An impact breaker bar showed signs of rapid wear. The nominal composition of this chromium alloy cast iron was Fe-2.75C-0.75Mn-0.5Si-0.5Ni-19.5Cr-1.1Mo. The measured hardness of this bar was 450 to 500 HRB. The desired hardness for this material after air hardening is 600 to 650 HRB. The microstructure consisted of eutectic chromium carbides (Cr7C3) in a matrix of retained austenite and martensite intermingled with secondary carbides. Analysis (visual inspection and 500x view of sections etched with Marble's reagent) supported the conclusion that the low hardness resulted from an excessive amount of retained austenite. This caused reduced wear resistance and thus rapid wear in service. Recommendations included avoiding an excessive austenitizing temperature and excessive cooling rates from the austenitizing temperature and controlling the chemical composition to avoid excessive hardenability for the section size involved.

A recuperator used for preheating the combustion air for a rolling mill furnace failed after a relatively short service time because of leakage of the pipes in the colder part. The 6 % chrome steel pipes used for the warmer part connected by means of welding with austenitic electrodes to the unalloyed mild steel pipe of larger diam. Visual inspection showed corrosion and deep, trench-like erosion over the entire circumference of the seam on the side of the thicker mild steel pipe. Examination using the V2-A solution for picral etch showed the microstructure of the unalloyed pipe had become coarse-grained and acicular, and the microstructure of the welding seam had become predominantly martensitic as a result of the mixing of the weld metal with the fused pipe material. The chrome steel pipe had become partially transformed to martensite or bainite at the transition to the weld. Thus, the failure occurred due to typical contact corrosion wherein the alloyed welding seam represented the less noble electrode. The martensitic structure may have contributed to the failure as well. Due to the typical nature of the failure, no recommendations were made.

A 44.5 mm (1.75 in.) diam type 321 stainless steel seamless tube in a power-generating turbine failed after 19,000 h in service. The tube was used to carry a mixture of approximately 25% steam and 75% hot air. Three fractured pieces and part of the tube containing the mating fracture surface were examined. Both fractographic and metallographic features revealed that the failure was by thermal fatigue caused by the presence of biaxial thermal stresses on the inner surface of the tube. It was recommended that the steam and air be thoroughly mixed prior to entering the tube to decrease the temperature fluctuations of the inner surface.

A 1-in. diam pump spindle fractured within the length covered by the boss of the impeller which was attached to the spindle by means of an axial screw. The pump had been in use in a chemical plant handling mixtures of organic liquids and dilute sulfuric acid having a pH value of 2 to 4 at temperatures of 80 to 90 deg C (176 to 194 deg F). The fracture was unusual in that it was of a fibrous nature, the fibers-which were orientated radially-were readily detachable. The surface of the spindle adjacent to the fracture had an etched appearance and the mode of cracking in this region suggested that failure resulted from an intergranular attack. Subsequent microscope examination confirmed the generally intergranular mode of failure. A macro-etched section near the fracture revealed a radial arrangement of columnar crystals, indicating that the spindle was a cast and not a wrought product as had been presumed. Spectroscope examination showed this particular composition (Fe-23Cr-18Ni-1.8Mo-1.2Si) did not conform to a standard specification and is apparently a proprietary alloy. It was evident that the particular mode of failure was related to the inherent structure of the material.

Occasional failures were experienced in spool-type valves used in a hydraulic system. When a valve would fail, the close-fitting rotary valve would seize, causing loss of flow control of the hydraulic oil. The rotating spool in the valve was made of 8620 steel and was gas carburized. The cylinder in which the spool fitted was made of 1117 steel, also gas carburized. Investigation (visual inspection, low magnification images, 400x images, metallographic exam, and hardness testing) supported the conclusion that momentary sliding contact between the spool and the cylinder wall caused unstable retained austenite in the failed cylinder to transform to martensite. The increase in volume resulted in sufficient size distortion to cause interference between the cylinder and the spool, seizing, and loss of flow control. The failed parts had been carburized in a process in which the carbon potential was too high, which resulted in a microstructure having excessive retained austenite after heat treatment. Recommendations included modifying the composition of the carburizing atmosphere to yield carburized parts that did not retain significant amounts of austenite when they were heat treated.

The outer tube, or stem, on a bicycle frame fractured after two years of use. Detailed investigation revealed that the lower stem bearing had been loose for some time and the bottom bearing cup contained many cracks. Metallographic examination of the chromium-plated cup confirmed the brittle nature of the cracks, located along prior austenite boundaries. The failure was attributed to hydrogen embrittlement due to improper manufacturing procedures following chromium plating. The cracking led to looseness in the bearing and consequent scoring, cracking, and overloading of the stem.

Valve seats fractured during testing and during service. The seats were machined from grade 11L17 steel and were surface hardened by carburization. Investigation (visual inspection, hardness testing, 59x SEM images, and 2% nital etched 15x cross sections) supported the conclusion that the fracture occurred via brittle overload, which was predominantly intergranular. The amount of bending evidence and the directionality of the core overload fracture features suggest that the applied stresses were not purely axial, as would be anticipated in this application. The level of retained austenite in the hardened case layer likely contributed to the failure.

Chain link, a part of a mechanism for transferring hot or cold steel blooms into and out of a reheating furnace, broke after approximately four months of service. The link was cast from 2% Cr austenitic manganese steel and was subjected to repeated heating to temperatures of 455 to 595 deg C (850 to 1100 deg F). Examination included visual inspection, macrograph of a nital-etched specimen from an as-received chain link 1.85x, micrographs of a nital-etched specimen from an as-received chain link 100x/600x, normal microstructure of as-cast standard austenitic manganese steel 100x, micrograph of a nital-etched specimen that had been austenitized 20 min at 1095 deg C (2000 deg F) and air cooled 315x, and micrograph of the same specimen after annealing 68 h at 480 deg C (900 deg F) 1000x). Investigation supported the conclusions that the chain link failed in a brittle manner, because the austenitic manganese steel from which it was cast became embrittled after being reheated in the temperature range of 455 to 595 deg C (850 to 1100 deg F) for prolonged periods of time. The alloy was not suitable for this application, because of its metallurgical instability under service conditions.

Stainless steel pipe (273-mm OD x 8-mm wall thickness) used in the fabrication of large manifolds developed crack-like decohesions during a routine inductive bending procedure. The imperfections, which were found near the outside diameter, were around 3 mm in length oriented in the circumferential direction and penetrated nearly 2 mm into the pipe wall. The pipes were made of titanium-stabilized austenitic stainless steel X6CrNiMoTi17-12-2. Six hypotheses were considered during the investigation, which ultimately concluded that the failure was caused by liquation cracking due to overheating.