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Loren Swensen

Trouble heat treating file knives

15 posts in this topic

I have been working on some knives im making from some old Nickelson files. Befor anyone gives me grief over not using known steel, I'm a collage student and can't afford any metal right now and I  got the files for free. So here is what I did. I annealed them, shaped them, then normalized them, and quench them following the normal procedure for 1095. Heat to 1475 F and quench in oil. I have access to a hardness tester and the blades came out arround 68 HRC. All seems to be going well. But here is where I start to be confused.

I tempered them in my kitchen oven at 450 F, two one hour cycles. But the bladescame out blue instead of straw colored. So I tested them again and they were all arround 43 HRC. I don't understand how that happened. How did they get so soft? So i desided to re-quench the blades and did everything the same as before. But this time the blades came out of the quench between 35-45 HRC. What happened?  Im going crazy trying to figure out why im getting these wierd results. First, why did my temper make theblades so soft? And second, why won't the blades get hard again? 

 

Edited by Loren Swensen

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A couple of questions that may help diagnosis:

  1. What kind of oil and what temperature was the oil at each time?
  2. How are you determining what temperature the blank is before quenching?
  3. How did you monitor the kitchen oven temperature (just the dial on the face or an actual thermometer)?  By the color and hardness you got it would appear you got it too hot.
  4. What was the maximum and minimum thickness of the blades before hardening?
  5. How long did you hold the blanks at the pre-quench temperature before the second quench, and how long did it take to get up to temperature (possible decarb if too long at heat)?
  6. Did you normalize again before the second quench?

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Old Nicholson files. If they're American made, they're either C10130 or 1095. If you got full hardness on one shot and not on the next, my guess is going to be that you're testing a spot of decarb. Check the edge with a file (and not another one of those old Nicholsons, some from the early years will cut full hard 1095) If the file is skating, then it's likely just decarb that is throwing the rockwell readings off. Rockwell testers read a puncture hardness where a file will give you a rough idea of the whole edge surface. Then get an oven thermometer. Ignore tempering color. Color is a guess and can be skewed by contaminants on the blade such as quench oil, finger prints, etc. Most ovens will be 10-15 degrees hotter than what they're set at and will often overshoot the temperature coming up. And 450 is too high for 1095 to begin with. 

Edited by Mark_Bartlett
typo
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Mark and Dan are spot on.  Your oven is running way hotter than you think it is, and temper color charts are not accurate.  Full blue means you got up around 500, unless you left oil on it, which will blue a blade at 350.  Which, by the way, is where you should have been. 

Finally, on the not hardening the second time: How long did it take you to get from the fire to the oil?  1095 MUST go from 1450 to below 900 in under one second.  If you miss that window it won't fully harden, with the degree of not hardening depending on the amount of time by which you miss the window. 

Oh, and I don't think anyone is going to give you grief about using USA-marked Nicholsons.  I just scored a pile of worn-out Black Diamonds myself!

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Thanks guys! Dan, I don't know what the oil is. I know its an actual quenching oil, but I don't know what kind. It was around 80 F both times. I use a Lusifer kiln with so I'm just trusting its calibrated correctly. I monitor my oven temp using a thermometer inside. Largest blade is 1/4 thick, the thinnest is 1/8. I brought the kiln up to temperature, put the blades in, and let them soak for 15 minutes.  I normalize the blades the first time but not the second time.

Does decarb happen just on the surface? Also, is their a limit on how many times you can re-heat treat a blade? 

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Bring the oil up to around 130 degrees, it makes for a faster quench.  Decarb can get pretty deep, and kilns are bad for that because of the oxidizing atmosphere.  Anti-scale compound really helps with this.

And there is no limit, but every time you do it the decarb gets worse.  Thus anti-scale compound again.

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There are three options off the top of my head.

1095 is a hypereutectoid steel, so if your normalizing cycle didn't do its job, the carbon won't be evenly distributed and instead be in little clumps of carbide, so that could be your problem.

Or, your oven is tempering much hotter than you think it is.

Or, you are measuring in a spot of decarburization. If this is the case, then you should be able to grind through the decarb and get a reading of about 58hrc

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There is no commercial quench oil that is capable of yielding maximum hardness in those sorts of steels except for Parks 50. If you're using Parks 50, DO NOT HEAT IT. Parks has a much lower flash point than most quench oils and actually provides better results at room temperature. Manufacturer recommendation for initial temp is from minimum 50 to a max 120 degrees F. It's one of only a few oils that it's not recommended to heat. And it's the only one I'll recommend for 10XX and W series steels. 

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Everybody stole the words out of my mouth. Your kitchen oven is running hot and you most likely have decarb. For tempering in a kitchen oven I always fill a drywall pan with sand and let that come to temp and then stick my knives in the sand to temper.

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I agree with all said. 

 

You didn't say if you preheated the tempering oven.  If not, it's possible that you had hot spots in  the oven while coming  up  to  temp that could  effect the temper.

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Good point, Mark.  Parks 50 clearly says room temperature.

I just had slower oil on the brain. I just scored some Parks AAA that clearly says 120-140 degrees, since it's a much slower oil. It is for 5160 and O-1, which have a much longer amount of time to get from critical to Ms and still fully harden.

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Parks is kind of an oddity. It actually slows when you heat it. Some believe that if you heat it it gets faster, but it doesn't. And some even the opposite that if you heat it, it slows enough to quench slower quench steels which is also incorrect. I had a brief conversation through emails with Kevin Cashen about it and he told me that while slow quench steels like 52100, 5160, 80CrV2, and O-1 will harden in Parks 50, they won't be as tough. 

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This one has me scratching my head.  Steel: old Nickolson, as quenched hardness: 68 HRC.  This right here let's us know your blades most certainly fully hardened, and decarb isn't an issue.

1095 that has been hardened to industrial standards, tempered to 450° should be 61-62 HRC.  I'm not sure how hot you would need to temper 1095 at to reduce the hardness to 43 HRC, my chart cuts off at 650° Fahrenheit which results in 53-54 HRC.... Did you temper them at 450° Celsius by any chance, because that would probably make sense.

A couple possibilities.  

Your kitchen oven is off by 250°+, or it's metric...

Your as-quenched Rockwell test was off....

I'm at a loss, really.

 

 

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6db.jpgAnother possibility: your files are case-hardened, and your first Rockwell test hit the outer skin, the subsequent tests hit the soft core.  This requires Nickolson to at some point to have made case-hardened files, which goes against everything I have heard about Nickolson files....

Edited by GEzell
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