MARKETING MATTERS CAST TIP
Cut Up Your Castings T
KEVIN FLEISCHMANN, AFS INSTITUTE
he first casting was poured more than 5,000 years ago. We’ve all made a lot of qual-
ity castings since then, so why can’t we make perfect castings every time? Variations throughout the casting process make each and every casting unique. When not enough variations are controlled in the process, or sim- ply not controlled tightly enough, the door is left open to produce scrap castings. While some defects come and go, others stick around. To reduce defects that are the most financially detrimental, root cause analysis should be conducted to de- termine what is precisely causing the problem then why, and lastly how to prevent it. Some defects are rather simple to identify from the surface while others may prove more challenging. Microscopic and chemical analysis consumes time and resources but makes defect identification defini- tive. Fracturing castings is the first step in defect and failure analysis. For thin castings, approach the suspect area with a saw blade and mechanically tear apart the remain- ing cross section. Oxides, inclu- sions and grain size can begin to be analyzed without any magnification.
Magnifying by up to 10x can show more clues as to what is really going on under the surface. Magnifying the surface also can help differenti- ate hot cracks from hot tears and shrinkage from inclusions. Using a microscope to further magnify the suspect area can start to differentiate various forms of porosity or voids, such as shrinkage and gas inclusions. If castings are too large to
fracture, sectioning should be done. Careful sectioning, without dam- age to the surrounding metal, also can be visually inspected. Obtaining multiple cross sections of the defect gives you the necessary three dimen- sional views to expose all evidence. For an even better view of the defect area, polishing can be done to reveal the microstructure of the surround- ing area. Inclusions can react with the base metal and alter the metal- lurgy of the casting, even producing other defects that one might assume is the cause, which actually is only a byproduct of the root cause. Fol- lowing the polishing step, a variety of etchants can be applied to the surface to reveal specific phases and grain boundaries in the structure. One of the last fronts for identi- fying defect root cause and casting
quality is image analysis. Having a computer program measure grain size and phase areas on a polished sample takes another layer of human error out of the equation. Ductile iron nodule size, shape and quantity also can be measured with careful input from the user. Oxide films, cracks and other porosity also are measured in all three dimensions. Images can be saved, overlapped, and compared to future analysis. Te precise determination of a
defect’s cause can give way to an easier corrective action in the end, when the guess work is minimized. Te information and knowledge gained also should be documented to apply in the future. You can’t see exactly what is happening inside the mold or the metal while it fills and solidifies. Turbulence can lead to oxide formation, and oxides chemi- cally react, leaving behind a complex puzzle of defects to the visual inspec- tor. Air entrainment, gas absorption, oxide films, shrinkage—where does it all go? Tese cracks and porosity are waiting to be revealed, and it often happens during machining. Cut up castings and learn from
your mistakes to avoid repeating them. Te microscope never lies.
Some defects may be easy to spot and identify on the surface, while others require microscopic of chemical analysis to study. April 2013 MODERN CASTING | 17
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