Optimum Hardness for A2

redsquid2

redsquid2

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Knifemaker / Craftsman / Service Provider
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I have the batch of blades just about ready to send out to H.T. I am wondering if they all should be hardened the same. I have got these, and also listing their thicknesses:

A hunter EDC .125" th., a couple of thin (.110) skinners, a utility wharncliffe .125, and a bushcrafter .125.

Would all of these be good at 60-61RC? I just think of 60-61, because it seems like that is a popular hardness for lots of knives these days. Would you go softer?

Any feedback appreciated.

Andy
 
I'm also curious what a good hardness is for A2. In my case I am making some big choppers from it. Looking at the data sheet it seems 60 is the sweet spot for edge retention and toughness. Does anyone know if this is correct?
 
Hmmm. Just looked at the data sheet, and tempering chart at alphaknifesupply.com . I don't understand completely, but I guess it is "soaked" at 1750 deg., then tempered at 400 deg., for optimum combination of hardness and toughness, to 61 RC.
 
My Ron Flaherty model 1 is Paul Bos heat treated to 60. I think this is perfect, but it's .200 thick if that makes a difference.
 
60 - 61 HRC is my target hardness for the majority of the A2 blades I produce.
 
I have talked to several great knifemakers that use A2 and 60Rc has been the consensus as the target. I even have my cryoed for good measure :)
 
redsquid2 said:
I have the batch of blades just about ready to send out to H.T. I am wondering if they all should be hardened the same. I have got these, and also listing their thicknesses:

A hunter EDC .125" th., a couple of thin (.110) skinners, a utility wharncliffe .125, and a bushcrafter .125.

Would all of these be good at 60-61RC? I just think of 60-61, because it seems like that is a popular hardness for lots of knives these days. Would you go softer?

Any feedback appreciated.

Andy
Click to expand...
All the same hardness.

One graph.
graphA2.jpg

Second graph.
graphA2vsO1.jpg

And a link to a crucible webpage
http://www.crucibleservice.com/eselector/prodbyapp/tooldie/airkoolt.html

Is seems that 60-61 is optimum hardness. You dont get any benefit from going softer.
 
Lycosa said:
fwiw, Hartsfield to some to 65 Rc.
Click to expand...

Thanks for sharing that detail about Phill Hartsfield's swords and knives. I didn't know he hardened them to 65 Rc. My late nephew, Amer Kubaisi, was one of Phill's most ardent fans. Amer told me that Phill's swords were legendary for edge retention. Perhaps the extra hardness is the reason for that.

I got to know Phill while writing an article about him. The article appeared in the February, 1999 issue of Knives illustrated.

Sincerely,

Arnold Howard
Paragon Industries, L.P., Mesquite, Texas USA
ahoward@paragonweb.com / www.paragonweb.com
 
You are welcome, Arnold.
You need to talk to Phill Jr. He might be willing to share, or not.
Thanks for the links.
rolf
 
I've had a lot of people tell me that about 60HRC is ideal for A2 and my personal experience has been that it's extremely tough and holds and edge well at that hardness. Whether that hardness offers the ideal edge-holding for your use I cannot say, I've tried an edge on A2 at 64.5HRC and it seemed to degrade faster during hard use, but had higher initial sharpness and may well be usable for a kitchen knife (no guarantees there as I didn't go that far with the testing, just thought it was interesting!)

With regards to those graphs: the hardness after tempering is also heavily dependent on the pre-temper hardness, so those graphs may not line up if you're not using the exact heat-treat that was used to produce the graph.

For instance my most recent test blades in A2 were austenitized at 1740ºF (400ºF/hr ramp, 20 minute hold), then oil quenched and sub-zero treated. I believe they were somewhere around 65/66HRC before temper (I got my hardness tester just after they were tempered unfortunately). One blade was tempered at 425ºF for 62.5HRC, the other has been tempered to 600ºF for 60.5HRC. This high level of hardness is likely because the fast quench and sub-zero treatment reduced the levels of retained austenite when compared to a more standard heat-treat. (NOTE: I haven't done toughness testing on these blades yet!)

Doing a fairly standard heat-treat (1775ºF austenization, 35 minute hold, quenched in still air, tempered at 400ºF) renders a blade at about 59.5HRC, and it is very VERY tough at that hardness. In fact it beat out CPM3V for impact toughness in the testing I did... Bear in mind that a lot depends on edge and blade geometry, so your results may differ. I imagine it would be hard to go wrong with an A2 blade at 60HRC though!
 
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With regards to those graphs: the hardness after tempering is also heavily dependent on the pre-temper hardness, so those graphs may not line up if you're not using the exact heat-treat that was used to produce the graph.

You will get different hardness, but in aspect of toughness those tempering temperatures will be optimal.
Although cryogenic quenching might introduce huge differences, as it martensite much more of austenite, so tempering won't generate as many epsilon and eta carbides, or some other strange proceses I don't understand. :)
 
idaho said:

With regards to those graphs: the hardness after tempering is also heavily dependent on the pre-temper hardness, so those graphs may not line up if you're not using the exact heat-treat that was used to produce the graph.

You will get different hardness, but in aspect of toughness those tempering temperatures will be optimal.
Although cryogenic quenching might introduce huge differences, as it martensite much more of austenite, so tempering won't generate as many epsilon and eta carbides, or some other strange proceses I don't understand. :)
Click to expand...

I'm honestly not sure how to interpret the toughness graphs in the case that the heat-treatment differs from the one used to generate the graph. It may be that you could replace the temperature scale at the bottom with rockwell numbers and it would be roughly the same... It may also be that it's simply not applicable at all given that the microstructure of the steel will be different.

I do think though that the graph will not be useful unless it's re-interpreted some way when applying to steel that's been heat-treated differently. ie: I don't think that a temper of 400ºF will line up with the peak toughness on steel that was really hard before temper. That's why they included the bit at the bottom of the graph about how the steel was austenitized and quenched.

I can test this experimentally soon as I have 2x A2 blades, one at 62.5HRC (tempered at 425ºF) and one at 60.5HRC (tempered at 600ºF). I plan on breaking them both probably tonight. That should help shed a little (informal) light on the situation!
 
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