Knife temper help?

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Eagle_Hawk41

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Jan 6, 2014
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Hello everyone! I have a batch of 7 knives made out of a mystery high carbon steel. The steel came from a farm implement disk, and is high carbon of some form. It throws long, feathery sparks when ground. I quenched the thinner ones in used motor oil, and ended up using hot water to quench the thicker ones after failing to skate a file out of the oil. All 7 knives, plus one from an older Nicholson file, skated a file after hardening. I put them all in my regular cooking oven for two two hour cycles at 400 degrees. As they were still too hard, I did one more two hour cycle, for a total of three so far. As they were still too hard, I then put them in for a two hour sit at 500 degrees. Now the file doesn't quite skate off of it, but certainly doesn't dig in like it should. I cleaned the scale all off after the first cycle, so I don't think it's that. Also, the two hour cycle was from after oven was preheated, not from when knives were at temp. So what's your best guess of what's going on, and how do I fix it? I know, mystery steel, motor oil, water... But I'm worried about getting these done, and I'll worry about a better quenchent and steel for the next group. (1084 and canola?) thanks in advance, I hope you can help!
 
Who said a file should dig in...? Adj your edge geometry, and put the knives to work....see if they hold up.

1084 is a good steel, but canola oil isn't the best quenchant for it......BUT if canola is
what you have, pre-heat it to 130 deg f. and agitate when you quench......

Also.....kitchen/cooking ovens aren't accurate....use a separate oven thermometer.
 
It might be your oven is running a little cool. Get one of those oven thermometers for it
 
I read where a file is supposed to " dig in a bit" when the steel is properly tempered. That is a bit misleading to some degree. It will grab more than it did at full hardness, but it won't really dig in any real amount. It mainly just feels and sounds different.

The best method is:
1) Test the edge at hardening with a good and fairly new file...it should slide along the edge with no bite at all. Take a second or third stroke if it seems to bite a bit. The decarb can fool you. If it becomes easier to dig in the file after a couple strokes, something is wrong. If it skates off after a few strokes, then it is just decarb.
2) Test the edge after temper - Sharpen the edge of the blade with a simple edge at a low angle, 12° per side is good. Test it on a brass rod for flex ( look up brass rod test) and cut some cardboard up.
3)If it flexes and returns to center ( mostly) and the edge holds up OK in a few dozen cardboard cuts....dull the edge and finish the blade. It is a good HT.
4) If the edge rolled over and stayed rolled, the HT wasn't right or the temper was too high. Dull the edge and re-do the HT, tempering at 40-50°F lower. Re-test as before after HT.
5) If the edge chipped out in the flex test, or chipped out in the cutting, it is still too hard. Temper another hour at 25°F higher than the last temper. Re-test and do a second (or third) additional temper, raising the temperature 25°F each time. When it isn't chipping out, it is right.

Final comment:
Many people think that tempering is done at 300-350°F. That is far below the proper range. If your knives perform well at that low of a temper, something was not right in the quench. Even the simplest knife steels will reach Rc 63-65 in a basic quench. After tempering at 350°F that should be too hard for almost any knife but a fine slicer kitchen knife. Most steels will need a temper between 400°F and 500°F to be robust enough to hold up in use. With 1084, a 450° temper will still yield a hardness of Rc60. I do my first temper at 390°F if unsure of a new batch of steel. On steels I have experience with like Aldo's 1084, I go straight to 450°F.
 
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Thanks guys! I figured it was just something I was overlooking or didn't know. I'm going to try the sharpen and test today, if I have time.
 
The other consideration especially with an unknown steel is to make some test blanks, I put in at least 4 small pieces of steel with every heat treat batch, I break one after quench to see if there is good grain, I expect it to break pretty easy, I then test others buring the tempering, I make one large enough to put and edge on an do some edge testing and cutting. If at some stage all is not right I can adjust my heat treat. This is very important when you are doing multiple blades.
 
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