Ramping during stress relief cycles.

HPD

HPD

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I'm probably being paranoid, but for some reason I was being a space cowboy tonight and ramped my blades from room temperature to 1200F for a stress relief cycle, not considering radiant heat. The oven ramped up pretty quickly, probably in less than half an hour, and then I soaked the blades at 1200F for a little over an hour. My concern is that radiant heat could have caused problems. I know the TC is covered with ceramic so it buffers radiant heat and therefore the temperature can be a lot higher when the coils are glowing. I did cover my blades with ATP-641 to prevent possible decarb and scale (I didn't want to use my expensive foil for a sub-critical operation), so that probably helped. In the future I will eliminate any ramp or ramp from a higher temperature. So two questions: 1) Did I damage the blades? 2) How do YOU heat your blades for stress relief cycles?
 
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I place a used piece of foil over top of the blanks now as a radiant shield but use not to and I never saw a difference in hardness even with Magnacut.

I also like to place the bevel edge down during stress releaving.
 
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DevinT said:
Nothing happens to those steels unless you get to over 1475’f.

Hoss
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Thanks, Hoss. Am I to assume the radiant heat never got over 1475? The oven reached temperature pretty fast.
 
HPD said:
Thanks, Hoss. Am I to assume the radiant heat never got over 1475? The oven reached temperature pretty fast.
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What brand of oven are you using?

Who is alarming everyone about radiant heat?

I doubt that any radiant heat would be ~300’f greater than what the TC is reading.

Steel takes time to heat through.

Hoss
 
I've been reading older posts. A little knowledge is a dangerous thing which is why I don't like to make assumptions and asked. I'm not going to sweat it now. I'll probably just follow Seedy's advice to sheild the blades next time even if it's not necessary.
 
Larrin said:
Radiant heat is a boogeyman. The thermocouple is also seeing the same radiant heat. It just causes people to imagine problems that aren’t happening.
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For some reason when you said “Radiant heat is a boogeyman” I flashed back to Heat Transfer class twenty some odd years ago. I can think of far worse things to call it. I’ve never worried about it in my oven though…
 
HPD said:
I've been reading older posts. A little knowledge is a dangerous thing which is why I don't like to make assumptions and asked. I'm not going to sweat it now. I'll probably just follow Seedy's advice to sheild the blades next time even if it's not necessary.
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I keep an un-shielded TC in my oven that responds fairly quickly to confirm the oven’s TC and I’ve never noticed it getting too far out of line while my oven is ramping. Certainly not enough to matter.
 
I stayed out of this so far. but I always consider radiant heat a myth. The TC gets just as much of it as the blade. The heat isn't a laser beam focused on the blade only. Can there be some degree of variability on the chamber during heat up- Yes. Is it hundreds of degrees - NO.

If you pre-heat the entire discussion is moot.
 
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FYI, I recently purchased a Grizzly hardness tester. The 61.8 test block it came with isn't as consistent as my blades (which have been pretty consistent) so I took 10 readings and averaged them. I took five samples before and after my blade readings. I even threw out the high and low for each set of five and compared those averages to the five sample averages, and the numbers were close. The before and after 5 sample averages were also close to each other, so I used the 10 sample average to calculate the correction. I got a correction factor of .72 Rc by doing all of this. I then tested an old AEB-L blade that I never finished. I took five readings off that blade. I also tested one of the AEB-L blades I finished yesterday that I was concerned about. I used identical temperatures and tempers for the heat treatments, as well as identical cold treatments. Both blades were similar thickness, .12 and .125 respectively. The only difference was I now use a carpenter's vice for my plates and get quicker quenches (and straighter blades). The old blade tested 61.5 after correction, and the readings were tightly grouped. The new blade corrected to 61.7 (62.3, 62.2, 62.7, 62.6, 62.25). I was worried about nothing.
 
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Update. The other two aeb-l blades tested at 62.4 and 62.3 Rc. The CD#1 blade tested 61 Rc.
 
Not sure how much you’ve messed with a hardness tester but whatever you’re testing needs to be flat and parallel with preferably about a 400 grit or better surface ground finish. Any angle or warp will affect your reading to some extent depending on severity. Decarb is also extremely soft so it needs to be ground off as well. The grizzly is more of a shop grade tester which is perfectly fine for knife making but I wouldn’t expect my readings to by right on top of each other with it. Precision costs a lot of money in hardness testers. The correction factor you mentioned is probably normal for that machine and that amount of spread is meaningless to the end user of the knife. I generally test 3-5 points per knife and if they all fall between 60-61 I call it 60 and go on with life.
 
ScarFoot said:
Not sure how much you’ve messed with a hardness tester but whatever you’re testing needs to be flat and parallel with preferably about a 400 grit or better surface ground finish. Any angle or warp will affect your reading to some extent depending on severity. Decarb is also extremely soft so it needs to be ground off as well. The grizzly is more of a shop grade tester which is perfectly fine for knife making but I wouldn’t expect my readings to by right on top of each other with it. Precision costs a lot of money in hardness testers. The correction factor you mentioned is probably normal for that machine and that amount of spread is meaningless to the end user of the knife. I generally test 3-5 points per knife and if they all fall between 60-61 I call it 60 and go on with life.
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I'm aware of the machine's limitations. I plan on getting a better test block and that should help. I sanded to 400 grit and kept everything flat; everything tested out to to around the target hardnesses for the temps used, so I'm fairly certain I'm +/- 1 point and probably a lot closer. More than anything the tester is just going to be another way to keep things consistent and make sure the oven is working. Honestly, I wouldn't feel good claiming "61.7" or "62.3" on my blades. I would probably say something like, "61-63" for the aeb-l blades and "60-62" for the CD#1 blade. I think that would be honest and fair.
 
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HPD said:
I'm aware of the machine's limitations. I plan on getting a better test block and that should help. I sanded to 400 grit and kept everything flat; everything tested out to to around the target hardnesses for the temps used, so I'm fairly certain I'm +/- 1 point and probably a lot closer. More than anything the tester is just going to be another way to keep things consistent and make sure the oven is working. Honestly, I wouldn't feel good claiming "61.7" or "62.3" on my blades. I would probably say something like, "61-63" for the aeb-l blades and "60-62" for the CD#1 blade. I think that would be honest and fair.
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I think showing a range is fair. That’s why I generally go with the lowest readings when I test. If I was shooting for 61 and I tested between 60.3-61 I’d call that good at 60. Between my SGA not being as good as a machine shop grade surface grinder and using a lower tier hardness tester like the Grizzly, I figure that’s about as close as I can expect to get to my hardness target number. We spent hours prepping our samples in materials lab back in the day and were using lab grade hardness testers. If they didn’t read almost spot on you knew you’d done something wrong somewhere. If it was way off you generally had bad heat treat. If it was “close” you generally hadn’t prepped the test coupon properly.

Main thing is now you know you’re getting a good hardness in your blades. If you aren’t doing so already, make a couple of test coupons and run them through your heat treat process first when using a new steel or a different thickness than you can normally use. Use them to dial in your HT recipe before you start cooking blanks. I do that and then break the pieces to check grain structure anytime I change something up. That has saved me a lot of headaches on a few occasions.
 
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