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.

Work rolls made of indefinite chill double-poured (ICDP) iron are commonly used in the finishing trains of hot-strip mills (HSMs). In actual service, spalling, apart from other surface degeneration modes, constitutes a major mechanism of premature roll failures. Although spalling can be a culmination of roll material quality and/or mill abuse, the microstructure of a broken roll can often unveil intrinsic inadequacies in roll material quality that possibly accentuate failure. This is particularly relevant in circumstances when rolls, despite operation under similar mill environment, exhibit variations in roll life. The paper provides an insight into the microstructural characteristics of spalled ICED HSM work rolls, which underwent failure under similar mill operating environment in an integrated steel plant under the Steel Authority of India Limited. Microstructural features influencing ICDP roll quality, viz. characteristics of graphite, carbides, martensite, etc., have been extensively studied through optical microscopy, quantitative image analysis (QIA), and electron-probe microanalysis (EPMA). These are discussed in the context of spalling propensity and roll life.

A steel bolt had been used to join the copper connecting strips between the poles of a 10-pole, series-connected, rotating field rotor of a synchronous motor. The exciting current was 155 amps. Failure of the bolt resulted in severe damage to the stator windings by the loosened ends of the strips. The bolt had fractured near the head, a location which probably coincided with the junction of the strips. A portion of the fracture surface was covered with copper that had been deposited in the molten state, while some was also present along the shank of the bolt, having apparently run in between the bolt and the hole in the strip. The bolt end adjacent to the fracture had been subjected to intense local heating. The extent of the grain-growth indicating that the temperature had been in the region of 1200 deg C (2192 deg F). When the temperature reached the melting-point of copper, 1083 deg C (1981 deg F), molten metal came into contact with the bolt, into which it penetrated along the grain boundaries, culminating in rupture.

Heating elements, consisting of strips, 40 mm x 2 mm, of the widely used 80Ni-20Cr resistance heating alloy, and designed to withstand a temperature of 1175 deg C, were rendered unusable by scaling after a few months service in a through-type annealing furnace, Although the temperature supposedly did not exceed 1050 deg C. Structural observations indicated a special case of internal oxidation. The required conditions for this were apparently provided by the moist hydrogen atmosphere of the annealing furnace, in which the chromium was oxidized, while the oxides of iron and nickel were reduced. Even the carbon suffered incomplete combustion and was enriched in the core. Thus, no protective layer could form or be maintained. The intergranular advancement of the oxidation may have been favored by the precipitation of chromium-rich carbides on the austenite grain boundaries. This form of internal oxidation is, in the case of Ni-Cr alloys, known as green rot. Alloys containing iron should be more resistant. As a preventive measure it was recommended to reduce the operating temperature of the strip sufficiently to allow the use of Fe-Ni-Cr alloys.

On 31 Jan 2000, a McDonnell Douglas MD-83 airplane crashed off the California coast while en route from Puerto Vallarta, Mexico, to San Francisco. Approximately 90% of the aircraft was recovered from a depth of about 700 ft. (213 m). Among the recovered components were parts of the jackscrew assembly, including the jackscrew with an internal torque tube, the gimbal nut, and the upper and lower mechanical stops. The jackscrew was connected to the horizontal stabilizer and controlled its movement. Multiple damage features, indicative of contact with another object, were observed on the upper surface of the lower mechanical stop. Damage to the spline teeth was also observed on the lower mechanical stop. The stripping pattern and offset circumferential marks were consistent with the lower stop being at two or more skewed angles to the splines of the jackscrew during stripping. This investigation is continuing.