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S35VN Positive Pressure Quench?


Brian Dougherty
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I though I should come back and close the loop on my solution.  I can't claim that this is a good solution, but it appears I got away with it.

 

Dealing with opening up foil pouches during a quench bugged me, and I didn't have anything available to use for a plate quench.  In the end I just oil quenched the parts pouch and all.  Since this is a borderline air hardening steel, I reasoned that the slower transfer through the stainless foil would still be a pretty fast quench.  I have no idea if that is good reasoning or not.

 

Here is a pic of the parts in the pouches.  I didn't like the air gaps that would slow down the thermal transfer, and make it irregular so I used my vacuum food sealer to squeeze the foil into a tight fitting package.

 

Before forming the pouches:

IMG_20191231_123943170.jpg

 

Here they are after.  I sealed them up in a food bag, and then used a leather forming tool to press all of the edges into place.

IMG_20191231_124634280.jpg

 

Then I just quenched the whole pouch in oil until cool enough to handle.  I resumed the normal heat treatment after that with 2 cycles at 600F for a couple of hours.

 

These knives were a rush job to get done for Christmas which is why I ultimately went rogue on the quench method.  Unfortunately the blade that got destroyed in my first attempt was going to be my test piece to see how the edge held up so I don't have much data on how effective this heat treatment was.  I do know that a R60C test file wouldn't scratch the surface before tempering, and could just barely mark it after.

 

The knives both took very good edges, and didn't seem that difficult to sharpen with Lanskey diamond stones.  After the fine diamond stone I went to 600 and 1000 grit synthetic stones.  Followed this up with some stropping and they shaved hair pretty well.  They were gifts for my brother-in-law and nephew, so hopefully I'll get some feedback on durability.

IMG_20191222_174001117.jpg

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-Brian

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Nice work Brian!

Just so you know, using a freezer will deal with some RA and is better than nothing at all. Larrin Thomas explained this in an article on his website. On steels like AEB-l, the freezer is enough to convert the majority of RA, though dry ice will be better.

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Hmm...

 

I easily can do dry ice and acetone to get to -80c, but I thought I had read that anything short of liquid nitrogen with s35vn wasn't helpful.

 

I've already shown that my ability to read the directions on this steel is a bit spoty.  I'll have to look into this alloy again to see, as that would be an easy step to add.

-Brian

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If you really want to try liquid nitrogen, you can probably get some pretty easily.  Just bring an insulated container (e.g. a styrofoam cup) to a welding gas supplier.  They should be able to fill it for you.  Not saying you should do this, but that you probably can.  Probably best to call ahead first.  

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Plus the liquid N2 treatment only needs to last untol the steel hits equilibrium.  When I was in college I knew a guy who carried a cup of liquid nitrogen down the strip at 10PM  on a Saturday.   Wish I had acces to the giant Dewar behind the physics building still...

Oh, and re: the vacuum packing: you're such an engineer! :lol:  But I am sure it helped. ;)

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2 hours ago, Alan Longmire said:

Oh, and re: the vacuum packing: you're such an engineer! :lol: 

:P (Guilty as charged :rolleyes:)

 

The sad thing is that I'm sure there is a lot of liquid nitrogen available in the chem labs where I work, but it's way off limits.

Edited by Brian Dougherty

-Brian

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Here's a good read.

https://knifesteelnerds.com/2018/12/03/cryogenic-part1/

 

We can see that in many cases, dry ice will do most of the job and in some cases, just a freezer will help too. 

 

The goal of his article was to demonstrate that "N2 or nothing" is a misconception of how cryo Works.

Edited by Joël Mercier
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interesting. It looks like you have to go from the tempering temperature to the sub zero temp fairly quickly.  Letting the blade sit around for an hour at room temp greatly impedes the conversion of retained austenite. 

-Brian

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On 1/1/2020 at 7:11 AM, Brian Dougherty said:

interesting. It looks like you have to go from the tempering temperature to the sub zero temp fairly quickly.  Letting the blade sit around for an hour at room temp greatly impedes the conversion of retained austenite. 

Their source for that was a paper from 1942.  

[6] Gordon, Paul, and Morris Cohen. “The transformation of retained austenite in high speed steel at subatmospheric temperatures.” Transactions of American Society for Metals 30 (1942): 569-591.

 

That isn't to say it is wrong, just that there is likely more information out there.  It is also for a single alloy.  I would bet that chart would be a bit like a TTT diagram, in that different chemistries will produce varying levels of austenite stabilization.  In other words: your mileage may vary.  

 

Side note:  During WWII there were nerds in a lab playing with liquid nitrogen... and helping the war effort.  If that doesn't make you feel better about humanity, I can't help you.  

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Just finished heat treat on a chef's knife with S35VN, and I think it was pretty successful in producing good hardness and wear resistance (based on how hard the finishing sanding has been).

 

I tried the following:

1) Wrapped the knives in foil, dusting the inside of the packets with talc and adding a bit of paper to help use up the oxygen in the packet.

2) Equalize oven at 1500F

3) Add the knives and equalize again.

4) Ramp as fast as possible to 1950. Hold for 30 minutes.

5) Remove and quench in the packets between two large aluminum plates, blowing compressed air between the plates. Cool to 125F

6) Temper at 450F for 2 hours.

7) Cryo with liquid nitrogen. I rigged up a sprayer, like a wart remover gun, and dosed the blades slowly to try to evenly and slowly bring the temperature down. I wasn't able to control this as tightly as I'd hoped to. Jay Fisher suggests that it needs to be no faster than 4 degrees a minute, but I was a lot closer to 40.

8) Temper at 450F for 2 more hours.

 

The metal quality seems good so far, but we'll have to work it further to see.  My major lesson learned is to put a much better finish on the blades before heat treating (I went to 220 grit, should have done 600), as sanding this stuff post heat-treat is a slooooow process.

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@graham_lancaster If you have the means, you might try ticking the temper level up to 600F.  Since the knives I just did were a rush job, I didn't do a lot of hand sanding on them, but the parts I did sand by hand didn't seem that bad.  Then again hand sanding a 2.5" pocket knife goes much quicker than a chef's knife.

Edited by Brian Dougherty

-Brian

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