casting tip C
Cut Up Your Castings Kevin Fleischmann, AFS and the Institute
be perfect every time? Variations throughout the casting
T
process make each and every casting unique. When not enough varia- tions are controlled in the process, or simply 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 fi nan- cially detrimental, talk to your casting provider about root cause analysis to determine precisely what is causing the problem, then why and, lastly, how to prevent it. Some defects are simple to identify
from the surface, while others may prove more challenging. Microscopic and chemical analysis consumes time and resources but makes defect identi- fi cation defi nitive. Fracturing castings is the fi rst step in a metalcaster’s defect and failure analysis. For thin castings, the suspect area is cut with a saw and the remaining cross section is torn apart mechanically. Oxides, inclu- sions and grain size can begin to be analyzed without any magnifi cation.
he fi rst casting was poured more than 5,000 years ago. With all of the quality castings produced since then, why can’t they
Magnifying by up to 10x can show more clues as to what is really going on under the surface. Magnifying the surface also helps diff erenti- ate hot cracks from hot tears and shrinkage from inclusions. Using a high power microscope to further magnify the suspect area can start to diff erentiate various forms of poros- ity or voids, such as shrinkage and gas inclusions. If castings are too large to
fracture, sectioning is done. Careful sectioning, without damage to the surrounding metal, also can be visu- ally inspected. Obtaining multiple cross sections of the defect provides the necessary three-dimensional views to expose all evidence. For an even better view of the defect area, polishing can be done to reveal the microstructure of the surrounding area. Inclusions can react with the base metal and alter the metal- lurgy of the casting, even producing other defects one might assume are the cause but are only a byproduct of the root cause. Following 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 hu- man error out of the equation. Ductile iron nodule size, shape and quantity can be measured with care- ful 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. T e precise determination of a
defect’s cause can give way to an easier corrective action in the end, when the guess work is minimized. T e 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 fi lls and solidifi es. Turbulence can lead to oxide formation, and oxides chemically react, leaving behind a complex puzzle of defects to the visual inspector. Air entrainment, gas absorption, oxide fi lms, shrinkage—where does it all go? T ese cracks and porosity are waiting to be revealed, and it often happens during machining. Ask your metalcaster to cut up
your castings, to learn from mistakes and avoid repeating them. T e micro- scope never lies.
Some defects may be easy to spot and identify on the surface, while others require microscopic or chemical analysis to study. 46 | METAL CASTING DESIGN & PURCHASING | May/Jun 2013
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