When bulging occurred in mortar tubes made of British I steel during elevated-temperature test firing, a test program was formulated to evaluate the high-temperature properties (at 540 to 650 deg C, or 1000 to 1200 deg F) of the British I steel and of several alternative alloys including a maraging steel (18% Ni, grade 250), a vanadium-modified 4337 gun steel (4337V), H19 tool steel, and high-temperature alloys Rene 41, Inconel 718, and Udimet 630. All the alloys evaluated had been used in mortar tubes previously or were known to meet the estimated minimum yield strength. The alloys fall in this order of decreasing strengths: Udimet 630, Inconel 718, Rene 41, H19 tool steel, British I steel, 4337V gun steel, and maraging steel. When cycled between room temperature and 540 to 650 deg C (1000 to 1200 deg F), only Udimet 630, Inconel 718, and Rene 41 retained yield strengths higher than the minimum. Also, these three alloys maintained high strengths over the tested range, whereas the others decreased in yield strength as cycling progressed. Analysis showed Inconel 718 was considered best suited for 81-mm mortar tubes, and widespread industrial use ensured its availability.

A series of poppet-valve stems fabricated from 17-4 PH (AISI type 630) stainless steel failed prematurely in service during the development of a large combustion assembly. The poppet valves were part of a scavenging system that evacuated the assembly after each combustion cycle. The function of the valve is to open and close a port; thus, the valve is subjected to both impact and tensile loading. Analysis (visual inspection, hardness testing, and stress analysis) supported the conclusions that the valve stems were impact loaded to stresses in excess of their yield strength. That they failed in the threaded portion also suggests a stress-concentration effect. Recommendations included changing the material spec to a higher-strength material with greater impact strength. In this case, it was recommended that the stems, despite any possible design changes, be manufactured from an alloy such as PH 13-8Mo, which can be processed to a yield strength of 1379 MPa (200 ksi), with impact energies of the order of 81 J (60 ft·lbf) at room temperature.

A 10-cm (4-in.) chain link used in operating a large dragline bucket failed after several weeks in service. The link was made of cast low-alloy steel (similar to ASTM A487, class 10Q) that had been normalized, hardened, and tempered to give a yield strength of approximately 1034 MPa (150 ksi). A hydrogen flake approximately 5 cm (2 in.) in diam was observed at the center of the fracture surface. Beach marks indicative of fatigue encircled the hydrogen flake and covered nearly all of the remaining fracture surface. The failure of this linkways caused by an excessive hydrogen content. Two steps were taken to combat this type of failure. First, when service conditions did not require high hardness to combat wear, the links were produced of a steel having a yield strength of about 690 MPa (100 ksi) rather than 1034 M

The fan drive support shaft, specified to be made of cold-drawn 1040 to 1045 steel, fractured after 2240 miles of service. It was revealed by visual examination of the shaft that the fracture had initiated near the fillet at an abrupt change in shaft diameter. The cracks originated at two locations approximately 180 deg apart on the outer surface of the shaft and propagated toward the center. Features typical of reversed-bending fatigue were exhibited by the fracture. A tensile specimen was machined from the center of the shaft and it indicated much lower yield strength (369 MPa) than specified. It was disclosed by metallographic examination that the microstructure was predominantly equiaxed ferrite and pearlite which indicated that the material was in either the hot-worked or normalized condition. An improvement of fatigue strength of the shaft by the development of a quenched-and-tempered microstructure was recommended.

The upper frame from a large cone crusher failed in severe service after an unspecified service duration. The ductile iron casting was identified as grade 80-55-06, signifying minimum properties of 552 MPa (80 ksi) tensile strength, 379 MPa (55 ksi) yield strength, and 6% elongation. Investigation (visual inspection, chemical analysis, unetched 30x images, and 2% nital etched 30x images) was difficult because the fracture surface of the frame section was obliterated by postfracture corrosion. Repeated attempts at cleaning using progressively stronger chemicals revealed that no telltale fracture morphology remained. However, the investigation supported the conclusion that the crusher frame failed via brittle overload fracture, likely due to excessive service stresses and substandard mechanical properties. Recommendations included additional quality-control measures to provide better spheroidal graphite morphology at the frame surface.

A sand-cast LM6M aluminum alloy sprocket drive wheel in an all-terrain vehicle failed. Extensive cracking had occurred around each of the six bolt holes in the wheel. Evidence of considerable deformation in this area was also noted. Examination indicated that the part failed because of gross overload. Use of an alloy with a much higher yield strength and improvement in design were recommended.

The paper describes the findings from a damaged propeller blade made from Mn-Ni-Al-bronze, commercially known as Superston 70 (ABS Type 5). The blade had broken at the 0.65 pitch radius location, and the fracture occurred in a brittle mode. The findings reported here point to two potential contributors to the propeller blade failure, viz., the presence of casting flaws at the low pressure side of the propeller blade and service stresses at this surface that reached approximately 400 MPa. This stress value exceeded the yield strength at the corresponding location of the unbroken blade by approximately 40%.

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.