast from AISI 420 grade ferritic stainless steel. Analysis revealed that the failure was caused by the presence of shrinkage cavities, which lowered the load-bearing capability. The failure occurred at the location where there was an abrupt change in the section thickness. A redesign to provide a smooth contour at the section junction was recommended along with optimization of casting parameters to avoid shrinkage cavities.

An engine cylinder head failed after operating just 3.2 km (2 mi) because of coolant leakage through the exhaust port. Visual examination of the exhaust ports revealed a casting defect on the No. 7 exhaust-port wall. A 0.9x examination of an unpolished, unetched longitudinal section through the defect indicated shrinkage porosity. This defect was found to interconnect the water jacket and the exhaust gas flow chamber. No cracks were found by magnetic-particle inspection. The gray iron cylinder head had a hardness of 229 HRB on the surface of the bottom deck. The microstructure consisted of type A size 4 flake graphite in a matrix of pearlite with small amounts of ferrite. this evidence supported the conclusion that the cylinder-head failure resulted from the presence of a casting defect (shrinkage) on the No. 7 cylinder exhaust-port wall interconnecting the water jacket with the exhaust-gas flow chamber. No recommendations were made.

Several back up rolls of 1400 mm barrel diam from a broad strip mill broke after a relatively short operating time as a result of bending stresses when the rolls were dismantled. The fracture occurred in the conical region of the neck at about 600 mm diam. The rolls were shaped steel castings with 0.8 to 1.0% C, 1% Mn, 1% Cr, 0.5% Mo and 0.4% Ni and were heat treated to a tensile strength of 950 N/sq mm. Because the bending stress on mounting was only 42 N/sq mm in the fracture cross section, it was evident at the outset that material defects had promoted the fracture. In the case of this roll and the other broken rolls, the cracking and fracture were promoted by various casting defects. Investigation of the rolls showed that both the breaking off of the neck and the disintegration of the barrel edges was caused by material defects, more exactly casting defects. The fractures on the other rolls examined were so badly rusted or contaminated that they were incapable of yielding any information.

Cluster bomb tailcone assemblies each containing two aluminum die-cast components were rejected because of the poor surface condition of the die castings. Numerous heat checks were found on the surfaces of the tailcones and radiographic inspection revealed inclusions, gas holes, and shrinkage defects in the castings. Most of the components failed to meet required mechanical properties because of these casting defects.

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.

A crack was discovered in a cast steel (ASTM A 356, grade 6) steam turbine casing during normal overhaul of the turbine. The mechanical properties of the casting all exceeded the requirements of the specification. When the fracture surface was examined visually, an internal-porosity defect was observed adjoining a tapped hole. A second, much larger cavity was also detected. Investigation (visual inspection and 7500x SEM fractographs) supported the conclusions that failure occurred through a zone of structural weakness that was caused by internal casting defects and a tapped hole. The combination of cyclic loading (thermal fatigue), an aggressive service environment (steam), and internal defects resulted in gradual crack propagation, which was, at times, intergranular-with or without corrosive attack-and, at other times, was transgranular.

A paper drier head manufactured from gray cast iron was removed from service as a result of NDE detection of crack-like surface discontinuities. This component was subjected to internal steam pressure to provide heat energy for drying. Investigation (visual inspection, chemical analysis, mechanical testing, as-polished 54x magnification, etched with nital 33x/54x/215x/230x magnification) supported the conclusions that the NDE indications were the consequence of a cold-shut condition in the casting. The cold shut served as a stress-concentration site and was therefore a potential source of crack initiation. The combination of low material strength and a casting defect was a potential source of unexpected fracture during service, because the component was under pressure from steam. Recommendations included removing other dryer heads exhibiting similar discontinuities and/or material quality from service.

Post-service destructive evaluation was performed on two commercially pure zirconium pump impellers. One impeller failed after short service in an aqueous hydrochloric acid environment. Its exposed surfaces are bright and shiny, covered with pockmarks, and peppered with pitting. Uniform corrosion is evident and two deep linear defects are present on impeller blade tips. In contrast, the undamaged impeller surfaces are covered with a dark oxide film. This and many other impellers in seemingly identical service conditions survive long lives with little or no apparent damage. No material or manufacturing defects were found to explain the different service performance of the two impellers. Microstructure, microhardness and material chemistry are consistent with the specified material. Examination reveals the damage mechanism to be corrosion-enhanced cavitation erosion, the most severe form of erosion corrosion. Cavitation damage to the protective oxide film caused the zirconium to lose its normally outstanding corrosion resistance. The root cause of the impeller failure is most likely the introduction of excessive air into the pump due to low liquid level, a bad seal or inadequate head. Corrosion pitting, crevice corrosion, and solidification cracks (casting defect) also contributed to the failure.

In the EMD-2 Joint Directed Attack Munition (JDAM), the A357 aluminum alloy housing had been redesigned and cast via permanent mold casting, but did not meet the design strength requirements of the previous design. Mechanical tests on thick and thin sections of the forward housing assembly revealed tensile properties well below the allowable design values. Radiology and CT evaluations revealed no casting defects. Optical microscopy revealed porosity uniformly distributed throughout the casting on the order of 0.1 mm pore diam. Scanning electron microscopy revealed elongated pores, which indicated turbulent filling of the mold. Spherical pores would have indicated the melt had been improperly degassed. Based on these findings, it was recommended that the manufacturer analyze and redesign the gating system to eliminate the turbulent flow problem during the permanent mold casting process.

Two investment-cast A356 aluminum alloy actuators used for handles on passenger doors of commercial aircraft fractured during torquing at less than the design load. Visual examination showed that cracking had occurred through a machined side hole. Fractography revealed that the cracks originated in hot tear locations in the castings. Microprobe analysis of fracture surfaces in the hot tear region indicated a much higher silicon-to-aluminum ratio compared with the overload fracture area. No microstructural anomalies related to the failure were found during metallographic examination. It was concluded that the strength of the castings had been compromised by the presence of the casting defects. Modification of the gating system for casting was recommended to eliminate the hot tear zone. It was also suggested that the balance of the castings from the same manufacturing lot be radiographically inspected.

This article focuses on the general root causes of failure attributed to the casting process, casting material, and design with examples. The casting processes discussed include gravity die casting, pressure die casting, semisolid casting, squeeze casting, and centrifugal casting. Cast iron, gray cast iron, malleable irons, ductile iron, low-alloy steel castings, austenitic steels, corrosion-resistant castings, and cast aluminum alloys are the materials discussed. The article describes the general types of discontinuities or imperfections for traditional casting with sand molds. It presents the international classification of common casting defects in a tabular form.

A gray iron cylinder head cracked after approximately 16,000 km of service. The head was cracked on the rocker arm pan rail next to the No. 3 intake port and extended into the water jacket on the rocker-arm side of the head. Microporosity was revealed in the crack in the sections taken from the water jacket next to the plug and the area next to the No. 3 intake port. A wave of microporosity travels midway between the inner and outer surfaces of the casting was observed and was concluded to have caused the cracking. The reasons and remedies for shrinkage porosity were discussed. Controlled pouring temperatures, improved design and use of chills were recommended to avoid the casting defects.

Failure analysis was performed on a fractured impeller from a boiler feed pump of a fossil fuel power plant. The impeller was a 12% Cr martensitic stainless steel casting. The failure occurred near the outside diameter of the shroud in the vicinity of a section change at the shroud/vane junction. Sections cut from the impeller were examined visually and by SEM fractography. Microstructural, chemical, and surface analyses and surface hardness tests were conducted on the impeller segments. The results indicated that the impeller failed in fatigue with casting defects increasing stress and initiating fracture. In addition, the composition and hardness of the impeller did not meet specifications. Revision of the casting process and institution of quality assurance methods were recommended.

An automatic press for making burlap bags had been used for several years. The press failed after being shipped by truck for a distance of about 400 mi. The objective was to determine whether failure occurred during or before shipment. The large piece which broke off the press included a section of the ways and a heavy adjustable mechanism which normally rides on these ways. The weight of the broken section was estimated at several hundred pounds. There was no support for the broken piece beyond the point of breakage. The material was a commercial cast iron, and the largest proportion of the fracture area was fresh and bright. It was concluded that this was a fresh fracture which occurred during shipment, and the crack itself was not present prior to shipment. The fact that a material defect of some sort was present and probably determined the location of the crack was apparently not significant as far as its usage was concerned. The failure could have been avoided by providing support underneath the overhanging member.

Examples of metallic inclusions in steels of various types are presented. The structure of an inclusion in an annealed Fe-1C-1.5Cr steel consisted of ferrite with lamellar pearlite. The carbon content of the inclusion was therefore considerably lower than that of the chromium steel and was adapted to the latter by diffusion only at the periphery of the inclusion. In another section of a hardened piece of the same chromium steel, the steel in this case had a structure of martensite with hypereutectic carbide, while the inclusions consisted of a very fine laminated eutectoid of the lower pearlite range (Troostite). In a pipe of 18-8 austenitic stainless steel a weakly magnetizable spot of limited size was found. This inclusion too was probably more alloy-deficient than the austenitic steel, similar to the ones described above. All three cases were casting defects.

Gears in a strip mining dragline failed in service. The material was identified as a low-alloy (NiCrMoV) steel. SEM analysis indicated that the initial fracture and subsequent fractures resulted from impact or a suddenly applied load. Mechanical testing indicated that the gears had low impact strength. Failure was attributed to low toughness caused by the absence of, or improper, heat treatment. Casting defects identified during metallographic examination were determined to be the fracture initiation site, but were considered less significant than the low as-received impact strength of the material. It was recommended that the equipment manufacturer implement an appropriate heat treatment to meet the impact requirements of the application.