multiple hardening cycles?

daizee

daizee

Knifemaker / Craftsman / Service Provider
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I mulled over how to word this before posting, and tried searching to no avail.
I'm worried this is another third rail of knifemaking forum lore...

What is the science behind multiple hardening cycles?
Does that science benefit a blade (as opposed to other types of tools)?
Is it only effective for certain types of steels, and what are they?
Or is it just a presumption that if multiple tempers are good, then multiple hardenings must be better?

My motive: pure curiosity. It sounds like an enourmous amount of work, and I don't plan to do it for my knives as I likely wouldn't be able to detect a difference if there was one. However I do want to be able to understand what I'm looking at when reading about knives that have had the process applied.


-Daizee
 
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My understanding is that if you follow the proper methods of the heat treat for the particular steel that you're working with, all you need is one quench. (Yes, even 52100.)
However, if you're working with an Air Hardening steel, multiple quenches may be the only way to refine the grain.
Someone kind enough around here posted a chart ref. multiple hardening cycles for D2 a while back; I'm going to try to post it on here.
/Users/mitchellrokicki/Documents/d2grainrefinement925104.jpg
 
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I ever saw a D2 progress by someone.after pre-queching at 1100C, then 980C queching and 240C tempering,the D2 had better toughness, which was increased 80% while maintaining hardness.
 
Crap! I have a new Mac computer and I can't do anything with it! Tried to attach the D2 grain refinement page out of a HT guide. Sorry guys.
 
yangguangzhu said:
I ever saw a D2 progress by someone.after pre-queching at 1100C, then 980C queching and 240C tempering,the D2 had better toughness, which was increased 80% while maintaining hardness.
Click to expand...

Yes, But the grain got more refined with the lowering of the subsequent austenizing temps, right?
- Thanks
 
Triple quench, triple drawn. Quench, anneal, quench, anneal, quench, temper. That's the method the old guy at the gun show does with his railroad spike knife to get them tougher than 5160. Some reading states that it reduces the grain size with each subsequent annealing. I suppose it has a bit of logic to it. . . . Not sure about 1040 steel ever being tougher than 5160, but I'm not a metallurgist.
 
I will make one post only, as I can predict how this will go.

Contrary to what you often read, there is no memory to the grain in steel nor is three full quenches better than one. When you heat steel above the critical temperature, the structure changes to austenite.
Upon cooling below the critical temperature one of two things can happen. Either it cools slow enough to enter the pearlite range and becomes pearlite....or it cools fast enough to miss the pearlite nose ( around 1000F) and becomes supersaturated austenite. This will start to become martensite at about 450F. Martensite is what we should be making knife blades from.

While above the critical temperature, the grains can grow. This is a function of time and temperature, with temperature heavily weighted. If the temperature is held at about 75-100F above critical, little grain growth will occur.

Now, there is a multiple cycling treatment that is often confused with a multiple quench. It is called thermal cycling. First you heat the blade to about 100F above critical and quench it. This will refine the large grains that formed in the forging steps.
Then you repeat the heat cycle with the temperature just hitting the critical (non-magnetic) point...and quench. This forms the finest grain possible for that steel type. No matter how many times you do this step, it is the last time that sets the grain. Any previous grain size is erased and re-formed.
Last, you heat the blade to about 1200F and let cool to 700F slowly in still air, then quench. This will produce a fine pearlite blade with very fine grain. This will be the starting point for the final quench to harden the blade. If the final soak to austenitize the blade is kept at 100F above critical, the grain will stay super fine and uniform.
Thermal cycling is a good idea on all blades, and will reduce stress and warping. It is of great advantage on higher alloy steels, like 52100. On any water or brine quenched blade it is mandatory.

This information is regularly posted and available in any metallurgy book. It is a fact that has been completely tested and proven. There are no ifs, ands, or buts over this. Some may see it different, but that is just their take on how to do this. The methods used are all just variations on thermal cycling.

I know that Ed Fowler and some of those who use his methods will disagree with me, and cite his results on triple quenches and placing the blade in a freezer overnight, but the plain fact is that his last quench is the one that does the hardening and locks the grain size in. Any previous quenches are just grain refinement, and do not improve the steel or make the blade get harder. His method will produce a great blade, but there is no magic in the triple quench...it is the last one that has to be nailed, and that is the one that makes or breaks the blade ( no pun intended).
 
One of the first questions I had when I signed on here 5 years ago. I specified O1 but the THEORY is well covered. Keep in mind, that is where I started. Soon after, I purchased my Evenheat & quench oils and quickly realised how primative my understanding & techniques were. And have continued to learn every since.

Many thanks again to Mr. Cashen for his ability to cut thru the crap and leave you with a load of facts.

'Working the three types of steel' [burried in the stickies] also covers a gread deal of info on this as well.


http://www.bladeforums.com/forums/showthread.php/438836-Triple-Quench-0-1
 
I will make one post only, as I can predict how this will go.
Click to expand...

That's what I was afraid of too, Stacy. Thanks for the detailed reply.

It sounds from your description that thermal cycling (as opposed to multiple quench/temper cycles) is most applicable for forged blades whose grain structure has been altered by the forging process in a way that is less desireable by some measure, or whose steel is of uncertain origin or in an uncertain state. Would you agree with that statement? (I understand if answering would violate your one-post declaration!)

Other than the above followup question regarding application, you have completely answered all aspects of my question, thanks!

I will go look into metalurgical texts to understand the exact definitions of and relationship between grain and structure so that I may grok in fullness.


-Daizee
 
from my understanding, multiple hardening cycles were used for knife makers that do not have the technology to do soak times becuase most knife steels have a soak time on the hardening treatment, and that it was done to ensure fully hardened steels to minimize large differences in hrc through out the blade.

so if you can do the soak time you should not have to do more then one hardening cycle.
 
Dewey, thanks for the reference thread.
 
OK,
Thermal cycling is beneficial for ground or forged blades. You have induced all sorts of grain boundry fractures and stresses in grinding,too.

Air hardening steels are a different animal, and they have to be treated individually, not as a group. As stated, some benefit from cycling, but in my oppinion, a full soak at the lower end of the austenitic temperature will produce as good blade. Too many trips across the austenite boarder can't be a good thing for the super steels. Kevin or Roman would be better ones to answer those specifics.
 
bladsmth said:
OK,
Thermal cycling is beneficial for ground or forged blades. You have induced all sorts of grain boundry fractures and stresses in grinding,too.

Air hardening steels are a different animal, and they have to be treated individually, not as a group. As stated, some benefit from cycling, but in my oppinion, a full soak at the lower end of the austenitic temperature will produce as good blade. Too many trips across the austenite boarder can't be a good thing for the super steels. Kevin or Roman would be better ones to answer those specifics.
Click to expand...

Thank you, Stacy. I appreciate the answers that you give. (I really don't want to bother the guys that you mentioned.)
 
All this helped me figure out what I was missing. Now after some extra-curricular reading, I think I've got a much stronger grasp of what happens during the heat treating process, and an inkling of why certain pieces might be varied. I'm sure there's way more, but this was a great re-start. Thanks!

-Daizee
 
Whether you do it earlier and call it normalizing, or you do it last and call it multiple quenching, that cycling through the transformations is what refines the grain.
 
I won't get into the argument about triple quenching VS. single, but what you posted makes the most sense to me Stacy. I do get better performance with a triple quench on some steels and I can see all three quench or harding lines in the steel. What you posted as a form of grain refinement makes a lot sense, thanks.

Best way to heat treat any steel is start with a basic industry standard method then test and branch out and try new methods and find out what works best for you and the knives you want to make. Listen to other makers, but don't take what anybody says as gospel, test it out for yourself, then You will know what works for you on your knives.
 
What's the best book on metallurgy so I can figure out exactly what you guys are talking about? I've just recently began forging my own blades and want the most knowledge possible. I know this is off topic and I'm sorry for hijacking your
Thread but there is a good discussion going here.
Sterling
 
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