Blueing/oil quenching of mild steel

from memory quencing in engine oil adds carbon and surface hardens the steel, old oil has more carbon and therefore hardens more, I did this as an apprentice years ago and i seem to remember they still rusted

From my often faulty memory from 40 odd years ago, wasn't blueing also just an aid for 'marking out' the work. An easily removable dye gave a blue surface colour on the 'shiny' metal on which you could scribe out lines, by cutting through the dye so that you could easily see them..

that was engineers blue

from the web Prussian blue is mixed with methylated spirits it forms a quick drying stain which is known as marking blue or layout dye. This stain is used in the marking out operation in metalworking.

No, it's the quenching (i.e. rapid cooling) which hardens the steel, as long as it contains enough carbon. For surface hardening you need to heat the steel to red heat in a carbon rich source for long enough for carbon to diffuse into the surface, and then you quench it. Look up case hardening.

Oil quenching is faster and better controlled than water quenching because you don't get the steam blanket, so it is more commonly used in industry, also it is better for some alloy steels.

You'll get some rust resistance from hot quenching, and more from proper chemical blue or chemical black treatments as used on guns, for example. Lots of proprietary suppliers or you can mix your own. Look up chemical black or blue

Not what the person is wanting - he is looking for an effect similar to that found on a gun barrel.

I recall processes such as Parkerising and Walterising - better than just blacking with oil.

I have a scribing block I made as an apprentice which is still a nice black - it was Parkerised.

I remember doing this at school, too. This was recently enough for the subject to be called "Design and Technology (Resistant Materials)", but was essentially still metalwork taught in the metalwork workshop by crusty old metalwork teachers :-).

The other day in my dad's garage I came across the mild steel gadget that I made in 3rd-form DT. The surface didn't look all that great (probably didn't when new either :-) ), but it wasn't rusty. So I reckon "moderate rust-resistance" is about right.

I do remember the aforementioned crusty old teacher telling us that the older the oil, the better.

I think so.

Pete

Hot oil: Heat them up (not glowing) and wipe with oil. Vegetable oils give browns, clean engine oil gives blues, old engine oil gives black. Not rustproof against weather. You only need a gas blowtorch to heat the steel up, then wipe with an oily paper towel. Use paper, as it's easier to extinguish than rags. Wear a good leather welder's glove when doing this, and not one with a hole in!

Cold blues: Selenium compounds. Phillip's is best (gunshop), Birchwood Casey a long way behind. Follow instructions. Cleanliness and degreasing with acetone is vital. Toxic. Not rustproof.

Browning. Controlled rusting to give the finish of old steelwork or shotguns. Read a pre-war gunsmithing book. Harder than it sounds to get good results.

Weatherproof bluing. Look up "Parkerising", buy the kit from Caswells. Complicated.

Hardening, tempering and quenching relies on using a steel with some useful amount of carbon in it, not mild steel. Carbon won't migrate from an oil quench. For a good text on hardening, knifemaking books from Barney & Loveless (cheap), Wayne Goddard or Jim Hrisoulas (excellent) are good primers. 1950s engineering textbooks are pretty good too. Weyger's book is crap.

If your steel has insufficient carbon to start with, add some by case- hardening. The wikipedia article is decent.