Quench issues

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EthanWS

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Feb 27, 2024
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I just recently started forging a couple knives (with relatively no experience).They were looking great until I tried to quench. I made sure to check if the farrier’s rasp I used was good. It snapped cleanly and had fine grain, but they would not harden in the canola oil. I tried heating the oil, but they were still soft. I finally tried quenching in water to see if the rasp was the problem and it hardened just fine. I don’t know if my oil is bad or something else is the issue. Should my blade and oil be at specific temperatures? And what oil would you recommend for a farrier’s rasp?
 
Canola oil is not a good quench oil. Get some Parks 50. “Knife steel nerds” did a YouTube video about it.

Hoss
 
The rasp is likely a shallow hardening alloy that requires a fast quench. You should get a couple gallons of Parks #50 if you plan on making knives regularly.

An alternative for oil on simple steels is a brine quench. Dissolve .75# of rock salt per gallon of water. I recommend making several gallons of brine in a drywall bucket, so use 3# salt and 4 gallons of water. Warm the brine to around 120°F before the quench.

Note:
Don't automatically assume the blade didn't harden just because a file digs in and doesn't skate. If the blade was forged, and the HT done in a forge, there can be a significant layer of decarb on the blade. File the edge five or six good hard strokes and see if it doesn't suddenly start skating and the sound pitch goes way up.
 
JoeBusic said:
What are the benefits of warming?
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Brine is a faster quench than oil, so I'm going to say that by warming up the brine, you'll slow the cooling rate just enough so that it doesn't crack the blade.
Please correct me if I'm wrong, thanks.
 
weo said:
Brine is a faster quench than oil, so I'm going to say that by warming up the brine, you'll slow the cooling rate just enough so that it doesn't crack the blade.
Please correct me if I'm wrong, thanks.
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Yes, it will be slower because of the vapor layer. Salt slows the forming of the vapor layer. So heating it would kill the benefit. If one want's slower, then pure heated water is just OK. Salt is to speed things up. And cooler the faster. Vapor layer is the enemy because the quench could be uneven. If it's to fast for one's liking, oil is in order. Of course there could be a benefit (thinking about it) that heated brine combined with vigorous movement in it of the blade could slow things up and maybe negate the unevenness of the quench. Is it? That's my point of asking.
 
I used canola oil as a quenchant for a while. However, after doing a fair bit of research, I lost confidence in it and decided to take the plunge and invest in Parks 50. It works very well and takes a lot of the guesswork out of the equation.
 
JoeBusic said:
Yes, it will be slower because of the vapor layer. Salt slows the forming of the vapor layer. So heating it would kill the benefit. If one want's slower, then pure heated water is just OK. Salt is to speed things up. And cooler the faster. Vapor layer is the enemy because the quench could be uneven. If it's to fast for one's liking, oil is in order. Of course there could be a benefit (thinking about it) that heated brine combined with vigorous movement in it of the blade could slow things up and maybe negate the unevenness of the quench. Is it? That's my point of asking.
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Don't be afraid to do some testing.
 
JoeBusic said:
Yes, it will be slower because of the vapor layer.
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It (120F brine) will also be a little slower than room temperature brine, just because of the higher temperature.
I think this is why Stacy suggested heating the brine.
 
120°F is the sweet spot between broken blades and whole blades.

There is a lot more than just cooling a blade down in a brine quench (or any quench) with a shallow hardening steel like the 10XX series steels.
If all that mattered was getting a blade cooled fast, quenching in liquid nitrogen would be superb ... but it isn't!

You would think that a cooler bath would prevent boiling and make a faster quench. However, boiling starts the second the blade enters the bath at 1450°F ... long before the blade gets near the 120°F bath temp. The boiling is part of how the blade cools. What we want is to collapse the vapor jacket around the bade as the thin film of water vapor envelopes it. That is what the salt does. I'm not going into how it helps collapse the vapor jacket, just know that it does. The right amount of salt is a 9% brine solution.

In quenching a 10XX steel blade:
First, you have to cool the blade fast enough to miss the pearlite nose at 1000°F in less than 1 second. The water does that (with or without salt).
Next, you have to drop down to 400°F at a reasonable rate, but not too fast. Again, the water does that as it boils around the blade. The percentage of salt controls how large the vapor jacket is and affects the cooling rate.
Finally, you have to go through the austenite to martensite transition between 400°F and 200°F without the blade tearing itself apart. The salt again controls the cooling rate and allows an even cooling without too much stress.

The problem is that the stress of the transformation from soft austenite to glass brittle martensite doesn't happen evenly in the blade. The thicker spine cools slower than the edge. First the edge converts to hard and brittle martensite. As it does this, it expands because martensite is less dense than austenite. The austenite spine is quite rubbery and compresses easily as the edge expands. When the spine converts a moment later it expands and tries to slightly bend the already brittle hard edge. If this happens too suddenly, the edge forms a small crack, which then propagates up the hardening blade to the spine and breaks the blade in half. Even if the crack does not break the blade in half, the blade is ruined.
That split second between the hardened edge and the hardening spine is when the warmer bath helps. It slows the rate just enough to allow a small amount of auto-tempering of the edge to prevent the cracks from forming.
 
Stacy E. Apelt - Bladesmith said:
That split second between the hardened edge and the hardening spine is when the warmer bath helps. It slows the rate just enough to allow a small amount of auto-tempering of the edge to prevent the cracks from forming.
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This is what I needed. I forgot about the geometry. I grind after HT and I forget things I don't need (too much on my mind already).
 
DeadboxHero said:
Was there any testing that generated data to come to that conclusion or are you just using your intuition?
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Screw intuition. I don't believe anyone who thinks intuition helps. A lot of testing. I grain refine extremely and things change a lot after that. With cold water I couldn't achieve more than 57HRC with 1060, so I had to go faster. Brine helped to get it to 65HRC but after 3 quenches again was not fast enough. In the end I had on occasion 2 buckets of brine and never quenched more than 4 knives a day. But went for stainless only, so all that is now history. Water and oil is not my thing any more. Except for titanium. Cold brine only.
 
Over quenching and too much grain refinement may make the steel so shallow hardening it won't harden without resetting the structures with a complete anneal and then re-growing the grain for a fresh start.
 
Stacy E. Apelt - Bladesmith said:
Over quenching and too much grain refinement may make the steel so shallow hardening it won't harden without resetting the structures with a complete anneal and then re-growing the grain for a fresh start.
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For some, it is advantageous. Another way to get a hamon. If bevels are ground before HT, it would be differentially hardened. Thick part ( I used 6mm) would be in lower 50'sHRC and the edge would get full 65HRC. And strength and toughness rises.
 
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