During the operation of tractors with cantilevered body, the lateral wall of the hypoeutectic cast iron cylinder blocks cracked repeatedly. Three of the blocks were examined. The grain structure of the thick-walled part consisted of uniformly distributed graphite of medium flake size in a basic mass of pearlite with little ferrite. But the thin-walled part showed a structure of dendrites of precipitated primary solid solution grains with pearlitic-ferritic structure and a residual liquid phase with granular graphite in the ferritic matrix. The structure was formed by undercooling of the residual melt. In this case, it was promoted by fast cooling of the thin wall and had comparatively low strength. The fracture formation in the cylinder blocks was ascribed primarily to casting stresses. They could be alleviated by better filleting of the transition cross sections. The fracture was promoted by the formation of undercooled microstructure of low strength in the thin-walled part. Similar damage appeared in a cylinder head, in which case, the cracks were promoted by a supercooled structure.

A tool used to stretch reinforcement wires in prestressed concrete failed. All eight individual jaws were broken. Visual examination of the fracture surfaces indicated that about half of the broken parts had a partially dendritic appearance. Further, fracture surfaces near the exteriors of the parts were clean and smooth, and there was evidence of a case. Examination of the flat surfaces of the parts revealed surface cracking where actual failure had not occurred. Chemical analysis showed the material to be a low-alloy carburizing steel. The microstructure was compatible with a steel which is cast, carburized, quenched, and tempered. The structure was generally satisfactory, except for the presence of severe shrinkage porosity. It was concluded that the presence of shrinkage porosity in critical areas was the primary cause of fracture. Extremely high hardness indicating a lack of adequate tempering was the secondary cause.

The specially designed sand-cast low-alloy steel jaws that were implemented to stretch the wire used in prestressed concrete beams fractured. The fractures were found to be macroscale brittle and exhibited very little evidence of deformation. The surface of the jaws was disclosed by metallographic examination to be case carburized. The case was found to be martensite with small spheroidal carbides while the core consisted of martensite plus some ferrite. The fracture was revealed to be related to shrinkage porosity. Tempering was revealed to be probably limited to about 150 deg C by the hardness values (close to the maximum hardness values attainable) for the core. It was interpreted that the low tempering temperature used may have contributed to the brittleness. The procedures used for casting the jaws were recommended to be revised to eliminate the internal shrinkage porosity. Tempering at a slightly higher temperature to reduce surface and core hardness was recommended.

The swivel head of a driving spindle of a four-high mill fractured. The fracture originated in a darkly stained spot on the bottom of the cylindrical part and then continued into the cylinder walls in the two directions. The fracture topography was of dendritic structure at the stained spot. This led to the conclusion that a shrinkage cavity was present. Metallographic examination confirmed that the fracture of the swivel head was caused or favored by a cavity.

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.

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.

Near-surface porosity in zinc die castings that were subsequently plated with copper, nickel, and bright chromium was causing blemishes in the plating. Identifying die casting turbulence and hot spots were keys to process modifications that subsequently allowed porosity to be greatly minimized.

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.

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.

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.

A cast steel bracket manufactured in accordance with ASTM A 148 grade 135/125 steel failed in railroad maintenance service. Ancillary property requirements included a 285 to 331 HB hardness range and minimum impact energy of 27 J (20 ft·lbf) at -40 deg C (-40 deg F). The conditions at the time of failure were characterized as relatively cold. Investigation (visual inspection, chemical analysis, and unetched 119x and 2% nital etched 119x SEM images) supported the conclusion that the bracket failed through brittle overload fracture due to a number of synergistic factors. The quenched-and-tempered microstructure contained solidification shrinkage, inherently poor ductility, and type II Mn-S inclusions that are known to reduce ductility. The macro and microscale fracture features confirmed that the casting was likely in low-temperature service at the time of failure. The composition and mechanical properties of the casting did not satisfy the design requirements. Recommendations included exerting better composition control, primarily with regard to melting, deoxidation, and nitrogen control. Better deoxidation practice was recommended to generate the more desirable Mn-S inclusion morphology, and reevaluation of the casting design was suggested to minimize shrinkage.

Two outer valve springs made from air-melted 6150 pretempered steel wire broke during production engine testing. The springs were 50 mm in OD and 64 mm in free length, had five coils and squared-and-ground ends, and were made of 5.5 mm diam wire. It was revealed that fracture was nucleated by an apparent longitudinal subsurface defect. The defect was revealed by microscopic examination to be a large pocket of nonmetallic inclusions (alumina and silicate particles) at the origin of the fracture. Partial decarburization of the steel was observed at the periphery of the pocket of inclusions. Torsional fracture was indicated by the presence of beach marks at a 45 deg angle to the wire axis. It was established that the spring fractured by fatigue nucleated at the subsurface defect.

Small paddles used to mix pulp had experienced a high incidence of breakage through the shafts. In some of the shanks, shrinkage was found relatively close to the surface where threads had been cut all the length of the shaft. Chemistries were within normal CF-8M ranges. Metallography showed the parts to be correctly heat treated. Cross sections of several of the parts showed pitting corrosion, and beneath the pits, stress-corrosion cracks in areas where the shafts had been bent during use. All the samples showed deep SCC in the areas where bending had occurred. In several cases, centerline shrinkage from inadequate risering had decreased life by reducing the cross-sectional area. Type CF-8M is not resistant to chloride SCC where the chloride concentration is considerable. The biggest problem was the bending of these parts. Deformed material with high residual stresses would always be susceptible to SCC. Redesign to lower stresses was essential. In addition, change to a high-strength duplex stainless steel with its higher strength and greater resistance to chlorides was recommended. Finally, the part must be adequately risered to produce solid shanks free from shrinkage.

A sand-cast steel eye connector used to link together two 54,430 kg capacity floating-bridge pontoons failed prematurely in service. The pontoons were coupled by upper and lower eye and clevis connectors that were pinned together. The eye connector was found to be cast from low-alloy steel conforming to ASTM A 148, grade 150-125. The crack was found to have originated along the lower surface initially penetrating a region of shrinkage porosity. It was observed that cracking then propagated in tension through sound metal and terminated in a shear lip at the top of the eye. The fracture of the eye connector was concluded to have occurred by tensile overload because of shrinkage porosity. Sound metal was ensured by radiographic examination of subsequent castings.

A polyoxymethylene gear wheel that had been in operation in a boiler room failed. Investigation (visual inspection and 305x images) supported the conclusion that failure was due to postcrystallization causing considerable shrinkage. Breakdown along the crystalline superstructure started mainly at the mechanically stressed tooth flanks. In addition, oil vapors, humidity, and other degradative agents could also have contributed to the observed failure. No recommendations were made.

A failed crosshead of an industrial compressor was examined using optical and SEM. The crosshead was an ASTM A148 grade 105-85 steel casting. On the basis of the observations reported and available background information, it was concluded that the failure began with the initiation of cracks at slag inclusions and sharp fillets in weld-repair areas in the casting. The weld-repair procedures were unsatisfactory. The cracks propagated in a fatigue mode. he casting quality was judged unacceptable because of the presence of excessive shrinkage porosity. It was recommended that crosshead castings be properly inspected before machining. Revision of foundry practice to reduce or eliminate porosity was also recommended.

A chemical storage vessel failed while in service. The failure occurred as cracking through the vessel wall, resulting in leakage of the fluid. The tank had been molded from a high-density polyethylene (HDPE) resin. The material held within the vessel was an aromatic hydrocarbon-based solvent. Investigation (visual inspection, stereomicroscopic examination, 20x/100x SEM images, micro-FTIR in the ATR mode, and analysis using DSC and TGA) supported the conclusion that the chemical storage vessel failed via a creep mechanism associated with the exertion of relatively low stresses. The source of the stress was thought to be molded-in residual stresses associated with uneven shrinkage. This was suggested by obvious distortion evident on cutting the vessel. Relatively high specific gravity and the elevated heat of fusion indicated that the material had a high level of crystallinity. In general, increased levels of crystallinity result in higher levels of molded-in stress and the corresponding warpage. The significant reduction in the modulus of the HDPE material, which accompanied the saturation of the resin with the aromatic hydrocarbon-based solvent, substantially decreased the creep resistance of the material and accelerated the failure.

This article describes the general root causes of failure associated with wrought metals and metalworking. This includes a brief review of the discontinuities or imperfections that may be the common sources of failure-inducing defects in bulk working of wrought products. The article discusses the types of imperfections that can be traced to the original ingot product. These include chemical segregation; ingot pipe, porosity, and centerline shrinkage; high hydrogen content; nonmetallic inclusions; unmelted electrodes and shelf; and cracks, laminations, seams, pits, blisters, and scabs. The article provides a discussion on the imperfections found in steel forgings. The problems encountered in sheet metal forming are also discussed. The article concludes with information on the causes of failure in cold formed parts.

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.

An investigation was conducted to determine the cause of numerous cracks and other defects on the surface of a cast ASTM A743 grade CA-15 stainless steel main boiler feed pump impeller. The surface was examined using a stereomicroscope, and macrofractography was conducted on several cross sections removed from the impeller body. Areas that appeared to have the most severe surface damage were sectioned, fractured open, and examined using SEM. The chemistry of the impeller and an apparent repair weld were also analyzed. The examination indicated that the cracks were shrinkage voids from the original casting process. Surface repair welds had been used to fill in or cover over larger shrinkage cavities. It was recommended that more stringent visual and nondestructive examination criteria be established for the castings.