Q: What causes chevrons or cavities in cold forged parts, and what can we do to avoid them?
A: Determining the specific causes of chevron defects in cold extrusions requires a rather lengthy analysis. However, ways to avoid them are pretty well known.
Consult Cold and Hot Forging: Fundamentals and Applications, published by American Society for Materials. It outlines the software approach to predicting chevron defects in steel forgings.
In my experiences, chevron defects are caused by four main factors: steel center quality problems (segregation); multi-pass extrusion with at least three passes; incorrect selection of die angles for the final reduction, possibly as a result of the dies wearing at a different angle than the design calls for; and final reduction of less than 15%.
Regarding the extrusion angle, a 45° angle with a light reduction (say 15%) invites chevrons, whereas a light reduction of 15-20% through a die angle of 10° on a side, or 20° included angle, will generally prevent chevrons. For example, there is a trend to use light reductions for the pitch diameter.
If there is a bar defect present, it may lead to infrequent defects. Remember, strand-cast steel tends to show centerline defects more than ingots. Bloom cast is an improvement. I avoided strand-cast steels (strands of less than 7 in. sq) when I specified steels for parts, when I was chief engineer of a cold forging company. We allowed the use of bloom-cast billets (greater than 12-in. blooms cast in curved molds). We made millions of automotive parts and solved our chevron problems with these kinds of restrictions.
H. James Henning answers forgers' technical and operational questions. For more than 40 years he held key technical positions in the forging industry, most recently as director of technology for the Forging Industry Association. He is president of Henning Education Services, Columbus, OH, specializing in customized education and training in forging technologies.
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