Many instances of casting porosity have been evaluated using the SEM. These include shrinkage porosity and gas porosity. One particular instance involved a cast iron crankshaft that failed catastrophically during use.
The following examples illustrate some of our contributions to this business sector:
Bronze Problem Solving
- The recurring problem of veined staining on the surface of ornamental bronze castings led to an investigation of the cause. It was found that shrinkage in heavier sections allowed lead to migrate to the cavity and to form a galvanic cell, which was activated when moisture was present. The galvanic action of the lead/copper cell produced copper oxide surface stains.
- A gray, putty-like substance was found to periodically clog the feed system of a toluene-sulfonic acid, no-bake sand system. GC/MS analysis of the foreign material identified it as a polymer of toluene-sulfonic acid. Examination of the MSDS sheet found the material to be incompatible with ferrous-based materials, especially fittings. The problem was traced to stainless steel fittings on the delivery system, which catalyzed the polymerization of the toluene sulfonic acid.
- Touchstone has performed numerous analyses of surface stains on decorative bronze castings including solvent blisters, edge bleeding (from sharp edges on characters) and surface casting defects. The evaluation of these defects is performed by SEM (scanning electron microscopy), light microscopy, GC/MS (gas chromatograph / mass spectroscopy) and Touchstone’s paint and coatings laboratory.
- Memorials were tested to determine the resistance of impact by mowing equipment used in cemetery maintenance. An impact tester normally used to test paint adhesion was modified to produce an impact force equivalent to that resulting from impact by a mower wheel. This effort led to the selection of an optimal design type and has helped prevent complaints due to damage.
Cast Iron Problem Solving
- Wear-resistant cast iron parts were evaluated to determine the best combination of chemistry and microstructure to improve service life. Improvement of service life gave a competitive edge that allowed the client to capture a significant market share and to more than quadruple sales over an eight-year period. This foundry is now a preferred supplier for many domestic users of wear-resistant cast iron parts.
- A cast iron guide was reported to be the cause of scratches in finished rod stock. Evaluation of the cast surface using both the stereo-microscope and the SEM found sand inclusions in the surface, which resulted from either dropout during mold closure or erosion during pouring. The findings led to an improved quality control plan to prevent the defect from reaching the customer and modification of the sand molding system to eliminate the defect.
- Many instances of casting porosity have been evaluated using the SEM. These include shrinkage porosity and gas porosity. One particular instance involved a cast iron crankshaft that failed catastrophically during use. While the metal met both elemental and mechanical specifications, shrinkage porosity at the failure initiation site provided a stress riser that led to the fracture.
- A Ni-hard, cast iron roll was examined to determine the cause of a large, saddle spall failure. Evaluation of the fracture found that one side of the spall had been heated more than the other as indicated by the depth of firecracks. Examination of rolling records found that a mill wreck had occurred just prior to the roll failure and that no examination of the mill rolls had been performed. The probable cause was established as overheating from long contact with the strip after the mill wreck.
- A second roll evaluation concerned a ni-hard roll that broke transversely near the center. Examination of the fracture surface and elemental analysis of samples at various depths found that the statically poured roll had experienced an arrest in the flushing operation during manufacture. This meant that the roll consisted almost entirely of hard, high-alloy iron, rather than a softer gray cast iron core.
- Touchstone has performed research projects worth more than a million dollars for the gray iron ingot mold industry, including molds from plants using either blast furnace, cupola, or electric furnace produced metal. Relationships between manufacturing practice, microstructure, and use conditions were explored to determine the best product for a given application. The work led to the development of more effective inoculation practices and methods for non-destructive inspection of castings prior to shipment.
Aluminum Problem Solving
- Two die-cast housings were examined using the SEM to determine the cause of cracks. In addition, they both were analyzed to determine whether the correct alloy was used. One was found to contain a dendritic structure over most of the fracture surface, indicating shrinkage porosity. The other crack was located at the site of a cold shut. Both problems were traced to variations in the die casting process.
- The manufacturer of a safety guard for woodworking equipment complained about a surface condition on a thin section of the die-cast aluminum part. The surface exhibited what is described in literature as “flow lines” and presented an unacceptable surface for painting. The results of the literature search found that low mold temperature is typically the cause of this problem.
- A die-cast aluminum equipment cover exhibited a brown stain on cooling fins that resisted cleaning and would not allow proper paint adhesion. Auger analysis of the stain found that it consisted primarily of carbon. A search of available literature found that excessive application of mold release agent often resulted in such a stain.
- Leakage of supposedly watertight castings led to evaluation of several specimens to determine the cause. SEM analysis of the defects found silica sand trapped in many of the pores. The report led to improvement of the sand practice and the addition of a wash to improve casting quality.
- Porosity in a regulator housing allowed leakage in an oxygen system. Evaluation of several specimens using the SEM found that the pores were nearly spherical and smooth, indicating gas porosity as the cause of the defect.
The above examples are only a sample of our work in this area. We would be pleased to provide any additional information you may require. We also invite you to review examples on our website of work that we have performed in support of other business sectors.
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