Blueing steel parts

[linamar]

Again, this is NOT for a volume production setup but for finishing small lots of parts, small screws in my case.
I know it is done in an oven, a thermo-chemical process that deposits a nice, thin, blue hemamtite layer on the surface of the parts.
What I don't know though, and I need your help with it, is the process: what media, concentration, temperature, soak time.
Thanks for your input.

 

[kenvlach]

Magnetite, Fe₃O₄, is ~blue. Hematite, Fe₂O₃, is reddish-brown. Heating actually will leave an invisibly thin hematite layer atop the magnetite.

This can be done by heating in air (need good air circulation in furnace for uniformity), furnace application of steam, or a chemical oxidation. There are 2 types of blue coloration:
Thin (translucent) oxide with coloration from refracted light. Can get practically any color by varying the thickness.
Thick, with coloration from the magnetite.
The thermal techniques are a bit sensitive, vary somewhat with alloy & I didn't find in either the Metal Finishing Guidebook or ASM Handbook vol. 5 Surface Engineering.

I suggest searching for "Gun Blue 1702 which meets the requirement of the aviation specification AMS2484" & available in 2 oz. bottles. Mentioned in

http://www.finishing.com/384/85.shtml

Try gun shops.
Let us know if/where available. I can further search for the thermal processes if needed, but you will have to fine-tune for your alloy.

Alternatives:
Zinc electroplate + blue chromate.

Make the screws from Al 6061 or 2024 & then anodize & dye blue.

 

[linamar]

kenvlach,
it is available online here:

http://shop.shooterscatalog.com/catalog.php?type=product&id=27327

 

[kenvlach]

They sure don't give much info on the Gun Blue 1702, do they? Request an MSDS if considering buying to make sure it's not mercuric nitrate. Also, their catalog has several other gun blackening & blueing chemicals; 'Formula 44/40 Instant Gun Blue' doesn't show a skull & crossbones on the front label.


The heat tint color formed on steel by heating in air, the so-called temper color, is a function of thickness. Constable, Proc. Royal Soc. A, vol. 117, p. 376+ (1928), reported the following for the first order interference colors for oxide films on iron:
Straw 0.046 micron (46 nanometers)
Reddish yellow 0.052
Brownish red 0.058
Purple 0.063
Violet 0.068
Blue 0.072

The color sequence repeats several times as thickness increases, but the luster keeps decreasing as the film become opaque.
Must be more such info in heat treatment books somewhere, but I didn't find in Metals Handbook vol. 4 (9th Edn.).
I did find some online interferometry info for silica films below, both from

http://www.ee.byu.edu/cleanroom/oxide.phtml

Re steam treatment: Metals Handbook vol. 4 (9th Edn.), p. 411 says steam treatment of ferrous metals at 345-650^oC will form a thin (1-8 microns), hard tenacious blue-black oxide [magnetite]. The steam should not enter the furnace until the parts are above 100^oC, and air should be purged from the furnace before reaching 425^oC.