This article provides a discussion on the metallographic techniques used for failure analysis, and on fracture examination in materials, with illustrations. It discusses various metallographic specimen preparation techniques, namely, sectioning, mounting, grinding, polishing, and electrolytic polishing. The article also describes the microstructure examination of various materials, with emphasis on failure analysis, and concludes with information on the examination of replicas with light microscopy.

The cause of the fatigue failure in the retaining ring of the compressor region of an aero-engine turbine was found to be the presence of a high concentration of nonmetallic inclusions. The results of chemical analysis were used to estimate the phases present. The most frequently observed inclusions were spinel solid solutions of the type MO middot; N2O3, where M = Fe, Mn, or Mg and N = Cr or Al. The detrimental inclusions were corundum, calcium aluminates, cristobalite, and silicates. The most detrimental phases were traced on the surfaces of the specimens fractured using impact loading; the comparison is being made with the polished surfaces and the tensile specimen fracture surfaces. The inclusions in the failed retaining ring were compared with the ones in a similar component obtained from a used engine. In the case of the latter, a large number of fine and elongated (Mn, Cr, Fe)S inclusions were present along with spinels. The nondeformable, rigid oxide particles are considered more undesirable than the sulfides as far as fatigue life of the component is concerned. It has been reported that the presence of sulfides may eliminate the stresses due to oxides.

Some 99.90 pure tin tubes (0.15 mm thick) used for packaging a chemical compound cracked on bending and underwent brittle fracture prior to filling, while others remained ductile and showed no sign of failure. Examination showed that specimens prepared by mechanical methods such as electrolytic and hand polishing and the vibration method resulted in poor edge and crack edge definition due to material thickness. Etching experiments involved a grain surface attack and hence produced a rather strong surface relief from which the grain boundary cracks could again not clearly be differentiated. The sections were therefore examined unetched in polarized light. The microstructure of the cracked tubes was shown to have much smaller grains than the ductile and showed cracks from the surface down along the grain boundaries. Material hardness also differed between the unusable tubes and the ductile, and chemical analysis showed a higher level of aluminum in the brittle specimens. Failure obviously occurred due to the high material aluminum content that increased hardness which then caused embrittlement at the surface which led to cracks or fracture on bending. Since no explanation of how the aluminum entered the tin was available, no recommendations could be made.

Metallographic examination is one of the most important procedures used by metallurgists in failure analysis. Typically, the light microscope (LM) is used to assess the nature of the material microstructure and its influence on the failure mechanism. Microstructural examination can be performed with the scanning electron microscope (SEM) over the same magnification range as the LM, but examination with the latter is more efficient. This article describes the major operations in the preparation of metallographic specimens, namely sectioning, mounting, grinding, polishing, and etching. The influence of microstructures on the failure of a material is discussed and examples of such work are given to illustrate the value of light microscopy. In addition, information on heat-treatment-related failures, fabrication-/machining-related failures, and service failures is provided, with examples created using light microscopy.

Fracture of a cadmium-plated accumulator ring forged from 4140 steel was discovered during inspection and disassembly of a hydraulic-accumulator system stored at a depot. The ring had broken into five small and two large segments. The small segments of the broken ring displayed very flat fracture surfaces with no apparent yielding, but the two large segments did show evidence of bending (yielding) near the fractures. In addition, some segments contained fine radial cracks. Analysis (visual inspection, optical microscopy on polished-and-etched specimens, hardness testing, and chemical analysis) supported the conclusion that the failure was caused due to brittle fatigue, as evidenced by the intergranular nature of the fracture path. Also, hydrogen penetration occurred during the plating operation and was not relieved subsequently as required.

A water tube boiler with two headers and 15.5 atm working pressure became leaky in the lower part due to the formation of cracks in the rivet-hole edges. The boiler plate of 20 mm thickness was a rimming steel with 0.05% C, traces of Si, 0.38% Mn, 0.027% P, 0.035% S, and 0.08% Cu. The mean value of the yield point was 24 (24) kg/sq mm, the tensile strength 39 (38) kg/sq mm, the elongation at fracture, d10, 26 (24)%, the necking at fracture 71 (66)% and notch impact value 11.5 (9.4) kgm/sq cm (the values in brackets are for the transverse direction). The specimen from inside surface of the boiler was polished and etched with Fry-solution, which revealed parallel striations formed due to the cold bending of the plate. The zones of slip were concentrated around the rivet holes. The cracks were formed here. The structure examination proved that the cracks had taken an exactly intercrystalline path, which is characteristic for caustic corrosion cracks. It was recommended that the internal stresses be removed through annealing or alternatively lye-resistant steel should be used.

A T-piece from a copper hot water system failed. Microscopic examination of a polished section revealed a main crack and branching transcrystalline cracks running from the outer surface of the pipe into the pipe wall. The crack appearance indicated disintegration by stress-corrosion cracking. Although copper is not susceptible in the pure state, it is prone to stress-corrosion cracking under tensile stress in the presence of other elements in a damp ammoniacal atmosphere. The material was not defective, but a phosphorus-deoxidized copper type. The residual phosphorus combined with oxygen to form phosphorus pentoxide. Hard soldering in turn prevented the formation of cuprous oxide, and hydrogen embrittlement occurred.

The curved parts of exit pigtails made of wrought Incoloy 800H tubing used in steam reforming furnaces failed by performance after a period of service shorter than that predicted by the designers. Examination of a set of tubes consisting of both curved (perforated) and straight parts revealed that the cracks initiated at the outer surface by a combined mechanism of creep and intergranular embrittlement. A smaller grain size resulting from cold bending fabrication procedures for the curved parts was responsible for accelerating the embrittlement. It was recommended that hot bending be used for fabrication of the curved parts. A change of alloy to a low-alloy chromium-molybdenum allay to protect against heat was also suggested.