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Rockwell hardness question

HPD

HPD

Gold Member
Joined
Dec 26, 2015
Messages
201
I'll try to make this as clear and concise as possible. I heat treated three blades, one blade each of CD#1, AEB-L and A2. I tempered the .108" CD#1 blade at 300F twice, the .091" AEB-L blade at 300F and 330F, and the .150" A2 blade at 300F, 330F and 400F. All tempers were two hours. I took over 10 readings with my test block and I got an average within .2 HRC of the nominal value of the test block. My average readings for the blades are 64.3, 63.95, and 64.43 respectively, way higher than anticipated. I can think of bigger problems to have, but I'm curious about why this happened. I now use a spring loaded carpenter's vice with my aluminum plates and compressed air, and I was very quick getting all the blades into a dry ice slurry that was at -103F. So my question is this: is it possible to get readings this high or is something else necessarily going on? I know something else could be going on, but I polished the blades to 600 grit and cleaned them before testing. Two of the blades were not perfectly flat, but were close (I have a granite surface plate and feeler gauges) and I think I compensated for that. The AEB-L blade was flat ground by Alpha Knife Supply and was dead flat. So is it possible that I just cooled the blades fast enough to get those readings? Any thoughts? It's not really a big deal but I'm curious.
 
Seedy Lot said:
Your getting 64 Rc with CD#1?
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Unless there's something wrong and my test block is off, I got 64 HRC with CD#1. It was thin stock and I was super quick with the quench, and I only tempered at 300F twice for two hours each time. I soaked at 1950F for 25 minutes. I was expecting 62 to 63 with those numbers.

I've heat treated more AEB-L than anything else and I get 61.5-62.5 when I use 1950F with a 15 minute soak and 330F tempers, but this was the first time I've used a carpenter's vice in combination with a dry ice slurry, and this was also the thinnest stock I've ever used by far. So that number isn't super surprising to me. Also, I got absolutely no warping with any of the blades which suggests the contact with the plates was rock solid. I manned up on the vice and clamped hard.

The A2 is also throwing me off; however, in the past I have gotten 63 HRC with thicker A2 stock tempered at 400F. So maybe my numbers are legit. I soaked it at 1775 for 30 minutes.

My tester is not in a climate controlled room. However, it was within the approved temperate range when I tested, but it was about 90F. Maybe that skewed the numbers up a bit as I usually test when it's 60-80F.

I might retest when its cooler, or I might just use the knives before attaching the handles and if they don't chip with normal use just run with it. I haven't decided.
 
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Seedy Lot said:
Your getting 64 Rc with CD#1?
Click to expand...
I retested my test block and it is about a point higher than it should be. I also retested my CD#1 blade and it's still reading 64 HRC. I also retested a CD#1 blade that I had previously tested when it was cooler, and it was also about a point higher. So I'm going to say I got 63 HRC with my heat treatment. Still pretty good and I'm going to keep the CD#1 and AEB-L blades as they are. The A2 I will probably do as Devin Thomas suggested and bump up the temper by 25 degrees. Quench speed and stock thickness matter.
 
Personally, I feel your temper is about 50° too low. I would do 350° as a minimum. Hardness is great, but not at the expense of a chippy edge.
 
Are you using a bench HR150 style Rockwell tester? Just asking as even when I have tried to get higher CD#1 hardness I always seem to end up with around 62 Rc.
 
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With certain steels there is a bump in hardness with a low temper. I think it’s a little less than 300’ but maybe you hit that peak.

Hoss
 
How did you measure the tempering temperature?
And what was your pre-austenising processing of the steels?

I would just temper a little hotter, but i don't chase precision
 
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Alex Topfer said:
How did you measure the tempering temperature?
And what was your pre-austenising processing of the steels?

I would just temper a little hotter, but i don't chase precision
Click to expand...
I tempered using my heat treating oven after it cooled down. I filed the bevels for the CD#1 blade before heat treating. For the other two I started the bevels to set my angles but removed very little stock. I did no stress relief step.

I'm going to try the CD#1 and AEB-L blades as is. If they chip they chip. Both steels have very good toughness so maybe they'll hold up. A2 is less tough so I think I'll take the advice to heat treat a little hotter. I think this is an interesting subject, nothing critical. I do this for fun.
 
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DevinT said:
Some heat treat ovens are not accurate at tempering temperatures. They are designed to be accurate at higher temperatures.

Hoss
Click to expand...
Thanks. I didn't think of that [edit: I didn't know that]. I have a thermocouple probe rated for the purpose. I will test and let everyone know my results.
 
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DevinT said:
Some heat treat ovens are not accurate at tempering temperatures. They are designed to be accurate at higher temperatures.

Hoss
Click to expand...
Bingo. The 300F setting tested 262F and the 400F setting tested 359F. I allowed the oven soak over 20 minutes at temp before taking the readings. I'll just bump everything up 40 degrees.
 
Well, it turns out the CD#1 blade is still 63+ even after retempering at a corrected 300F. The AEB-l is at 62.3, about right for an actual 330F temper. I'm currenty cooking the A2 at corrected 400F. In the past I've usually tempered with just the toaster oven. Thanks to all who helped me. I appreciate you guys.
 
Use an oven thermometer. That way you know what is happening inside. Digital read out is not always accurate.
 
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