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I've read in different places that putting a blade through 2 or 3 heat/soak/quench cycles produces a more thoroughly hardened edge (particularly if you are using a limiter block to do an edge quench).

Can anyone substantiate this? Wouldn't the process of re-heating the quenched blade effectively "wipe out" the hardening process from the initial quench?

Just wondering.
Thanks!
- Dan Hurtado

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At the risk of starting a fight, my view of triple quenching is that it's a method developed to make people think that complicating heat treating makes a better knife. The idea is that one progressively dissolves more carbon into the the austinite, which it will, and keep you from dissolving too much carbon and forming excess retained austinite, which it won't. Too much time at temperature will form excess retained austinite whether you do it in three stages or all at once. Also hypoeutectic, eutectic, and slightly hypereutectic steels don't have enough carbon in them to form enough retained austinite to cause a problem so all you would be doing is austinizing the steel more than necessary if you have also done multiple normalizations. For steels like 1095, 52100, or high carbon W series of steels it would be far better to use a regulated heat source to austinize the steel and soak for around 10 minutes at under 1485° and only quench once.

 

The problem with steels like these that are strongly hypereutectic is that they form almost all plate martensite. When plate martensite plates comes together they have a tendency to form microscopic cracks, and by microscopic I mean that you could need a scanning electron microscope to see them. This is a know property of using these steels. It will happen upon quenching. I see no reason to give the steel three chances to form these cracks. Do I know that multiple quenches will create more of these cracks? No, I actually don't. But I don't know that it won't happen. I don't have the money that it would take to test this so I prefer to assume that multiple quenches of high carbon oil or water quenching steel will produce more cracking until I can find out otherwise. I have that responsibility as a knife maker.

 

Now if you are wanting to minimize this cracking caused by these high carbon oil and water quenching steels, for one, keep the grain size in the steel small. These cracks are only as long as the interface of the two plates of martensite. Another thing is to reduce the stress of quenching. Marquenching would be good but you have to have the right oil for that, or you could do an interrupted quench; something like 6-8 seconds in the quenchant then air quench for a few seconds and back into the quenchant. Of course, marquenching will prevent using water or brine to quench in as neither can be gotten hot enough.

 

As Jake said, this method can be used for refining grain size. I've seen nothing that says that this is better than triple normalizations and triple normalizations will introduce less stress into the steel. With either method, you will have to perform the operation properly. Both ways, if you heat the steel too hot for too long for the alloy that you are using you will grow the grain.

 

Doug

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I would consider it a good alternative to a ten minute soak, for a smith using hypereutectoid steel without a well regulated heat source.

 

What I don't understand is that a few well known smiths that own knife ovens still triple quench with a torch for a heat source. You can't really hold or even accurately measure an even heat with a torch for a 10 minute soak, so triple quenching makes sense there. (sort of).

 

But why use the torch when you have better methods at hand? To localize heat at the edge I suppose, so that the blade won't fully harden.

 

But, if you are edge quenching the knife anyway, why not just heat the whole blade up at once, in an oven, with a 10 minute soak, and edge quench it one time for a differential hardening?

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I've wondered the same thing, Salem, about people who have the equipment to heat treat with a regulated heat source and still attempt something like a triple quench. It would really floor me to learn that they take out the rosebud burner on an acetylene torch to austinize with. To me there is no reason for that than wanting to make life harder than it needs to be.

 

Admittedly, it could be easier for some makers to localize the heat where they want the steel to convert to martinsite and full quench than to austinize the whole blade and edge quench. However, a high carbon, mildly complex alloy is still a high carbon mildly complex alloy and they tend to have rather narrow temperature ranges and times to heat treat at without running into grain growth and retained austinite.

 

Doug

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I think it's horse-hockey.

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There's eight million things you can do to infinitesimally increase some aspect of a blade's performance, but they all have trade offs, and sometimes the increase, while scientifically measurable in some quantity, amounts to very little in the practical world. I'd put the triple-quench in that category, but what do I know?

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Repeating a heat treatment to get the hardness right is one thing. You'll usually be letting the steel cool in air at least once which will allow most things to be erased from the previous heat treatments. With triple quenching you dissolve carbon into the austinite and then lock it in place in the quench then austinize again to increase the amount of carbon into solution. If you have a regulated heat source that you can soak the steel at a given temperature for a given time, something that has been established by previous testing, you don't have to hope an pray that you're getting it right. If you can't achieve proper temperatures for the time required, use a steel that will match the equipment that you have. That will be carbon steel and other simple alloys.

 

Doug

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Well gentlemen, here's the thing. If you want the grain size refinement to go as fast as possible from BIG to small, then quenching to make martensite causes the next batch of austenite grains formed on the way up to be smaller, relatively, than if you started with pearlite.

 

It ain't the holy grail, nor is it useless. It is more risky than air cooling for grain refinement, and it is 'not' neccessary to get ultimate grain refinement. But, it will get you there faster than air cooling. In less cycles.

 

 

Carry on...

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Well gentlemen, here's the thing. If you want the grain size refinement to go as fast as possible from BIG to small, then quenching to make martensite causes the next batch of austenite grains formed on the way up to be smaller, relatively, than if you started with pearlite.

 

It ain't the holy grail, nor is it useless. It is more risky than air cooling for grain refinement, and it is 'not' neccessary to get ultimate grain refinement. But, it will get you there faster than air cooling. In less cycles.

 

 

Carry on...

Thanks for the clarification Mr. Clark

I think I will continue to normalize since I can afford the extra time involved.

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