Q. We need to add a slot to a PH13-8MO H1000 part. Do we need to repassivate? What is our risk if we use carbide tools only for the rework? —M.W.
A. There are generally two purposes to passivating any stainless steel. The first one is to remove near surface iron from the part. Two common sources of iron would be from welding or machining/forming. Even if using the correct stainless filler to weld stainless, there will often be some amount of iron that segregates and solidifies. Another name for this is delta ferrite. The other common source of free iron may be from tool steel used in manufacturing parts. Passivating a part will help to dissolve and remove this free iron that would act as a contaminant and easily corrode on an otherwise good surface.
The other reason to passivate is to assist in building a passive chromium oxide layer on the part. While the chromium in stainless steel naturally oxidizes providing the stainless corrosion resistance, it is a time- and atmosphere-dependent process. In addition to removal of the free iron, the passivation process will aid in establishing an oxide layer with a greater and more uniform thickness.
Use of the carbide tooling will minimize the introduction of free iron into your stainless part. However, the newly machined surfaces will have a thinner oxide level in comparison to the balance of the part that was already passivated. You should refer to you customer specifications and testing requirements to determine if this naturally occurring oxide layer will be sufficient to meet them. I would expect the newly machined surfaces to perform poorer than the balance of the part that was passivated, although it may still be acceptable in meeting the requirements of your customer specifications.
Having issues achieving a consistent color with the application of black oxide? Start by looking at your pretreatment. Connor Callais of Hubbard-Hall offers advice for troubleshooting your black oxide process.
