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Magnacut Preheat/Ramp Time?
- Thread starter ak650
- Start date
- Joined
- Feb 7, 2023
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- 254
I’ve had a bit of mild sticking in the past nothing bad but the last round of heat treating it stuck like crazy. My only thought was I washed all the blades with soap and water before wrapping and heat treating. Maybe the cleaner surface helped adhesion.
My metal was super clean as well as being surface ground/smooth.
Sanded the surfaces of the small blade and coupons flat & smooth to 240, and tested on the Ames Model 1. Took measurements at the tip, middle( this one on the opposite side ), and tang, and got what I would call 61( it was over 60 and under 62 ) consistently. Same on both coupons. Did a warmup on a piece of scrap, then got a good 62 on the factory 62 test block, and it repeated at 62 on the test block after checking the blade/coupons. Pretty confident in the recipe I used now, and will do the wife's future kitchen knife heat treat tomorrow. In the mean time, making a stand to hold the Ames, which should free up a hand, and keep the instrument at a consistent angle at all times. The factory stand is damn near $400, so I'll gladly spend a couple of hours making one.
Thanks for the tip Hoss! Did another blade today( another Magnacut ) and gave it a rub with soapstone, as I have plenty in the welding cart. The foil peeled right off, easy as you please.
Interesting find, when I checked the hardness of this blade. It was about as long a blade as I can do in my oven, and the tip was about and inch or so from the back wall, in the gap between the heating elements. So, diagonally, the tip of the blade was about 1-1/2" from the elements. Measured the hardness at the handle, mid blade, and the tip. The handle and mid blade were the same 61( which is what the data sheet predicted for a 2000°F/350°F/Dry Ice recipe ). The tip was a solid point higher at 62, maybe even a smidge over. Apparently a little hotter there than the 2000°F that I was set at. The data sheet predicted 62 for a 2050°F austenizing temp. Down the road, I can see buying a knife oriented oven( mine is geared towards machine shop type use, which is what I originally bought it for ). The new Evenheats look like they only have elements down the sides, and none on the back wall, which would likely help prevent this kinda thing.
- Joined
- Feb 7, 2023
- Messages
- 254
It could vary that much or not it’s hard to say there’s a lot of factors that influence the results from the hardness tester. Small warps can push a result higher or lower, a patch of decarb. I’ve seen it vary from side to side of the blade. If the blade isn’t flat and parallel the results can get skewed.
- Joined
- Mar 22, 2014
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- 5,366
Yea, decarb removal, surface finish and flatness always have to be ruled out first before making conclusions about hardness readings.
Having used an Ames, the resolution for readings is lower along with lower repeatability and reproducibility meaning readings within 1-2rc could be just be scatter.
Having used an Ames, the resolution for readings is lower along with lower repeatability and reproducibility meaning readings within 1-2rc could be just be scatter.
Agreed. I really don't see much point in even taking a reading unless the test area is decarb free, flat, and polished. And with the Ames, being aware of being consistent with how you manipulate/hold the instrument, viewing the dials, etc...Yea, decarb removal, surface finish and flatness always have to be ruled out first before making conclusions about hardness readings.
Having used an Ames, the resolution for readings is lower along with lower repeatability and reproducibility meaning readings within 1-2rc could be just be scatter.
- Joined
- Mar 22, 2014
- Messages
- 5,366
Well that's the problem. Nobody aware of best practices knowingly tries to test on decarbed, non-flat, unfinished surfaces.
What happens is a lack of ruling things out when unusual readings come up and jumping to conclusions.
Rule out = process of elimination
When unusual readings occur, there should be a checklist that goes through a person's mind of eliminating the most common possibilities which will only leave one possibility.
It's pretty difficult to get things really flat in practice.
Before I got a surface grinding attachment, it was difficult to get pieces flat, there was some extremely shallow, hidden convexity which was having an effect on the readings.
It was "flat" but wasn't flat.
Even the backside of the part facing down on the test anvil has to be flat. I had a friend send me some pieces in CPM 4V for hardness testing.
He was quite perplexed and was getting strange hardness readings that didn't follow the typical hardness curve.
All this despite the fact of having a beautiful surface finish from a large surface grinder with a 320 grit finish (well beyond the 120 grit minimum)
What was going on?
Was his hardness tester broken?
Was there something wrong with the steel?
Was there something wrong with the heat treatment?
Was there something happening that changed fundamentally our understanding of how things work?
Nope. Rule it out.
The part that faces the anvil still had mill scale on it and was not completely flat which was throwing off the readings.
Doh
Once the back side was surfaced and flattened the hardness curves lined up perfectly.
Absolutely. I'd add that ideally, the surfaces should also be parallel. A small surface plate is a big plus to have in the shop, along with an indicator and height gauge; for checking flatness and parallelism. I also do a confidence check on a test block (closest in hardness to the items under test ) both before and after a series of measurements.