How to Gray Steel

[dbb-the-bruce]

The coating is black oxide, @benmychree describes the classic process. There are several others that operate at lower temperatures.
Here's the one I've used recently.

Went down the path of trying out "blueing" steel. Finally settled on just heat and dip in canola oil multiple times. I use my muffle/heat treat oven. Give it a good soak depending on size. Don't recall the temp off the top of my head, but think it was in the range of 600-800 F.

This method is simple, takes some time and doesn't involve storing and handling chemicals. Nothing against the other methods, but this worked great for me on hot rolled steel.

 

[ChazzC]

Now that you mention it... But that was just some of what I gleaned from the methods described obfuscated by patent-legalese and utterly buried in this illuminating document: https://patents.google.com/patent/EP0977908B1/en

That even made my head hurt!

 

[pontiac428]

That even made my head hurt!

No doubt! But it does say depositing manganese in the presence of nitric acid on the surface conditions and seeds the growth of phosphate grains for a strong barrier that absorbs oil. The best info is in the descriptions of the existing technology, not so much the one they are proposing for patent.