Super duper CPM-s3v heat treatment recipe?

About LN2 soaking, there is a paper online that shows soaking period and hardness levels at certain intervals, D2 I think. It looked like a sine wave until you get past 8 hours, which the curve continues to go up and then level out....don't recall the exact times, tho.
 
to confuse you more - you think that converting more RA to martensite will do more for wear resistance ? NO ! what does the work is the creation of eta carbides !
Told you cryo was weird . That's why only metallurgists can understand it !! LOL ! :rolleyes:
 
I have always read that you should always temper your blade ASAP after hardening. I've never found an explanation why it should be done ASAP Anyone know why ?
 
Quenching, the conversion from austenite to martensite can produce very high stresses. If you don't reduce those stresses by tempering The steel may reduce stresses by fracturing !! Even if the stresses don't fracture the blade there may be small internal fractures that will break the blade in use !!
 
During hardening, the steel is heated to form Austinite, then rapidly cooled to for Martensite. Some of the Austenite doesn't convert to Martensite immediately (retained Austenite). Since austenite and martensite are different molecular structure and different density, when the retained Austenite does convert it can causes extreem stress and lead to structural failure.
This is a simplified explaination, I hope it answers your question.
Jim Arbuckle
ABS JS
 
Ok, but would it make any difference if it was tempered within a couple hours of quenching or a couple days ?
 
As said, the quench creates very highly stressed martensite. The newly formed martensite is very brittle and needs tempering immediately. Also there is some amount or retained austenite. The retained austenite will become stable if left too long. The brittle martensite also converts to a more stable form better when done as soon after quench as possible.

If left untempered for any period of time, disruptions can occur along the grain boundaries. These can start to shear and grow into cracks or even catastrophic failure where the blade breaks apart just sitting on the bench. Releasing these stresses as soon as possible will avoid this shearing. In most cases, the blade may look fine, but there may be microscopic shears along the grain boundaries that may make the blade less strong and easier to break later in use. Once formed, these shear lines can not be repaired in tempering.

The first temper changes the brittle martensite into tempered martensite. This removes the stresses between the grains. It also converts some of the RA into new martensite ... which is brittle.
The second temper tempers the new martensite and also converts some more of the RA into martensite. On steels with high RA ( like 3V), a third temper is used to make the process more complete. Most knife steels do fine with only two tempers.

BTW, it is best to rapidly cool the blade between tempers by cooling in water. There are several metallurgical reasons, but the basic one is getting the newly formed martensite below 200F as fast as possible or it may slip back into RA.

So, the answer to your question is that doing the tempering ASAP after quench will give a stronger blade with less stress and lower RA.
 
I have an add on question to the OP's. When doing stainless that you are going to cryo in LN, some sources say to do the cryo right after quenching and before tempering, others say to temper first, then cryo, then temper again. All that I have read agree that you should temper after cryo and I understand that, you are converting the high RA that stainless has into martensite. So, my question is, is it better to do cryo right after quenching and then temper, or quench, temper once, then cryo, then temper again, and why?
 
If you are the nervous type you may first " Snap Temper " but no higher than 300 F !! Then into cryo , then temper 2X or 3X. Each time bringing it down to room temperature
then up to the second temp ,etc.
Eta carbides only form after the cryo bath, when the tempering occurs.
The whole process is part of the HT. The cryo is NOT an add on. The process is not quite the same as other types of tempering. The -300F is not just to reduce RA ,it's to form the eta carbides ! Do the whole process right after the quench don't wait.
 
Mete is of course correct.
The quench begins the Continuous Cooling Curve - CCC for short. The optimal word is CONTINUOUS. There should be no rests or changes in the downward curve. In stainless steels, the temperature when reaching the Mf is -100F. The Martensite finish point is often referred to as sub zero, but it is 100 degrees below zero. A dry ice slurry is the right temperature for sub-zero. Once you reach that point, the quench is completed as far as martensite formation.
If you want the added gain of eta carbides, you have to go down below -300F, which is referred to as cryo. Liquid Nitrogen (LN) is the standard medium for that.
The rise to the first temper should be smooth and immediate after the sub-zero or cryo. Subsequent tempers are not as critical time wise, but good metallurgical practice dictates that they are done as one continuous wave on a graph.
 
Ok, that makes sense. One last question, I've read some places where longer soak times in cryo is better, like 8+ hrs, but on some spec sheets, they state no soak is necessary. Does the longer soak times lead to increased ETA carbides?
 
The martensite transformation at sub-zero is complete the moment it gets there. No need to soak.

The eta carbide transformation at cryo takes 6 to 8 hours, and doesn't actually form until the first temper.
 
Just finished cryo this morning and started tempering 20mins ago.

When I did the cryo, the entire blade wasn't submerged in the liquid nitrogen. About 3in of the handle stuck out. The entire blade/edge was submerged though. I figure, as long as the entire blade is submerged, I should be okay, right?
 
You guy's are a wealth of information. Yesterday I tempered 3 blades within 3 hrs of hardening just because I needed to take the dog for a walk. Next time I'll get a earlier start . Thanks for all the replies.
 
The temp in the chamber is all below the conversion end point (Mf). With the majority of the blade in the LN, you are fine. As long as you still have LN in the tank, you will reach Mf, even if the blade isn't in the liquid.
 
Just so you don't get crazy about it, in most cases a few hours won't matter at all. I would say that doing it within 12 hours is safe enough.
Now, a water quenched Hitachi #1 white steel blade should go from quench tank to oven as fast as you can clean it up and stick it in.
 
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