A2 impact resistance and edge retention

[jdm61]

When you see the comparison charts, typically A2 is "rated' at 60 Rc. I may be wrong, but IIRC, A2 may have been one of the steels that Mr. Cashen talked about where you should "take what the chemistry gives you." I don't know if it has any quirks when left too soft like say CruForgeV, but I wouldn't want to risk finding out.

I run mine at 59-61.

 

[Lycosa]

A Hartsfield A2 kwaiken was Rc 65, when Pohan Leu tested it.
rolf

 

[Cobalt]

If you have to leave pretty much any 'high" carbon steel that soft in a knife you are probably using the wrong steel.

I don't think CRK was using the wrong steel. Those old knives made his name. According to crucible(I take everything they say about non CPM steels with a boulder of salt) the Toughness of A2 at Rc of 60 is the same as A2 at an Rc of 57. This makes no sense. If it was 40 ft lbs at 60 it is likely a hell of a lot tougher at 57, maybe on the order of double that number. Which would make a huge difference. Rc for CRK A2 was listed as 55-57. At 55 I would not be happy, but at 57 I would be ok with that on a hard use knife.

 

[marthinus]

Here is a nice pdf from Bohler on A2.

http://www.bucorp.com/media/A2_data_sheet_09032013.pdf

 

[me2]

I don't think CRK was using the wrong steel. Those old knives made his name. According to crucible(I take everything they say about non CPM steels with a boulder of salt) the Toughness of A2 at Rc of 60 is the same as A2 at an Rc of 57. This makes no sense. If it was 40 ft lbs at 60 it is likely a hell of a lot tougher at 57, maybe on the order of double that number. Which would make a huge difference. Rc for CRK A2 was listed as 55-57. At 55 I would not be happy, but at 57 I would be ok with that on a hard use knife.

A2 suffers from temper embrittlement. The relationship between hardness and toughness is no where near linear, particularly in the hardness range of knives.

 

[Charlie Mike]

S7 is another favorite of mine. I usually run it at 58 but have done 61 on request.

 

[Cobalt]

A2 suffers from temper embrittlement. The relationship between hardness and toughness is no where near linear, particularly in the hardness range of knives.

You sure avout that? I found izod testing that shows a linear relationship between toughness and hardness.

 

[me2]

Yep. Most steels used in cutlery show it, from 1095 to the high temp stainless steels (154CM type). The problem comes from the relative low toughness of basically all cutlery steel, with a couple of exceptions. As the over all toughness decreases, the ability of some toughness tests, like Izod and Charpy, to detect it decreases. What is obvious in 1040 is less so in 1055. Now, that could mean the problem is going away, or it could mean the test used just can't detect it. In this case some of the toughness tests aren't sensitive enough to detect it.

 

[noob93]

S7 is another favorite of mine. I usually run it at 58 but have done 61 on request.

I would love a large Bowie type knife made of s7. I just think for a shorter blade a2 would hold its edge quite a lot better wouldn't it. Also I just saw your last post and I'm not surprised. You make your knives the way you like your knives strictly business and tough.

 

[Cobalt]

Yep. Most steels used in cutlery show it, from 1095 to the high temp stainless steels (154CM type). The problem comes from the relative low toughness of basically all cutlery steel, with a couple of exceptions. As the over all toughness decreases, the ability of some toughness tests, like Izod and Charpy, to detect it decreases. What is obvious in 1040 is less so in 1055. Now, that could mean the problem is going away, or it could mean the test used just can't detect it. In this case some of the toughness tests aren't sensitive enough to detect it.

Valid points. But I am still going to contend that on average between the typical knife hardnesses, the graph of toughness to hardness will be fairly linear and not show the wierd hump crucible gives A2. The page below is the data sheet for A2 from carpenter instead of crucible. check out page 4 and see how linear that line is. I think this will be typical across the board. I would agree that at some point things change but if you look at comparisons between say 55 and 61, I would bet most curves will be fairly linear. But who knows. According to this, below, A2 is linear.

https://www.alphaknifesupply.com/Pictures/Info/Steel/A2-DS-Carpenter.pdf

 

[Charlie Mike]

I would love a large Bowie type knife made of s7. I just think for a shorter blade a2 would hold its edge quite a lot better wouldn't it. Also I just saw your last post and I'm not surprised. You make your knives the way you like your knives strictly business and tough.

S7 stuff...

 

[Cobalt]

I would love a large Bowie type knife made of s7. I just think for a shorter blade a2 would hold its edge quite a lot better wouldn't it. Also I just saw your last post and I'm not surprised. You make your knives the way you like your knives strictly business and tough.

This is what S7 can do when thru hardened to 58 Rc with a top HT.

 

[noob93]

This is what S7 can do when thru hardened to 58 Rc with a top HT.

That's pretty impressive although I doubt I would ever nee to do that with a knife lol. Those blades are like Jim carry's face in the mask

 

[DeadboxHero]

give use more information about what the knife is used for so we can help

 

[Jakers]

This is my totally unscientific observation of a2.

I have a 7" Bowie made from a2 with a 30 degree bevel and ground a little thinner behind the edge than my esee 6. It holds an edge WAY longer and despite its keener edge is really tough. I went kinda wild with it in the woodpile for a good 30 minutes chopping dirty 2x4's and batonning through seasoned knotty elm. Most of the wood had a thin layer of old mud on it too. It really surprised me, not even a rolled edge. I would probably call bs if I hadn't done it myself. It makes me wonder what Nathan's 3v is capable of!

I don't usually beat the hell out of my tools but just wanted to test out the geometry and heat treatment. Btw, it was hardened to 60 and cryo treated by peters.

 

[me2]

Valid points. But I am still going to contend that on average between the typical knife hardnesses, the graph of toughness to hardness will be fairly linear and not show the wierd hump crucible gives A2. The page below is the data sheet for A2 from carpenter instead of crucible. check out page 4 and see how linear that line is. I think this will be typical across the board. I would agree that at some point things change but if you look at comparisons between say 55 and 61, I would bet most curves will be fairly linear. But who knows. According to this, below, A2 is linear.

https://www.alphaknifesupply.com/Pictures/Info/Steel/A2-DS-Carpenter.pdf

Fair enough. Toughness is highly dependent on heat treatment, so some differences are bound to happen. However crucible shows the same peak / trough behavior in more than just A2. Torsional toughness testing as well as fracture toughness testing is more likely to show it. In torsional toughness A2 has a dual peak, one at about 400 F, and another at a higher temperature, near 750 F, with corresponding valleys between and on the higher side of the second peak. There is temper embrittlement behavior shown in plain carbon steels, but as the carbon content goes higher, the peak flattens and eventually can't be seen on a graph of tempering temperature vs toughness, likely somewhere around .75 % carbon. However, all the mechanisms for temper embrittlement are still there, and indeed the peak/valley show again during more sensitive testing. The same is true of A2. The mechanisms still there, but Izod may not be sensitive enough to find it.

I do find that graph to be a bit odd. I think merely because of the scale of the graph, any peak/valley behavior would be very likely hidden. Graphs of tempering temperature vs toughness are what is normally used to show this phenomenon. I think we've discussed before how I look at hardness as a by-product, and look more at the tempering temperatures, as that is what controls the changes that cause various properties, including hardness. Tempered martensite embrittlement (TME) is the full name for what we are generally discussing, and it occurs in certain temperature ranges across a huge variety of steels, from 1040 to 440C, A2 included, though it is different enough that it is steel specific in its high and low temperatures, though the middle of the range is around 450 to 500 degrees F. It has long puzzled me why many knives have hardnesses that indicate possibly tempering temperatures right in the embrittlement range. The basic answer is that the bottom of the toughness trough is still not zero, and the blades are tough enough for their intended purpose, so the embrittlement goes unnoticed.

 

[noob93]

Fair enough. Toughness is highly dependent on heat treatment, so some differences are bound to happen. However crucible shows the same peak / trough behavior in more than just A2. Torsional toughness testing as well as fracture toughness testing is more likely to show it. In torsional toughness A2 has a dual peak, one at about 375 to 360 F, and another at a higher temperature, with corresponding valleys between and on the higher side of the second peak. There is temper embrittlement behavior shown in plain carbon steels, but as the carbon content goes higher, the peak flattens and eventually can't be seen on a graph of tempering temperature vs toughness, likely somewhere around .75 % carbon. However, all the mechanisms for temper embrittlement are still there, and indeed the peak/valley show again during more sensitive testing. The same is true of A2. The mechanisms still there, but Izod may not be sensitive enough to find it.

I do find that graph to be a bit odd. I think merely because of the scale of the graph, any peak/valley behavior would be very likely hidden. Graphs of tempering temperature vs toughness are what is normally used to show this phenomenon. I think we've discussed before how I look at hardness as a by-product, and look more at the tempering temperatures, as that is what controls the changes that cause various properties, including hardness. Tempered martensite embrittlement (TME) is the full name for what we are generally discussing, and it occurs in certain temperature ranges across a huge variety of steels, from 1040 to 440C, A2 included, though it is different enough that it is steel specific in its high and low temperatures, though the middle of the range is around 450 to 500 degrees F. It has long puzzled me why many knives have hardnesses that indicate possibly tempering temperatures right in the embrittlement range. The basic answer is that the bottom of the toughness trough is still not zero, and the blades are tough enough for their intended purpose, so the embrittlement goes unnoticed.

So I'm not a metallurgist or anything but from reading the chart I'm taking it you would want to temper a2 right around 640 to get both toughness and edge retention?

 

[me2]

Which chart? The A2 I've had great treated was tempered at 400 twice for a hardness of 61-62. This was for a fine cutting blade. For a tough blade I'd chose 750 or so, if I had to use A2. If toughness is the top priority I'd choose different steel if possible.

 

[Charlie Mike]

I place wrapped knife in oven and ramp (as fast as possible) to 1800F and hold for 15 minutes. Plate quench and temper for 2 hours at 325F, let cool for a few hours and temper again at 375F for 2 hours.

 

[noob93]

I place wrapped knife in oven and ramp (as fast as possible) to 1800F and hold for 15 minutes. Plate quench and temper for 2 hours at 325F, let cool for a few hours and temper again at 375F for 2 hours.

Awesome that you shared that info with everyone I'm hoping the exchange gets better very soon so I can gift myself a cmftw Xmas present! Maybe I could even talk you into grinding both sides haha