Dull Polish in 440c and 420B NIMO

Question:

We are ball making people. We find dull polish problem during passivation of 440c and 420b nimo steel. J.S.

Answer:

Not having much to go on with your question, there could be a couple possibilities. First is that the balls would need to be clean (oil and grease free) before passivation. This would involve some sort of process that would wash the parts with a solvent or an aqueous based solution with a detergent.

Second could be the removal of heat tint and/or scale from the parts. If this is present on the part, it will not be removed by most passivating solutions. Most cleaning solutions intended to remove oxidation, scale or heat tint from parts will include hydrofluoric acid along with nitric acid to provide an actual etch to the base stainless.

The next possibility is the passivation solution. The same passivating solutions used for austenitic stainless steel (300 series) are not necessarily recommended for ferritic (400 series stainless steel). The ferritic stainless steels, especially those with higher carbon content such as those mentioned will typically start out with 20% by volume nitric acid and 5% by weight of sodium dichromate. You will need to use caution with this mixture due to its corrosivity, oxidizing potential and the toxicity of the chromate. Make sure that the process complies with workplace and environmental regulations.