Search Results for Grain structure

During a routine inspection, cracks were discovered in several aluminum alloy (similar to either 2014 or 2017) coupling nuts on the fuel lines of a missile. The fuel lines had been exposed to a marine atmosphere for six months while the missile stood on an outdoor test stand near the seacoast. A complete check was then made, both visually and with the aid of a low-power magnifying glass, of all coupling nuts of this type on the missile. Investigation (visual inspection, spectrographic and chemical analysis, and metallographic examination) supported the conclusion that the cracking of the aluminum alloy coupling nuts was caused by stress corrosion. Contributing factors included use of a material that is susceptible to this type of failure, sustained tensile stressing in the presence of a marine (chloride-bearing) atmosphere, and an elongated grain structure transverse to the direction of stress. The elongated grain structure transverse to the direction of stress was a consequence of following the generally used procedure of machining this type of nut from bar stock. Recommendations included changing the materials specification for new coupling nuts for this application to permit use of only aluminum alloys 6061-T6 and T651 and 2024-T6, T62, and T851.

The lower receiver of the M16 rifle is an anodized forging of aluminum alloy 7075-T6. Degradation of the receivers was observed after three years of service in a hot, humid atmosphere. The affected areas were those in frequent contact with the user's hands. There was no question that the material failed as a result of exfoliation corrosion, so an investigation was undertaken, centered around the study of thermal treatments that would increase the exfoliation resistance and still develop the required 448 MPa (65 ksi) yield strength. The results of the study concluded that rolled bar stock should be preferred to extruded bar stock. Differences in grain structure of the forgings, as induced by differences in thermal-mechanical history of the forged material, can have a significant effect on susceptibility to exfoliation corrosion. Regarding thermal treatment, the results show conclusively that large changes in strength and exfoliation characteristics of 7075 forgings can be induced by changes in temperature or time of thermal treatment. With regard to the effect of quenching rate on exfoliation characteristics, a cold-water quench below 25 deg C (75 deg F) would appear to be far superior to an elevated-temperature quench to minimize exfoliation for 7075 forgings in the T6 temper.

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.

In 1979, during a routine bridge inspection, a fatigue crack was discovered in the top flange plate of one tie girder in a tied arch bridge crossing the Mississippi River. Metallographic analysis indicated a banding or segregation problem in the middle of the plate, where the carbon content was twice what it should have been. Based on this and results of ultrasonic testing, which revealed that the banding occurred in 24-ft lengths, it was decided to close the bridge and replace the defective steel. The steel used in the construction of this bridge was specified as ASTM A441, commonly used in structural applications. Testing showed an increase in hardness and weight percent carbon and manganese in the banded region. Further testing revealed that the area containing the segregation and coarse grain structure had a lower than expected toughness and a transition temperature 90 deg F higher than specified by the ASTM standards. The fatigue crack growth rate through this area was much faster than expected. All of these property changes resulted from increased carbon levels, higher yield strength, and larger than normal grain size.

A throttle arm of an aircraft engine fractured and caused loss of engine control. The broken part consisted of a 6.4-mm (1/4-in.) diam medium-carbon steel rod with a thread to fit a knurled brass nut that was inserted into the throttle knob. The threaded rod had been welded to the throttle-linkage bar by an assembly-weld deposit made on the rod adjacent to the threaded portion. The fracture surface exhibited a coarse-grain brittle texture with an initiating crack at a thread root. The throttle-arm failed by brittle fracture because of the presence of cracks at the thread roots that were within the HAZ of the adjacent weld deposit. The heat of welding had generated a coarse-grain structure with a weak grain-boundary network of ferrite that had not been corrected by postweld heat treatment. The combination of the cracks and this unfavorable microstructure provided a weakened condition that resulted in catastrophic, brittle fracture under normal applied loads. The design was altered to eliminate the weld adjacent to the threaded portion of the rod.

Following a freight train derailment, part of a fractured side frame was retained for study because a portion of its fracture surface exhibited a rock candy appearance and black scale. It was suspected of having failed, thereby precipitating the derailment. Metallography, scanning electron microscopy, EDXA, and x-ray mapping were used to study the steel in the vicinity of this part of the fracture surface. It was found to be contaminated with copper. Debye-Scherrer x-ray diffraction patterns obtained from the scale showed that it consisted of magnetite and hematite. It was concluded that some copper was accidentally left in the mold when the casting was poured. Liquid copper, carrying with it oxygen in solution, penetrated the austenite grain boundaries as the steel cooled. The oxygen reacted with the steel producing a network of scale outlining the austenite grain structure. When the casting fractured as a result of the derailment, the fracture followed the scale in the contaminated region thus creating the “rock candy” fracture.

During periodic inspection of the tubes of a reformer furnace, a soapy water leak test with the tubes pressurized with nitrogen was being carried out by site personnel in a manner contrary to the policy of the organization when one of the tubes suddenly disintegrated with explosive violence. The tube approximately 30 ft. long by 6 in. diam, was constructed of three spun cast sections butt welded together. The material specified for tubes for this service was basically a 25% chromium, 20% nickel, cast stainless steel containing 0.4% carbon to optimize creep resistance. Failure initiated in the region of the tube where the dark fracture surface and columnar grain structure were evident. These features indicated the presence of a defective zone or progressive cracking which had occurred during service. Microscopic examination of sections through the zone revealed extensive creep cracking. The cracking was intergranular and followed the interdendritic columnar structure adjacent to the outer surface.