A2 steel - History and Properties

Thanks Larrin.
This article could not have come at a better time as I have been kicking around the idea of using some A2.
Definitely gives me some more to think about while trying to figure out which way to go.
 
Another article that I enjoyed reading. Concerning the possibility of some A2 having larger than expected carbides, SB Specialty metals has ESR A2 available upon request. "Electro Slag Remelt". This remelting process is said to be tougher and cleaner than conventional A2, and I would think also an elimination of those possible undesired larger carbides. They sent me a small flat bar of 1/16" ESR A2 to play with, and will be using more of it in the future. It was my first experience with A2, but it seemed very clean and stable in HT, as well as performing great in paring knives at 63-64HRC.
 
Another great article, although A2 steel was never in my radar. Probably as demanding to heat treat as cpm3v and other non stainless pm steels (cpm4v, V4e, ZWear), but these will be much tougher, more wear resistant and, at least, as “stain resistant”. But it have earned its place in knifemaking history.
 
hugofeynman said:
Another great article, although A2 steel was never in my radar. Probably as demanding to heat treat as cpm3v and other non stainless pm steels (and cpm4v, V4e, ZWear), but these will be much tougher, more wear resistant and, at least, as “stain resistant”. But it have earned its place in knifemaking history.
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I usually think of air hardening steels as being easier to heat treat because quenching is less critical. And 5160 and CruForgeV toughness testing we have done shows that low alloy steels are highly sensitive to overheating during austenitizing. I think the reputation of low alloy steels to be "easy" to heat treat is kind of misguided. Easier to harden with a torch or a forge, yes, easier to get a good heat treat no.
 
Larrin, some food for thought. Some of the history I have seen over the years regarding A2 and D2 were on the topic of retained austenite. Both of the grades by design have some retained austenite after "normal" heat treatment. This was done to balance the growth during transformation with the intent to have a neutral result. The limit in growth was done since finish grinding and machining was a lot more difficult 70 years ago and limiting the amount to remove made the tool makers jobs easier. Some of the very old data sheets had this minimal growth as a benefit to the grades vs the older traditional high carbon steels.
 
NYMet said:
Larrin, some food for thought. Some of the history I have seen over the years regarding A2 and D2 were on the topic of retained austenite. Both of the grades by design have some retained austenite after "normal" heat treatment. This was done to balance the growth during transformation with the intent to have a neutral result. The limit in growth was done since finish grinding and machining was a lot more difficult 70 years ago and limiting the amount to remove made the tool makers jobs easier. Some of the very old data sheets had this minimal growth as a benefit to the grades vs the older traditional high carbon steels.
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Thanks Ed. I haven't written much about size changes since in general knife makers don't care much.

For background on the effect for others: Martensite is larger than ferrite, while retained austenite is smaller than either. So the part as a whole will increase in size slightly when it is transformed full to martensite. But a certain percentage of retained austenite leads to no net size change. Alternatively, for some companies/parts size changes during use are unacceptable, and the transformation of retained austenite during the lifetime of the part would lead to small size changes. It depends on the concerns of the application. It's not unique to A2 or D2 so I didn't cover it in these articles. Perhaps it is worth writing about on its own.
 
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Larrin said:
I usually think of air hardening steels as being easier to heat treat because quenching is less critical. And 5160 and CruForgeV toughness testing we have done shows that low alloy steels are highly sensitive to overheating during austenitizing. I think the reputation of low alloy steels to be "easy" to heat treat is kind of misguided. Easier to harden with a torch or a forge, yes, easier to get a good heat treat no.
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Got it! So good furnaces (with PID controllers) both for Hardening and tempering (not the kitchen oven:D) is the way to go to maximize steel attributes, even with “simple” steels as 5160.:thumbsup: Even so, I think that there are very skilled bladesmiths that can deliver a very tough knife heat treated with a charcoal forge (or gas forge) and “simple” oil, as mankind used to make blades in the past. Theses guys have very trained eyes and can probably (without scientific proof, at least that I know of) achieve the toughness peak of the steels by the colors and probably by trial/error. But no doubt that a controlled heat treatment setup would deliver allways same (good) results, if all the steps are the same, of course, and the steel is from the same (very clean and high quality) batch.:thumbsup:
 
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I don't think the forge heat treaters can do A2, it would be very hard to get right without PID. Also, the forge heat treaters temper down to below 60rc so there is a lot of room for not nailing and holding correct temps.
 
cotdt said:
I don't think the forge heat treaters can do A2, it would be very hard to get right without PID. Also, the forge heat treaters temper down to below 60rc so there is a lot of room for not nailing and holding correct temps.
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You’re right, I’m talking about 5160 and 6150 steels. I should have posted this in the 5160 steel article, sorry!:(
 
That is very interesting. I would have not expected A2 to be lower on the toughness scale than high hardness CFV as A2 is generally promoted for its inherent toughness at the expense of some abrasion resistance compared to say D2. It looks like it has more of the latter than expected but considerable less of the former.
 
Looking at the article again, 52100 is tougher than A2, and A2 is supposed to be tougher than O1. I thought O1 was a bit tougher than 52100. We have a bunch of O1 coming up too to tease this out.
 
A number of years back, I asked Kevin Cashen what commonly available carbon/tool steels he would use for say a mid tech line. He recommended O1 for blades under 5 inches and 52100 for bigger blades because both made for good "slicers" but the 52100 was tougher, whereas the O1 had better abrasion resistance. From these recent charts, it looks like CFV might fit into that Goldilocks zone between the two go them.
Willie71 said:
Looking at the article again, 52100 is tougher than A2, and A2 is supposed to be tougher than O1. I thought O1 was a bit tougher than 52100. We have a bunch of O1 coming up too to tease this out.
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Larrin said:
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Is that the O1 I did?

I’ve got more of the NJSB 52100 and more of the AKS 52100 to run a few more comparable samples. Carbon levels are different, and Larrin and Devin’s samples showed 1475f might have been too low of an austenitizing temp for 52100. I had a bunch of cancellations this week in my real job, so the oven will be running for quite a few hours. I have quite a few coupons cut out ready to run.

I can see why I much preferred 52100 for fine edge blades compared to O1.
 
It's not obvious from the composition why 52100 > A2 > O1 in toughness. Maybe it's the quality of the steel itself?
 
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