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

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  #16  
Old 09-29-2014, 12:16 PM
Kevin R. Cashen Kevin R. Cashen is offline
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Unfortunately a post temper Rc reading alone lacks any reference point from which to evaluate your heat treatment. As was touched on earlier, if your as-tempered blade reads 59Rc but your as-quenched blade was only 62Rc when it should have been 65HRC, you will never know that your hardening operation needs some adjusting. Tempered martensite makes a great knif,e a tempered pearlite/martensite mix is not going to quite cut it in comparison (pun intended). But if you have the numbers to show you have a 59-60HRC post temper and that is after a 65HRC as-quenched, you have all the information to be confident that you are at least nailing the hardness aspects.

I say ?at least the hardness aspects? in order to emphasis that no one test will tell you everything you need to know and the more stand-alone the test is the less it tells you. Enlarged grain will harden more deeply so even great Rc numbers can still occur in a overheat situation, but if you know your grain size is good, you as-quenched numbers are good, and your post temper numbers are where you want them, then you are on a pretty good heading. Also the more HRC tests you do the more accurate they are as the number should always be an average. I always do at least five readings and average it for the final number, if there is a range the average helps, if there is no deviation in the five you give yourself a pat on the back and proceed with a good feeling about the whole process.

Edited to add- oops there were two more posts in the time I was writing mine and the quality of both of them was such that it tells me I am not really bringing much more this conversation as you folks got it pretty well figured out, and I am just being redundant.

Last edited by Kevin R. Cashen; 09-29-2014 at 12:19 PM.
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  #17  
Old 09-30-2014, 09:23 AM
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Don Robinson Don Robinson is offline
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That is the result of testing on the edge. As someone else pointed out, the Rc test must be made on parallel surfaces. Otherwise this can happen. Also, the test result will not be accurate if done on a surface that isn't square to the penetrator. That's why it showed softer than it should.

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  #18  
Old 10-01-2014, 07:52 AM
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Andrew Garrett Andrew Garrett is offline
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This thread has led me to the conclusion the edge hardened (clay-backed) blades cannot be tested accurately with a standard tester. Back to files.

Thanks all for the lively discussion.


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  #19  
Old 11-06-2014, 06:07 AM
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Why not take a second piece of the same steel as a "control" sample? Grind it to the same thickness and finish as the area on the blade you want to test, only with parallel sides. Heattreat at the same time under the same parameters (normalizing, clay coating, quenchant temps, etc) and do your testing on the "control". You should get more consistent and reliable data from the tests and thereby know more about what you think is going on with your HT.

Side note: Have to agree with the others on the grain depiction in the original pic. When it manifest that bold something got a little too hot. Bonus that steel should is still salvageable with some careful thermalcycling.

Never trusted the files for accuracy after the first couple of uses, too many human error variables that are effected on the process from day to day. Mental temperament being just one of them.
Example: Just prior to testing you manage to step in your neighbor's dog's deposit in your yard.... or the other neighbor's goat is in your garden eating the last of your prized okra.... or the boss just cancelled your long awaited vacation...
Any of these will affect just how hard you apply the files to the steel.....blown test results.


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  #20  
Old 11-06-2014, 11:27 AM
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I considered that approach Crex, but decided that their are still too many variables, to assume one piece of steel will harden exactly like the other. My forge has hotter and cooler zones and two blades cannot occupy the same space.

Because of the way I edge harden, I see no other option than file testing.

Note: Since the failure, I destruction tested another two blades to check grain structure. On those, it was creamy and pale gray and I couldn't even detect a graininess. Hence, I am forced to conclude that the failed blade that prompted this discussion did indeed get too hot. It had just been so many years since I had a failure that I did not recognize the large grain when I saw it. I'm glad it failed.

I have also put together a much larger peanut oil quench tank with carefully controlled temperature (130*). End result: This event and this discussion has inspired improved HT in my shop. I thank all who participated for that. It is easy to get complacent with certain procedures over the years. Sometimes we just need a kick in the butt to get refocused.


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  #21  
Old 11-07-2014, 06:45 AM
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As long as you feel your HT process is consistent all the time, just do a few test pieces as described, separately. It will give you an idea on what kind of hardness you are attaining from your process.

Don't fret about it too much....most of us don't know what grain enlargement problems are from watching U-tube....
Most the time when someone ask me about "how I Know", I respond with "I read a lot" (which interprets into - Been there Done it. This kind of stuff happens so we can learn and refresh our procedures - a good thing all considered.

I'd be focusing on getting rid of the hot/cold zones in the forge, even if it requires building a separate forge for HT purposes or get an oven.
Good thing on the quehchant, larger controlled heat volume supplies greater median temp control with multiple quenches and bigger blade mass = better consistency.


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  #22  
Old 11-07-2014, 07:34 AM
WBE WBE is offline
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Hi Crex. Go ahead and get an oven. You will wonder how you ever did without it, and it opens new possibilities in HT.
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  #23  
Old 11-07-2014, 09:52 AM
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Andrew Garrett Andrew Garrett is offline
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Actually, I have a Paragon KM-24D that I use for stainless and air-hardening steels (which I choose to plate quench).

I have two forges: a Johnson Gas pedestal model that I picked up from Bob Warner in Texas years ago and rebuilt. I have never used it, lol. My other forge is my favorite. A guy on ebay used to sell them. He called it a 'Poor Boy'. It's a small atmospheric model with two burners. It's super easy to use and is far quicker than the oven.

For the record: Anyone want to buy a nice forge and maybe a Rockwell hardness tester?


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Last edited by Andrew Garrett; 11-07-2014 at 09:58 AM.
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  #24  
Old 11-07-2014, 08:54 PM
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WBE, I have both a Paragon and a separate tempering oven (was a suggestion for Andrew, which he has addressed).
But for what it's worth.........I wonder about a whole lot of things.

I think if we do something a lot, we get complacent about procedure and need a goof to shake our beans up a little. Happens to all of us.


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