Cutting oil

[Harvey 2]

I have been told that to harden steel after its been heated to cool it in used engine oil, the heat draws the carbon into the steel and makes it hard.
They say it only works in used oil, as new oil doesn't have the carbon in it.
If all that is true, would used oil be better for cutting and drilling than regular cutting oil?
If the used oil adds carbon to the steel, wouldn't it make the drills or cutters more hard?
What are you all's thoughts on the subject?
Thanks in advance:
 

[Greg K]

I think you're putting too much thought into it! However you are probably right unless the drill bit already has more carbon than it can absorb from this method.
 

[RGMartin]

New oil has lots of carbon in it. Generally guys use used oil because they are cheap. That said, the carbon is already in the steel. Water works fine for a quench on high carbon steel, but since it cools the steel faster, it can make it brittle in some cases. You can heat and quench mild low carbon steel in any kind of oil you want and it doesn't harden.
 

[MarkB_MI]

Hmm. Isn't oil a hydroCARBON? Don't worry, those dead dinosaurs will do the trick.

Actually, quenching in water (which has NO carbon) will harden steel as well.
 

[Dick L]

all steel is not oil hard. You have air and water hard steel as well.
The difference would be in the time it takes it to cool not what is in the oil.

Air hard steel uses a longer cooling time where water hard steel uses a quicker cooling time. Oil hard is between the two as far as how fast you need to cool the steel.
Air hard would have a timer on the oven after the heat was turned off and let cool inside the oven.
 

[Bob M]

Someone's feeding you bunk...

It's the CARBON CONTENT of the steel and RATE OF COOLING that determines steel's final hardness after heat treatment.

As for quench cooling rates, brine (salt water) cools fastest. This is followed by plain water then oil and finally air. But even cooling in oil happens so quickly there's no time for the steel to absorb any carbon. Rather to absorb significant carbon a part must be maintained at very high temperature (1,500+) for several hours in a carbon-rich environment such as a metal cyanide salt bath, carbon monoxide gas, etc.

Incidentally aside from cost there is no advantage quenching in used motor oil vs new. I suspect used oil will likely smoke and smell worse however.

----

Regarding cutting oils it is extreme pressure and temperature additives that make an oil effective for cutting metal. Years ago these additives were primarily lard and sulfur - thus the distinctive smell inside every tool and die shop. I have no idea what's used in cutting oils today.
 

[Skaning]

Actually, what happens when you heat and quench steel is the metal goes through changes in it's crystal structure and the amount of carbon in the steel to begin with governs how it hardens. Low carbon steels won't harden, higher ones will, generally the more carbon, the harder the quench will take it. The hardened steel is stronger but more brittle. Adding other metals like chrome, nickel, moly when the steel is made will change the strength and toughness of the steel and how it hardens. What you quench it in has no bearing on carbon content. That's built into the steel at at the mill.
 

[Harvey 2]

Thanks to all who contributed to the question.
I failed the course in metallurgy at the school of hard knocks.
 

[Bob - MI]

The technical answers are spot on. Your drills are already hardened so they will hold the edge and cut.

To do the hardening action the steel has to be the right chemistry and be heated into the red/orange range and then quenched.

Use the best oil for the job. It's the cheapest approach in the long run.
 

[Tom-Pa]

When you case harden steel, like a centerpunch for instance, you heat it up red hot and put in a container of carbon. We all had to make centerpunches in shop class too many years ago to count....I also redo chisel and centerpunches when they are sharpened too much and the softer metal is showing up after the hardened carbon metal outer layer is gone.