Summary for silicon bluing process Discussion
Silicon and the bluing process are discussed in terms of why investment castings and higher-silicon steels can be harder to blacken. Shawn Mckinney asked whether silicon in 4130/4140-type castings hurts bluing performance and why. Dan Brewer replied that silicon is detrimental because it will not blacken like the surrounding metal, so the finish can come out uneven. His practical advice was to first activate the parts in a fluoride-containing acid, such as hydrofluoric acid, to remove the surface silicon before blackening. He noted that the blackening chemistry itself is not designed to react with silicon, which is the root of the problem.
Q. I have heard that Silicon has a detrimental effect on the bluing process. I’m interested in bluing some “Investment castings” e.g., 4130, 4140 alloys, which inherently have higher levels of silicon for foundry performance such as fluidity. Can anyone lend some information as to why this may be.
Thanks in advance.
Shawn Mckinney
– Ephrata, Pennsylvania, USA
2002
A. Yes, silicon has a detrimental effect in that it will not blacken the way the surrounding metal will. You must first activate the parts in a fluoride-containing acid (e.g., hydrofluoric) to remove the surface silicon, and then blacken. The blackening chemistry is not designed to react with silicon.
Dan Brewer
chemical process supplier – Gurnee, Illinois
2002