Heat Treat Problem - Help Requested

M

MichiganMike

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Feb 2, 2019
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Yesterday I heat treated six Horace Kephart five inch knives. They were all made of 1084. I use a temp controlled electric furnace. The temperature was 1475 degrees F. After coming up to temperature I quenched them in Parks AAA, which has given me excellent results in the past.

After cleaning them up, the went into our kitchen oven at 400 degrees for two hours. I removed them and cooled them to room temperature and back in the oven again. Here is where the problem lies. Either I left them in the oven too long, or left them in the oven and turned off the oven. The end result is that I have six blades that are too soft. They barely rate 50 Rockwell using the Tsubonsan hardness testing files.

My question: Is it possible to run these blades through the heat treatment process again and get a good result? Thank you for any help you can provide.

MichiganMike
 
My thoughts:

Did you normalize and thermal cycle? Some steel comes in a spheroidized state and needs to be normalized and thermal cycled. This could possibly be the culprit.
Did you grind through all the decarb? You might get a false hardness reading if there's still decarb.
Could it be some other steel other than 1084? I've mislabeled a couple of bars of steel in the past myself.
 
Removed them and let them cool to room temperature…..that is probably a contributing factor there. At the end of each tempering cycle, regardless of steel, I’ll dunk in water to stop the tempering process so to speak. Otherwise, if you just “let it cool down, it’s annealing the steel to a small extent.

I’d try to reheat myself.
 
Cooling slow to room temp isn't going to anneal the steel. Its cooling from 400, so the tempering is at 400, so getting cooler doesn't soften the blade?
 
M MichiganMike I would suggest you go through a normalizing cycle before trying to achieve austentizing again. Regardless of the issue, the steel is no longer annealed and ready to go through hardening in it's current condition. I don't personally see anything major wrong with your process....so assuming maybe the actual original temp at hardening/austentize would be my suspect. 2nd cycle tempering should not have that kind of effect unless you went well over the temperature setpoint.

So, short answer...yes, you can redo and have good results but I think running a normalization cycle first is wise. Many recipes for that on the internet.

Good luck!
John
 
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When did you put them in the oven to GET them to 1475? Did you wait until the oven had heated up to that temp first?
When I use the oven for practically anything, I let it hit the target temp and then HOLD that temp for 15-30 minutes to equalize the entire oven's mass.
 
godogs57 said:
Removed them and let them cool to room temperature…..that is probably a contributing factor there. At the end of each tempering cycle, regardless of steel, I’ll dunk in water to stop the tempering process so to speak. Otherwise, if you just “let it cool down, it’s annealing the steel to a small extent.

I’d try to reheat myself.
Click to expand...
Yeah that’s not how annealing works
 
I assume you're from Michigan, which is pretty cold right now. Did you pre-heat your AAA? It has a listed minimum working temp of 100F. It also has a max working temp as well.
As others have also pointed out, soak time is also important, not just for the blades, but the oven as well. I let my oven soak at temp for around 45 minutes or more.

If you had a good critical heat and a proper quench, no amount of tempering should drop your hardness to 50HRC.

Did you clean your blades well before testing with the files? You want to remove any scale and decarb and start with a relatively high grit finish.
I have a set of tsubosan hrc files and I JUST got a hardness tester. While these files have a 5HRC range, each file can be +/- 2HRC if I remember the spec sheet correctly, and in practice, I think the "ballpark" range can be even greater than that if you don't understand how they work. A softer file can still scratch the surface of the blade a bit, but it may not be cutting like a harder file would.
I tested a knife on my tester at 62HRC, but was also getting a little bit of bite from the 60HRC tsubosan file, for what it's worth.

After quench, I'd take a regular file and do a skate test. I also put a secondary thermometer in my oven to help monitor temps, and you also want to let your tempering oven soak at temp for a little while. A large thermal mass in the bottom of the oven can also help reduce some temperature swings, though again, even if you overshoot by 100F, it shouldn't affect your hardness to the extent you're seeing.

Anyway, to answer your question, yes you can re-heat treat the blades, though you may want to throw in a normalizing cycle or two.
 
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godogs57 said:
Removed them and let them cool to room temperature…..that is probably a contributing factor there. At the end of each tempering cycle, regardless of steel, I’ll dunk in water to stop the tempering process so to speak. Otherwise, if you just “let it cool down, it’s annealing the steel to a small extent.

I’d try to reheat myself.
Click to expand...
As Scrawnylumberjack noted, that isn't how annealing works. It is technically best to do a quick cooling from temper to room temp when doing tempering, but the difference is mostly time convenience. Without Larrin's lab you could not tell how the steel was cooled between and after tempering.

IMHO, hardness files are not a good way to tell hardness without a lot of experience. Also, you have to grind the edge clean of all decarb before testing.

There are many variable that may be at play with your blades. I would grind the edge area of one clean of all decarb and test the edge again with your files. Then, sharpen the edge at around 20DPS and test it for roll in dent with some bone chops. That will let you know if it is the testing files or the actual hardness.
If you decide to re-harden them, normalize and re-do the HT paying attention to these details:
1) make sure the HT oven has fully heated for at least 15 minutes.
2) Make sure the blades sit in the oven for 5-10 minutes after it returns to the set point.
3) Quench fully in 130°F AAA and keep in the oil until it stops smoking ... around 30 seconds.
 
My two thoughts are first as other have said a layer of decarb. I’ve had some blades with a layer of decarb .02 thick.

My other thought is if you didn’t leave them in the oil long enough to sufficiently cool you could’ve had the blades auto temper.

Example you oil quench but when you pull the blade out it’s still 600-800 degrees and you let it air cool you’d essentially temper them at that higher temperature range.

You can certainly heat treat them again but I would run a DET anneal first.
 
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