Why "Used" oil?

ricky_arthur

Well-Known Member
In all the discussions about quenchants, Motor oil is inevitably thrown into the discussion because, well, that is what some people have available to use. But the suggestion is always "USED" motor oil. Now, I do not use Motor oil but I do use Canola oil. I run a restaurant and have access to tons of "used"canola oil, but I have been using New Canola oil, Just because I can get it fairly cheap.

My question is this... Is there something about the oil; Motor or Canola that makes USED oil a better quenchant than new? Or is it simply a cost issue?

Also Wondering.

Heating the oil doesn't reduce the rate of cooling, It increases it. The thinner viscosity causes it to cool faster. In fact, the only reason I could find for keeping the oil temperature below 150 is because it begins to break down the oil and no one wants to shorten the life of expensive oil for a diminishing return. Even that reason makes little sense since the Canola oil I use spends and entire week about 10 hours a day at 350deg before it needs to be changed. I can't imagine that an hour at say 200 degrees would have enough of an effect to be worried about.

If oil at 130-150 degrees quenches faster than oil at room temperature, Then is oil at 175-200 degrees not going to cool it even faster? In my head it would seem so. (But I have never tested the idea, I always quench at 130-140 because I was told to do so)

To make sure my original question is being understood.

People always use the word "USED" oil when referring to using motor oil as a quenchant. Do they specify "Used" because it is better? or because it is cheaper?

If it is because it is better, Then could the same apply to non-petroleum oils? Is a used oil a better quenchant than a new one? I did a google search and couldn't find an answer to either question.

I'm simply asking because I have access to alot of both new and used Canola oil. I Have used both and with the simple methods available to me to test the results I can see no measurable difference. That most definitly doesn't mean there ISN"T a difference, only that I would be unable to tell a difference of say 1 or 2 or maybe 3 HRC difference. But if useing one over the other would RESULT in a few points HRC difference I would like to use the best of what is available to me.

Anyone have any answers to these questions?

Used VS New oil

125-150 Degree quenchant VS 150-200+ ?

Thanks
 
Because used motor oil, or I guess used canola runs from cheap to free. Motor oil is a super poor quench oil. New or used. It's quality as a quenchant is just a degree above lard and bacon grease. It will harden some 10xx steels, but not as well as some other options, and it's fumes are quite toxic. Most quenching oils work best between 120° and 140°, with the exception of Parks #50, and some similar oils, which is best at between 70° and 100°. If your oil is much over 140°, you will not cool fast enough to achieve a quality result. The oil is heated in order to get a thinner viscosity, and not to raise the temp just for the sake of hotter oil. Hotter oil has no benefit, and starts to be detrimental with most oils, after the temp passes 140°. The thinner viscosity cools faster because it reduces the vapor jacket effect faster. Vege oils seem to produce a lesser vapor jacket than most petro based oils, so they could be used at a lower temp. More is not always better.
 
Canola and the other vegetable oils work best fresh. The main problem with the vegetable oils is relatively poor stability compared to the petroleum oils. Canola should look clear with a slight yellow/green tint. When it starts to turn brownish, like the color of weak tea it’s time to change it out.

There is virtually no vapor jacket with the vegetable oils.
 
I noticed a difference in oil life when I started adding vitamin E to the tank. If you don't have access to engineered quenchant or just prefer organic oils for health/environmental reasons, canola is a decent substitute.

As for "used" oil... I think that people looking for answers, are sometimes drawn to the unknown. So many anomalies at play give the illusion of anything being possible. I find that many knifemaker's seek definitive answers(or magic recipes) when they should be broadening their knowledge base to arrive at their own conclusions.

I would much rather find out I was wrong by misinterpreting the facts, than to be successful and not know why or how.
 
If you added vitamin E to it for stability and extended life,… wouldn’t that qualify it as an engineered quenching fluid, basically?
 
... guess it's not worth bothering with the vitamin E. Just change it out when needed.
 
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