Thread: Oil Blackening Steel

That is a nice finish there Chris.

It's hard to get good black oil these days unless you have a diesel.

john

Thanks John. I don't know what the oil came out of- it's filthy. Not my car thankfully! I got it from the bloke who fixed my brakes today.

As an interesting comment, yesterday I purchased some high tensile 1/2"x20 bolts from a bolt supplier - length was 6"- they were similar coated as per the oil quench shown.....I asked the bloke what the coating was and he informed that was how all larger high tensile bolts came - he didn't know how it was applied or what it was but told me they could be identified in the bolt head with 5 or 6 indents to indicate they were high tensile steel. I'd not come across that before as not having much use for high tensile stuff.

I had to extend the thread on them and noted when I cut them, the coating had penetrated the metal to about half a mm and was definitely not a spray on finish.....they looked exactly the same as Chris's oil quench and other oil quenches I have done in the past - it was just the depth of penetration was greater and there was no residual oil coating you usually get........unfortunately I didn't hang around to ask the bloke if he knew what sort of process they underwent..............perhaps others here can shed some light on how high tensile bolts are processed - are the oil quenched/treated?????

They certainly looked oil quenched to me..............
Lee

They certainly looked oil quenched to me..............

Black Oxide Metal Finishers, Tool Makers Victoria | Bluing, Surface Protective Coatings, Electroplaters

Many black coated steel items - including bolts and nuts - are black passivated zinc plated. If you sand it the zinc and the the steel show through.
According to my electroplater its the most corrosion resistent passivation. The other common forms are 'blue' (silver) and 'yellow' (gold).

This link will take those interested to numerous other links discussing hot and cold black oxide finishing.

The Top 5 Finishing FAQs

Bob.

Originally Posted by jhovel

Many black coated steel items - including bolts and nuts - are black passivated zinc plated. If you sand it the zinc and the the steel show through.
According to my electroplater its the most corrosion resistent passivation. The other common forms are 'blue' (silver) and 'yellow' (gold).

"Zinc plated" and thereafter additionally "black passivated" is an entierly different, industrial, zinc plating process: Black Zinc Plating
But the high tensile black machine screws that we all know are only hot black oxyde finished. Definitely not zinc plated and then black passivated.


In the home shop, there are several ways to blacken steel items:

- cold chemically, by polishing and using a blueing or blackening or browning agent as sold by gun shops. I use Birch Casey products which would be best known. You can mix your own if you have easy access to chemicals/acids. I use this on parts trhat I need to remain dimensionally stable (like homemade toopost/yoolholdes for the lathe). You only cook the parts in boiling water to degrease prior to application, so not hot enough to distort. And the chemical blackening does not build up a coating, it rather leaches a coating into the metal without changing dimension.

- the classic hot way, by polishing and heating a workpiece to below blue color, then quenching in a clean oil that has the ability to polimerize easily, like linseed oil. The process is repeated several times until the desired browning/blackening occurs. There is very little heat distortion, there is very little build up of a coating to add to final dimension. Old guns were made this way.

- by brutally heating a workpiece to red color and then quenching in old black engine oil. The resulting black coating is actually a coating of carbonized oil (aka sooth) of substantial thickness. The heat required is sure to distort any accuracy out of the workpiece, the sooth buildup will do the rest to destroy any accuracy lefty. Besides, the fumes from burning old engine oil contain all sort of poisonous nasty compounds, so only use this method outdoors. This method is more suited to rough blacksmithing type items, rather than for things like T-nuts.

Chris

I just went and had a look at one of the t-bolts I blackened under a very large magnifying glass. The extremely fine machining marks are still clearly visible and well defined. I'd say the layer of "soot" is so thin as to have negligible effect on the dimensions of the part. Certainly not measurable with any of my measuring tools. I can't comment on the amount of distortion introduced other than to say the bolt is still straight and still fits nicely into the t-slot.

You are right about the nasty fumes. I did the first piece in the garage as it was very windy outside. The garage was soon full of very unpleasant smelling fumes.

If you are looking for a way to make a non-critical part very black, very quickly, I'd say give this method a go.

Originally Posted by jack620

I just went and had a look at one of the t-bolts I blackened under a very large magnifying glass. The extremely fine machining marks are still clearly visible and well defined. I'd say the layer of "soot" is so thin as to have negligible effect on the dimensions of the part. Certainly not measurable with any of my measuring tools. I can't comment on the amount of distortion introduced other than to say the bolt is still straight and still fits nicely into the t-slot.

You are right about the nasty fumes. I did the first piece in the garage as it was very windy outside. The garage was soon full of very unpleasant smelling fumes.

If you are looking for a way to make a non-critical part very black, very quickly, I'd say give this method a go.

Just be careful Chris. The oil can catch alight.

Will do Bob. Definitely an outside job. It also spits a bit, so safety glasses are the go.

Thanks Chris for the explanation and the links.....I have been oil blackening for quite a few years and never achieved a consistent result that would say I perfected the process.....now I know why.....as an aside on the safety aspect, could I share my process which might be helpful to others who want to give it a go and maybe deterred by what's involved....the usual safety criteria applies here with due diligence

Without a dedicated kiln or furnace, I find it difficult to gauge the colour variation on the heated metal outdoors due to sunshine/cloud whatever so I heat the metal in the garage where the light is controlled......probably would help if I got an infra-red heat sensor but judging the colours is quite accurate for what I want.......also having a printed colour chart nearby helps as lots of people see "cherry red" quite differently...

I have my quenching tin/bucket half filled with old oil just enough to cover the piece and then that bucket sits in a large metal tray to collect any over spills or "frothups" as I call it..as well as catching the piece from the tongs if I drop it..........that's all placed outside the shed on a concrete area clear from anything with the old fire extinguisher handy if needed....it's an open area with clear access...

It's not uncommon for most of my quenches in the oil catching fire so I have a metal lid made from gal iron with a long handle that I just place over the bucket/tin to smother any flames or smoke...it will get hot so you need a handle - the lid needs to be metal and a good fit over the tin so no air can get in to the flames or smoke escape...but just sitting it on top of the bucket/tin works, you don't need to weigh it down......that generally keeps all that nasty stuff from smoking out the neighbourhood

By smothering it as soon as you quench, you stop most of the smoke escaping and also smother the fire without the need for other extinguishers...........I know others who leave the piece in the oil to cool, I don't so once the flame/froth/smoke has died out, I remove the piece and leave it on the tin tray to cool.

A work of caution here - sometimes if you remove the lid too soon, it will re-ignite...........but always remember it is oil, b**dy hot, and wants to spoil your day as well as your job.....so please take care and use proper safety processes, including having the correct fire retardant for oil just in case .....water won't help!!

My process isn't high tech, but it stops the smoke and flame problems.........now all I have to do is perfect the colour....

Hope that helps others..............Lee