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Heat Treating and Metallurgy Discussion of heat treatment and metallurgy in knife making.

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  #1  
Old 01-13-2013, 10:16 AM
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ricky_arthur ricky_arthur is offline
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Why Used oil?

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?

Wondering.
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  #2  
Old 01-13-2013, 11:09 AM
Doug Lester Doug Lester is offline
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Strain out the chunks and use it. There have even been some people who have rigged oil burners to their forges and burned used cooking oil in it. Actually though, I would discourage people from using any motor oil due to toxic fumes given off during quenching. I think that the used element was just repurposing waste material.

Doug


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  #3  
Old 01-13-2013, 12:24 PM
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I'm going to assume that if used is better than new. That it is because of reduced viscosity?
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Old 01-13-2013, 02:51 PM
Doug Lester Doug Lester is offline
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Don't know about that. What gives the oil better viscosity for quenching steel is heating it slightly. Somewhere around 120-140? will do fine without reducing the rate of cooling appreciably.

Doug


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Old 01-13-2013, 03:49 PM
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ricky_arthur ricky_arthur is offline
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Doug, 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 noone 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? Im 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)

I'm not 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.

Is that any more clear?

Last edited by ricky_arthur; 01-13-2013 at 04:01 PM.
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Old 01-13-2013, 04:31 PM
metal99 metal99 is offline
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I think "used" motor oil was the rave because its got more "carbon" in it. That's what I gathered from all the reading I've done in it. They even did a used motor oil quench on myth busters lol. I think they were trying to case harden hammer heads if I remember correctly. I prefer to keep my oil new and free of contaminants.

I had a blade crack on me because the oil was way to hot. The hotter the oil the faster the quench.


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Old 01-13-2013, 05:52 PM
WBE WBE is offline
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Quote:
Originally Posted by metal99 View Post
I think "used" motor oil was the rave because its got more "carbon" in it. That's what I gathered from all the reading I've done in it. They even did a used motor oil quench on myth busters lol. I think they were trying to case harden hammer heads if I remember correctly. I prefer to keep my oil new and free of contaminants.

I had a blade crack on me because the oil was way to hot. The hotter the oil the faster the quench.
I seriously doubt that was the reason your blade cracked. If what you say was true, then we would be boiling the quench oil for some steels. Much over 140? and you will retard martensite formation which will start at around 400/450? and needs to transform relatively fast when it begins. At that temp, you can quench in water and get a tiny, albeit, small degree of more Martensite. Air cooling, in most steels, will be enough. Extra heat will not help this transformation, but retard it because of a slow start. A blade is not going to crack until Martensite begins, unless it already had a problem and over heated oil would not be the problem because of any faster cooling, but because it will not be faster, it would slow the cooling and allow pearlite transformation the closer it gets to the oil heat. With 1095 you have about .8 of one second for the steel to drop from about 1350? to under 450?. With 01, you have as much as 9 or 10 seconds to get there. More is not always better in quench oil heat. As far as case hardening, there is not enough carbon in used motor oil to case harden at any measurable degree anyway.
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Old 01-13-2013, 06:40 PM
Ed Tipton Ed Tipton is offline
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Ricky Arthur...I think the reason for suggesting the use of used motor oil is simply because it is easily acquired by almost everyone, it is available at no cost, and it is an effective quenchant. If you are going to use oil as a quenchant, you do need to have a means of smothering a fire when one develops. ATF is a little better since it has a higher flash point and is therefore somewhat safer...but I'm not sure if it is a better quenchant or not.
The argument sbout heating the oil above 140 degrees being better comes from the old philosophy that "If enough is good, then more is better, and too much is just right". While that theory might be OK for sex and ice cream, it doesn't apply to quenching mediums.
Listen to WBE.

Last edited by Ed Tipton; 01-13-2013 at 06:46 PM.
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  #9  
Old 01-13-2013, 06:45 PM
metal99 metal99 is offline
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The higher your oil temp is the faster the rate if cooling is. I don't have a doubt about my blade cracking from the hot oil. I did a batch of knives and they were all fine untill the last one. The oil was very hot come the last knife. Another member here on the forum had the same problem when doing a batch of 1084 blades.
The higher the oil temp the less vapor barrier=more time for cooling.

1095 has to be dropped to under 900 from critical on less then one second doesn't it?. Not 450, even with a brine quench I think getting it lower then 450 in that time would be very tough to do.


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Last edited by metal99; 01-13-2013 at 08:45 PM.
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Old 01-13-2013, 08:35 PM
Doug Lester Doug Lester is offline
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I think that the rate of cooling with hot oil might be true to a point but according to more expert people in the field there comes a point where that might not hold up. That's why I've been told to get a marquenching oil for my experiments in austempering. Of course with that your are talking about really hot oil, possibly over 525?.

Doug


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Old 01-14-2013, 06:51 AM
WBE WBE is offline
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99 you are correct. That was meant to be 900?, not 450?. At that point the big race is over, but a too hot oil will retard martensite formation and leave more austenite in the structure than would be desired for most blades. Martensite begins at around 400? to 450?. If it is not allowed to continue at a relatively fast rate, it is my understanding you will end up with a kind of martensite/bainite mixture, lacking any of the advantages of either.
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  #12  
Old 01-14-2013, 07:59 AM
Tai Google Tai Google is offline
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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.


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Old 01-14-2013, 08:00 AM
metal99 metal99 is offline
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Ahh ok I was kinda wondering if I was misunderstanding something or not.


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Old 01-14-2013, 12:03 PM
Imakethings Imakethings is offline
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90% of he reason I use it is cost. The other 10% is that I don't have to replace it as often.
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  #15  
Old 01-14-2013, 03:03 PM
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Another reason NOT to use used motol oil is the fumes. A lot of nasty compounds form in used motor oil which will produce some very unhealthy fumes when heated like during a quench. Saving a few bucks isn't worth your health.

If you can't afford a commercial quenchant then stick with canola oil.

Gary


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