Harden A2 twice?

[Nate Johnson Arts]

I have some A2 blades that are 60rc. I want to reharden them to 62-63rc. Would I have to anneal before rehardening, or can I just go for it? I can't find any info on this for air hardening steel, other than a few threads where people talk about hardening some types of steel twice with no cycling in between. Thanks.

 

[Sean Yaw]

I re-hardened A2 successfully just by "going for it".

 

[Stuart Davenport Knives]

Personally, I would do a temper anneal. You've hardened it once already, it's martensite now, so bring the oven to 1200°F and then insert the blade and start a 2 hour timer. Let it air cool and then re-harden. A2 is a little more complex than simple carbon steels that can just be hardened again.

 

[Sean Yaw]

I've also re-hardened MagnaCut without doing any pre-austenitizing steps.

 

[Nathan the Machinist]

Personally, I would do a temper anneal. You've hardened it once already, it's martensite now, so bring the oven to 1200°F and then insert the blade and start a 2 hour timer. Let it air cool and then re-harden. A2 is a little more complex than simple carbon steels that can just be hardened again.

I don't think that's going to do much.

 

[Nathan the Machinist]

I re-hardened A2 successfully just by "going for it".

I wouldn't feel comfortable selling something like that without having validated the process, but this actually works okay more often than you might expect. It is bad practice because as you are getting back up to temperature you are potentially dissolving the carbides that are pinning the grain boundaries but, it actually often works okay once. No promises. Just be aware you do not need a long soak, the carbon is already in solution. You just need to get it up to temperature, let it soak maybe 10 minutes and do your quench.


If the steel was overheated in a previous heat or if it was austenitized at a high temperature and you are re-austenitizing at a high temperature again, you can get explosive grain growth. Or you may get grain refinement. It's a crapshoot.

 

[DevinT]

It’s best to do a subcritical anneal first before rehardening.

Hoss

 

[HSC ///]

Personally, I would do a temper anneal. You've hardened it once already, it's martensite now, so bring the oven to 1200°F and then insert the blade and start a 2 hour timer. Let it air cool and then re-harden. A2 is a little more complex than simple carbon steels that can just be hardened again.

I think 1200 F is appropriate for stress relieving, but an anneal for A2 is in the 1600F range and slow cool

edit - I see sub critical anneal has already been mentioned,

 

[Nate Johnson Arts]

It’s best to do a subcritical anneal first before rehardening.

Hoss

Thanks for the response, could you elaborate on this? Just trying to sort through the diversity of answers here!

 

[Nate Johnson Arts]

I wouldn't feel comfortable selling something like that without having validated the process, but this actually works okay more often than you might expect. It is bad practice because as you are getting back up to temperature you are potentially dissolving the carbides that are pinning the grain boundaries but, it actually often works okay once. No promises. Just be aware you do not need a long soak, the carbon is already in solution. You just need to get it up to temperature, let it soak maybe 10 minutes and do your quench.


If the steel was overheated in a previous heat or if it was austenitized at a high temperature and you are re-austenitizing at a high temperature again, you can get explosive grain growth. Or you may get grain refinement. It's a crapshoot.

Interesting, thanks for the insight. I appreciate the molecular explanation. These were professionally heat treated (Bos). I just want to do a few at higher RC in my own heat treat oven (1750F). So overheating will not be an issue. Can you elaborate on what the danger is exactly, if temperatures are controlled and grain growth isn't an issue? Trying to sort through the info and understand exactly what risks / issues are. Thanks.

 

[Nate Johnson Arts]

I've also re-hardened MagnaCut without doing any pre-austenitizing steps.

Interesting. Anecdotal experience like this is intriguing. I'm trying to understand why I shouldn't just reharden...as in what are the issues that would come up assuming temperatures are spot on. In the other hand, if it works then it works.

 

[Stuart Davenport Knives]

If you start from a pearlite structure, then a soak at 1200F would indeed only stress relieve. But the OP had already hardened the blade, thus it is martensite. When you "temper" martensite at 1200F for 2 hours, is that NOT called a "temper anneal"??

Sure you can do the whole process anneal, starting at 1600F and slow cooling it down. But I was under the impression that A2 was sort of on that border line between benefiting from a temper or subcritical anneal vs a full process anneal.

 

[DevinT]

High alloy steels don’t do well with multiple quenches from full austenitizing temperature. Pre-quenches are sometimes used to refine the grain but are done at a lower temperature.

Quenching from the same austenitizing temperature without some kind of anneal causes irregular fish scale type grain growth.

Full cycle or subcritical annealing increases the amount of carbide volume allowing for the pinning of the grain boundaries, keeping the grain size small when re-austenitizing.

The critical temperature for A2 is around 1460’f. To do a subcritical anneal, heat to 1375’f for 8-12 hours and air cool. Probably best to foil wrap.

This is a good condition to austenitize from.

Hoss

 

[Nate Johnson Arts]

High alloy steels don’t do well with multiple quenches from full austenitizing temperature. Pre-quenches are sometimes used to refine the grain but are done at a lower temperature.

Quenching from the same austenitizing temperature without some kind of anneal causes irregular fish scale type grain growth.

Full cycle or subcritical annealing increases the amount of carbide volume allowing for the pinning of the grain boundaries, keeping the grain size small when re-austenitizing.

The critical temperature for A2 is around 1460’f. To do a subcritical anneal, heat to 1375’f for 8-12 hours and air cool. Probably best to foil wrap.

This is a good condition to austenitize from.

Hoss

Hey thanks so much, that's exactly what I was looking to hear. Do you have a resource that has this kind of information in it? I don't know where to access this level of detail outside of a forum like this.

 

[DevinT]

Hey thanks so much, that's exactly what I was looking to hear. Do you have a resource that has this kind of information in it? I don't know where to access this level of detail outside of a forum like this.

Larrin is my oldest son, he has a PhD in metallurgical science. Look up knife steel nerds and there is a bunch of great information. He also wrote the book Knife Engineering which is also a great resource.

Tool Steels 4th edition and Heat treater’s guide are also good. And John Verhoeven’s book Metallurgy for the non-metallurgist is very good as well.

Go slow, it’ll make your brain hurt.

Hoss

 

[Nate Johnson Arts]

Larrin is my oldest son, he has a PhD in metallurgical science. Look up knife steel nerds and there is a bunch of great information. He also wrote the book Knife Engineering which is also a great resource.

Tool Steels 4th edition and Heat treater’s guide are also good. And John Verhoeven’s book Metallurgy for the non-metallurgist is very good as well.

Go slow, it’ll make your brain hurt.

Hoss

Ha nice! I've spent a *lot* of time on Knife Steel Nerds lately. A lot. That little piece of info you gave me I didn't encounter however. I think it's time to get some reference materials, thanks for the start!