Tempering questions (general)

PIndelicato

PIndelicato

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Sep 16, 2002
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I have a couple of general questions about tempering that have been banging around in my brain today (lots of free space in there, apparently):

1) If the tempering device (kitchen oven, kiln, toaster oven, etc.) has a temperature swing of say 30 degrees, at what temperature am I actually effectively tempering? For example, let's say I want to temper at 375 and my average temperature is 375, but it cools down to 360 and heats up to 390 to maintain that average temperature, is the blade experiencing a temper of 375, or 390, or?

2) If you temper a blade once at 350 for 2 hours, then a second time at 375 for 2 hours, have you effectively completed your "twice for two hours" temper at 375, or would it need to be done again at 375?

Thanks in advance for any enlightenment!
 
Paul,

Tempering is a function of time and temperature. It is hard to say what exactly is happening.

I use a 3/8" thick steel plate in my tempering oven and lay my blade on it. By adding more heated mass the swing is not as drastic. You could stack pieces of steel to get the same effect.
 
Z that solves the issue nicely! A pizza stone should work too. I think I might even use aluminum foil, that would just delay conduction from slightly overheated air from contact with the edges/tip.
 
You can also just bury the blade in dry sand in a suitable cake pan. The sand will greatly help balance temp flcuations.
 
Well this is one of those "doh!" moments for me. I've certainly heard of using dry media to help with slow-cooling a blade when annealing, and liquid media for even heating, but I never thought of using solid mass to keep temps even for tempering. Makes sense to me, thanks guys.

wnease said:
I think I might even use aluminum foil, that would just delay conduction from slightly overheated air from contact with the edges/tip.
Click to expand...

I could be wrong, but I doubt foil would have much affect at all. Unless you wrap about 40 layers around the blade, in which case, sand or a slab of scrap steel would be cheaper and easier to re-use. Just thinking out loud.
 
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Aluminum is a good insulator of radiant heat and poor at insulating against conduction and convection. Something is better than nothing.
 
They have touched on several ways to even out the heat applied to the blade. As to your second question about multiple tempers:

When two tempers are done, the highest temperature in either is what will most affect the hardness - regardless of order ( it is a bit more complex than this, but for a simple statement it is true).
Most of the time, when you see the second temp given in a post as a higher number, that usually indicates the person tested the hardness after the first cycle, and wanted to lower it some...thus the higher temperature on the second cycle. In normal tempering, there would be no reason to temper at different temperatures.

As said, tempering is a function of time and temperature. While a few seconds with a torch and looking at the color will have some tempering effect, a couple hours in an oven will be far more complete and even.

Ovens are made to bake larger thermal masses than a small piece of steel. Ovens swing up and down a bit when they cycle on and off, but the cake/turkey/pie doesn't change temp as fast as the air, so it swings very little. If the dials have been calibrated at the factory, the food will stay close to the set temp.
Increasing the thermal mass of a blade will thus help keep the blade temp more even. This can be done in the ways suggested. Most folks don't bother at all, or at most put a plate of steel in and set the blade on it. If you are a real worrier, a bread pan full of sand will work very well. Just remember to pre-heat the sand for 30 minutes to bring it up to temp first.

If you use the same oven all the time, you should find the temp to set it at that works right by testing the edges...........you are occasionally testing the edges for roll and chip....aren't you? Roll = set oven lower; Chip = set higher; Flex and return to straight = just right.
 
I appreciate the responses guys! I was wondering about these concepts more from a 'theoretical' standpoint than necessarily about any specific problem or how to fix it. I think the underlying concept I'm having trouble with is these two seemingly opposing ideas:

1) Tempering is a function of time and temperature (ie, "twice for two hours, cool to room temp in between") and,

2) If you are grinding a blade post HT, and it changes color, you've ruined your temper...this seems to involve almost zero time (ie, make a pass on the belt, dunk it in the bucket, look and see pretty colors you didn't want to see). Along the same lines of this idea are the practices of heating with a torch until the colors run/change to temper, soften a tang, drawback the spine, etc.

I suppose I might be able to find more worthwhile ways to spend my time thinking ;)
 
Hi Paul - time at temp is the problem you're wrestling with. If you have a steel that tempers to 59RC at 450F, heating to 700F will get it to 59RC faster and then it will surpass it. Now, I contend that there's no good evidence to show that briefly surpassing your tempering target will "ruin" a knife, but since you don't know how long it takes at whatever unknown temperature you reached on the grinder, it's best to just try and keep it below your tempering temp.
 
not that it matters much but when I suggested foil I meant as a tent, not a wrap. That way higher temp air can't directly contact the blade
 
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