The thermal treatment of metals can include such processes as through hardening, surface hardening, annealing, stress relieving and sintering to name a few. In some instances the performance of the component in service can be adversely affected by errors committed during heat treatment. Optical microscopy of metallographic cross sections is often used alone or in conjunction with other tests to determine the cause of the poor performance. The following gives a few examples of actual case histories of failed components that have been examined by the author.

Fig. 1. Photograph at 10x showing cracking within the broadening of the microstructurally-transformed induction-hardened case. (2% Nital)

Case History I - Induction Hardening

One of our clients is engaged in the business of manufacturing diesel locomotive gears. The process involves both machining and induction surface hardening of the gear-tooth profiles. On one particular day, their facility experienced several brief surges of electrical power and numerous quench cracks were found during the final inspection of the gears. Several of the quench cracks occurred in the root areas between adjacent gear teeth. We were asked to find evidence of a direct link between the power surges and the quench cracking. After preparing several metallographic specimens in conventional orientations, it was decided to try a cross section through a quench crack parallel to the tooth root and at a location close to the maximum depth of the induction-hardened case at the root location. This orientation provided the sought for evidence (Fig. 1), where it can be seen that the crack lies primarily within a brief broadening of the darker etching band of transformed martensite. The broadening of the martensitic band would have been caused by a greater heat input at this point along the coil scan and thus could only be attributed to a power surge.

Fig. 2. Photograph showing internal fissures within a copper filter particle. (As polished)

Case History II - Sintering

Another of our clients manufactures filters made from small copper particles that are sintered together to form a porous final product. It was reported that filters made from one lot of copper particles had exhibited poor flow rates, indicating that the filters were becom-ing clogged. Metallographic cross sections through both good and bad filters were prepared and examined. The bad filters showed numerous internal fissures within the individual copper particles (Fig. 2). The clogging of the filters was determined to have been caused by the eventual breakdown of the copper particles. It was also determined that the copper particles within this lot of material contained cuprous oxides that had reacted with hydrogen in the sintering-furnace atmosphere to produce internal steam pressure that created the internal fissuring.

Fig. 3. Photograph showing the dealloyed surface layer of the casting. (60% Nitric acid)

Case History III - Furnace Atmosphere Control

A manufacturer of small stainless-steel casting hardware had several parts returned from the field after they displayed severe uniform corrosion in what should have been a benign environment. Again the sample was examined metallographically. In the as-polished condition, internal oxidation was observed at the exterior surfaces. After etching, the microstructure of the casting was observed to consist of islands of delta ferrite in a matrix of austenite except at the exterior surfaces that were deficient in the islands of delta ferrite (Fig. 3). Furthermore, a faint line of demarcation seemed to separate the surface layer from the internal bulk of the casting. The elemental chemistry of the bulk and surface layers were analyzed using a scanning electron microscope (SEM) that was equipped with an energy dispersive spectrometer (EDS). Compared to the internal bulk of the casting, the surface layers were found to possess lower levels of chromium, which impart to stainless steel its corrosion resistance. Chromium is also a ferrite-stabilizing element, and its presence in much lower concentrations explained the missing delta-ferrite islands near the surface. It was concluded that the stainless-steel hardware had been heat treated in an oxidizing atmosphere that essentially depleted the surface layer of chromium and produced a dealloyed layer – that was no longer stainless – having very poor corrosion resistance.

Fig. 4. Photograph showing intergranular carbide precipitation at the surface of the stainless steel tube. (10% Oxalic acid)

Case History IV - Pre-cleaning of Stainless Steel

Another example of variations in surface chemistry affecting a material is illustrated in this last example. A manufacturer of very thin-wall stainless-steel tubing annealed his product after the mechanical forming operations. He did not clean the tubing prior to heat treatment, however, to remove the forming and drawing lubricants. Afterwards, metallographic examination of the product and etching, in accordance with the parameters of ASTM A262 Practice A (oxalic acid etch test), revealed the presence of shallow intergranular chromium carbide precipitation (Fig. 4). Because of occasional failures of his product that may or may not have been attributable to the presence of the intergranular carbide precipitation, he decided to clean the tubing to remove the forming lubricants. Thus the intergranular carbide precipitation after annealing was also prevented, eliminating one line of attack by litigating attorneys.

Conclusion

Metallographic examination of heat-treated components can often discern the true cause of failure of a material. Metallography has been practiced for over a century now and will remain one of the most useful methods of investigation for the failure analyst. IH

For more information: Edward S. Larkin is vice president of the materials engineering group for Matco Associates, Inc., 4640 Campbells Run Rd., Pittsburgh, Pa. 15205; tel: 412-788-1263; fax: 412-788-1283; e-mail: ed.larkin@matcoinc.com; web: www.matcoinc.com

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