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That's sort of how I see it too.  My toaster oven does not overshoot, though, ever. I have a thermocouple in it because the dial is not at all accurate.  It's also full of hard firebrick.  I bring all that mass up to heat (usually 350-400 F), let it stabilize, and when I put a blade(s) in it it doesn't even flinch.    The temp might drop ten degrees with the door open, but it never goes over the high set point.  It will drop up to 30 degrees below that point, though.  %0 without the brick chunks in place.  I do the preheat because I figure the radiant heat off the elements during initial heatup is far more likely to stress and overheat thin parts than just plopping room-temp steel on a hot brick.  

 

I've used the Evenheat to temper stainless folder springs, since the toaster oven can't handle the 1140 degrees F required...  And also since it takes the thing too long to cool down from hardening heat to 350, but it gets down to 1100 by the time the springs are ready to come out of cold treatment.  But stainless isn't O1.  

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5 hours ago, Jerrod Miller said:

This is not normalizing.  Normalizing requires a phase change.  Keeping it below critical will help with stress relief, but won't do much for grain size reduction.  

Quite right. I was typing slower than I think. Normalizing O1 is at roughly 1600*F  followed by a stress relief at 1200-1250*F.

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6 hours ago, Jerrod Miller said:

This gets you a little bit of blue brittle martensite, which is bad.  If you don't harden it you get a phase change gradient, which isn't too bad at all.  Metallurgically speaking, your way is worse

Jerrod, this is not meant as a disrespect, but I believe it was Kevin Cashen who said "One test is worth a thousand expert opinions"

 

Proof of concept-process (2) V2.jpg

 

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Your proof of concept/proof of process picture doesn't show anything to prove that it was good, let alone better than differentially hardening.  Also, you skipped out on quoting the very next sentence from my post:  

 

6 hours ago, Jerrod Miller said:

Practically speaking though, I would doubt that it is significantly, or even measurably different.  

 

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1 hour ago, Jerrod Miller said:

Your proof of concept/proof of process picture doesn't show anything to prove that it was good, let alone better than differentially hardening.  Also, you skipped out on quoting the very next sentence from my post:  

 

 

The point is you said “a little bit of blue brittle Martensite, which is bad” and then went on to say the extra bit about marginal difference.

 

my point is this: what do you base this assessment on? Is it empirical test data you have done, witnessed, and repeated?

Let’s start with this

what is the purpose of differential hardness in the tang area to begin with?

Edited by Joshua States
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I'm thinking this is getting a bit off topic for this thread so if we really want to go further on this topic we should start a new thread.  But to put it bluntly:  Yes I have seen temper embrittlement and have broken the test bars (tensile, Charpy, and bend, as well as some actual broken parts) that failed with poor results due to it.  Dozens of them.  And only dozens because it is well understood by every metallurgist that it happens and we try to avoid it.  It only occurs when something went wrong in the process.  Could be the wrong HT program was used, bad weld procedure, or any number of operator errors.  Nobody in their right mind that understands that temper embrittlement is a thing would argue that it is not a bad thing.  The question is just "how bad is it?" for any given scenario.  And as I have said here, in this scenario: probably negligibly bad.  And that goes for it being anywhere.  Blades, tangs, gears, pry-bars... anything.  The point of a differential hardness in parts is to prevent catastrophic failure (e.g. a crack starting on an edge but stops in the softer spine, preventing the whole blade from breaking), and/or leaving part of the steel hardened, and part of it in a softer state for further working (e.g. drilling, peening, etc.).  So for any reason one would want a differential hardness they can achieve it by either through hardening and tempering part of it back, or by preventing the intended soft part from hardening in the first place.  The latter is metallurgically more sound.  Depending on circumstances, either method may be more practical.  

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I have done my own tests, although they were specific to knives and the tangs. My mentor told me to do the testing myself. He recommended through hardening and draw back, rather than differential hardening, but encouraged me to do the testing myself. I did just that and would encourage all makers to do the same. Do two basically identical knives. One method on each and see which one resists deformative failure better. Don't take anyone's word for it. Do the test. Repeat the test. Make a choice.

I have made mine. I choose tempered martensite.

Edited by Joshua States
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