Thread: Question regarding oil blackening.

Don't know about the oven, but with heating you don't need to go that hot to change the properties of it for it to work in oil.
I always just leave the part in for 30 seconds to a minute and pull it out, the oil then dries out with the heat still left in the part.

Dave

Hi,
It may look the same as before but you may find it is less liking to rust.

I black/brown/blue on the stove or in the oven. Hot as I can get it on the stove for black. Straw starts around 225C through brown purple to blue about 315C. If you want a good blue you need a fine finish and heat control is tricky, but if you just want to slow rust up it doesn't seem to matter much.
Dropping in oil after heating and leave it to cool. Just how smokey is it oiling it first?

Here is a bad picture of my cmalock key that is trying to show the changes in color you can get.


If its heat treatable steel, I wouldn't go over blue as after that you have no way how hot it is until its gets to about 700C which is getting a little close to the temperature you would be going to to harden it.
If the steel has been heat treaded anything over about 150C may effect the temper.*

Stuart

*I swim in the shallow end when it comes to heat treatment

Com-VC, the two processes you describe are not quite the same so will probably give slightly different results .

When the red hot object is plunged into oil the lack of oxygen combined with the high heat rapidly decomposes the oil to carbon that deposits carbon onto the surface which together with the small amount of black iron oxide and iron carbide is why it looks really black and can look crusty. Even when the black crust is dusted off it it still looks reasonably black.

When an oven is used (unless oxygen is excluded) the amount of oil that is decomposed to carbon is small compared to that from cooling a red hot object in oil. In an oven the oil painted on the surface slowly burns to CO2 which does does not add much Carbon to the surface. I also reckon doing it for 30 minutes is too short a time. The blackening obtained with this method tends to be much fainter. There could also be a problem with your oven. Most domestic ovens tend to read temp quite incorrectly and few can even get to 250ºC

I use a BBQ oven to do my tempering colouring and monitor the temp with a thermocouple the probe of which I sit under the objects being heated.

Com,

My rough and ready method is to heat the object till it's blueish purple and then lower it into some sump oil from a 300,000 km Corona motor. The sort of motor that requires topping up rather than an oil change. This photo is from my clamp thread of a couple of days ago.

BT

Stuart: When I coated them in oil and placed them in the oven, I had a fair bit of smoke coming from the oven. I had the oven in the shed, next time I would probably do it outside as the smell was quite bad. I wasn't in there at the time though

I will have to have another go at doing it and this time heat till blue.

May try the oven again or maybe even the bbq and leave them for a longer period.

I actually bought a kiln sometime last year which would probably be ideal for this, but just haven't had a chance to play around with it yet.