S35VN Positive Pressure Quench?

[Brian Dougherty]

Recently I saw a folder I made for a guy earlier in the summer.  He carries it every day and is very 'Proud' of the knife, but he also knows how to take care of tools.  Even with that I was surprised to see some rust forming around the pivot area.  It's made me think about trying some stainless for a folder. (Shudder)

 

I have a commercial knife with an S35VN blade.  It is the one production knife I have had in the last 10 years where I wasn't disappointed in the blade performance so I thought I would start looking there.

 

I have a small heat treating oven, so I am not too worried about complex thermal schedules.  However, the quench process is new to me.  Here is what I copied from a supplier website about hardening:

 

HARDENING

Preheat: Heat to 1550-1600°F (845-870°C), Equalize.
Austenitize: 1900-2000°F (1035-1095°C), hold time at temperature 15-30 minutes.
Quench: Air or positive pressure quench (2 bar minimum) to below 125°F (50°C), or salt or interrupted oil quench to about 1000°F (540°C), then air cool to below 125°F (50°C).
Temper: Double temper at 400-750°F (200-400°C). Hold for 2 hours minimum each time. A freezing treatment may be used between the first and second tempers. Freezing treatments help to attain maximum hardenability and must always be followed by at least one temper.
NOTE: For optimum stress relieving, CPM S35VN may be tempered at 1000-1025°F (540-550°C). Tempering in this range may result in a slight decrease in corrosion resistance.
Size Change: +0.05 to +0.10% when fully martensitic. The presence of retained austenite may reduce the net growth. When tempering at 400-750°F (200-400°C), freezing treatments may be necessary to minimize retained austenite.

 

I take it this is an air hardening steel.  Am I right in thinking that the positive pressure is to increase the air density to pull the heat out faster than it would at ambient pressure?  I'm not sure how I would pull that off, but I could do the interrupted oil quench fairly easily.

 

This stuff isn't' that expensive.  Now that my curiosity is up, I'll probably end up buying a little to play with. (Shudder again)

[Alan Longmire]

I'm in that same boat, and what I've managed to find is that plate quenching (pressing the parts between a pair of thick aluminum plates) is what most stainless folder guys are doing.  

[Gary Mulkey]

Back in my stock removal days I was one of the first to get any S30V which I would  think would be quite similar as to H/T'ing.  I was having trouble with the H/T so I called Ed Severson who  invented it for Crucible Steel.  He suggested that I do a plate quench and said that aluminum plates weren't necessary and that steel plates would work fine.  I got some 2" thick plates from the  local scrap  yard and had good success with them.  He also recommended doing a cryo treatment between tempers (4 hours in dry ice).  He explained that this would bring the austenitic conversion from 91-92% to over 99%.

[Jerrod Miller]

4 hours ago, Brian Dougherty said:

Am I right in thinking that the positive pressure is to increase the air density to pull the heat out faster than it would at ambient pressure?  I'm not sure how I would pull that off, but I could do the interrupted oil quench fairly easily.

Yes, and this is done in a vacuum oven (which can also run at positive pressures).  So if you are already heat treating in such an oven, when it is time to quench it you can't just open the door and pull it out, that will ruin your elements.  You have to purge/fill the chamber with gas.  Here they are telling you that you should back-fill the chamber with at least 2 bar to be effective enough.  

[Gerhard Gerber]

Hi Brian

 

I have one word for you: Elmax 

 

Or at least, look at other steels out there.  I would be very surprised if you could not get VERY good results with S30/S35 using foil pockets and plate quench.

 

My mentor has standardized on Elmax for mono-steel blades for several reasons, and I believe it performs almost the same or slightly better than S35.

 

 

I know this might be seen as heresy here, but my recent experiences with Sandvik 14C28N has been so positive I'm seriously considering trying better stainless steels.

 

Working with the foil was not pleasant, but also not the end of the world.  As my mentor pointed out during lunch today, lot's of advantages to getting to know a steel, especially if you have a Rockwell tester like in his case.

 

[Alan Longmire]

 

I borrowed this chart from Larrin Thomas's Knifesteelnerds site article on AEB-L.  Look where Elmax is on the hardness vs. toughness scale.  Plain S35 is right up around CPM-154, and AEB-L up at the top of it all is the same as 13C27, and very comparable to 14C28N.  14C28N is even more corrosion-resistant, though.  The "new steel" on this chart is a niobium steel under development https://knifesteelnerds.com/2019/02/18/niobium-alloyed-knife-steels/ that sounds a lot like S35VN.  Elmax is much more corrosion-resistant, though.  

 

I sound like I know what I'm talking about here, but it's all from Larrin's work plus some anecdotal stuff I've seen elsewhere.  I plan on a line of folders and kitchen knives out of 14C28N in the future, based on maximizing toughness at high hardness, but with ease of sharpening taking priority over corrosion resistance.  I do not like CPM154 or S30V because they're so darned hard to sharpen.  I imagine Elmax is in that same realm, since it has such high wear-resistance compared to AEB-L.  

 

Then again, I also plan to do some kitchen knives out of bloom steel just for fun.     Those will have to find the right owner.  Stainless in the kitchen is for the uneducated masses.  

[Joël Mercier]

Here's a more recent chart from the same source. We can see S35VN isn't a particularly tough stainless steel.  I have a benchmade in s30v and I'm not impressed by it's edge holding either. 

 

Like Alan said, the main issue with those steels is to get them properly sharp. You either have to make a rough toothy edge or use a dmt or cbn stone to actually cut those V carbides instead of tearing them out. That's the only way to get an edge that's smaller than the carbides while keeping full abrasion resistance. 

[Brian Dougherty]

OK, so I bought some of the S35VN to goof around with just to see what was what, and my first attempt at heat treating was a complete failure!  I made a small (~2.5") folder blade just as a test piece. 

 

After the soak at the Austenitizing temp, all I had left  was a flaky pile of what looked like scale.  The pile barely had enough structural integrity left to held together when I picked it up out of the oven.

  

I didn't have any heat treating foil at the time, so I thought I would try without it.  However, I have a hard time believing that even without foil the blade would have oxidized 100% through.  I have foil ordered, but I'm wondering if I missed something else.

 

After rough grinding, I followed the data sheet from Crucible exactly:

 

1. Stress relieve by heating to 1200F and holding for 2 hours before cooling is still air.  (this seemed to go fine)

2. Preheated at 1550F until equalized (Seemed OK at this point, but I was just peeking in the oven so I can't be sure)

3. Ramp up to 1950F and hold for 2 hours (At this point, the blade had become a pile of flaky scale)

4. Quench - I didn't get this far

 

Any thoughts?

 

FWIW, I am exploring this more for my own amusement than anything else.  I'm not sold on this as a stainless option for folders, it was just an easy steel for me to get to play with.

DataSheet S35VNrev12010.pdf

[Alan Longmire]

An electric furnace without a controlled atmosphere is the most oxidizing atmosphere of all the usual HT methods, but yeah, if 309SS foil can stand 2 hours at 1950 without falling apart it seems odd that bare S35VN would scale itself to death...  Evenheat will sell you the kit to flow argon, but I've heard that shortens the life of the elements.  I'd love to hear Jerrod's thoughts on this.  

[Brian Dougherty]

I'm  wondering if 2 hours was overkill.  It's what the datasheet says, but the folks over on the tacti-cool forums that lust after super steels seem to all soak for around 30 minutes.

 

Old school tool steels all have heat treating regiments that are a bit off for blades because of the thin cross section.  Maybe this is the same?  I assumed that S35VN is mostly for blade use, so didn't think to question the soak time.

[Jerrod Miller]

Yeah, soaking for that long at that temp is pretty much going to require protection from oxygen.  304 foil holds up better thanks to the Ni in it, which helps for higher heat.  S35VN is designed to be used with optimal HT equipment (given the previous discussion, vacuum seems to be their intent).  There is a lot of V (and thus VC) in there, and those do NOT like to dissolve (same with Mo and MoC).  That is why the high temp for so long.  It would be interesting to know what the microstructure looked like from the manufacturer.  If the carbides are fine and well distributed, then an extended soak wouldn't be necessary (assuming stock removal only).  

[Brian Dougherty]

Thanks Jerrod.  I made a few of the blade blanks, and will try another one once I have the foil.  If you are curious about the structure of the steel, I could send you a cut-off to look at.  (Don't do it for me, but I'm happy to mail out a piece if you would like to satisfy your own curiosity.)

 

I'll have to re-think my quench when I use the foil.  I can't imagine I can dunk the whole pouch in oil.  A lot of folks plate quench it foil and all, but I don't see how the edge will get cooled very quickly that way.  (Assuming the bevels are already roughed in)  I may have to get some big gloves so I can cut open the pouch and get the blade out quickly.

 

Making my own pattern welded steel is much easier than all this   (Much prettier too!)

 

[Jerrod Miller]

Thanks for the offer, but I am not currently set up for doing any micros (got a couple decently usable microscopes, but not the sample prep).  Also, to be completely honest, I don't really care about these types of alloys enough to put any real effort into it.  I don't think the benefits outweigh the drawbacks to them.  

 

Also, I just noticed that your soak time was supposed to be 15-30 minutes based on the spec sheet.  And I don't think you have to cool this down too terribly quick, so oil in the foil is probably fine, though it will probably be hard to ensure that it is even enough to prevent cracking, especially since the oil is supposed to be interrupted at 1000F.  

[Gary Mulkey]

2 hours ago, Brian Dougherty said:

 

 

I'll have to re-think my quench when I use the foil.  I can't imagine I can dunk the whole pouch in oil.  A lot of folks plate quench it foil and all, but I don't see how the edge will get cooled very quickly that way.  (Assuming the bevels are already roughed in)  I may have to get some big gloves so I can cut open the pouch and get the blade out quickly.

 

Making my own pattern welded steel is much easier than all this   (Much prettier too!)

 

 

When I was doing stock removal and using S30V (similar) I used to quench the packet until it showed no color and then quickly into a plate quench which seemed to work well.

[Chris Christenberry]

Gary, are you saying you quenched the packet the blade was encapsulated in?

[Brian Dougherty]

3 hours ago, Jerrod Miller said:

...Also, I just noticed that your soak time was supposed to be 15-30 minutes based on the spec sheet....

 

Doooh!  Not sure how I messed that up.  

 

Being an idiot and not reading the instructions carefully probably isn't helping my situation any 

[Gary Mulkey]

2 hours ago, Chris Christenberry said:

Gary, are you saying you quenched the packet the blade was encapsulated in?

 

Yes.  The blade was actually press quenched.  I dunked the packet (w/blade) just  long enough to cool the foil.

[Chris Christenberry]

Man, do I ever have a lot to learn about this process................no, a lot more than a lot!  If the blade was "press quenched", then why does one waste time with quenching in oil/water in the first place?????

[Gary Mulkey]

4 minutes ago, Chris Christenberry said:

Man, do I ever have a lot to learn about this process................no, a lot more than a lot!  If the blade was "press quenched", then why does one waste time with quenching in oil/water in the first place?????

 

I wanted to remove the heat from the foil so that the press quench would be that much quicker.

[Gerhard Gerber]

Been said already but 2 hours is way too long, my mentor has been to a (the?) Bohler factory specifically for advice on heat treating, and he varies soak time according to the general size and thickness of the blade.

A 6mm thick 330mm long blade soaked the full 30 minutes, 4mm and half the length 15 minutes.

As he put it, whatever is going to happen inside the steel has happened already.

 

He puts a layer of newspaper around the blade, and what really surprised me is the top of the foil envelope where the wire hook protrudes is not really sealed at all, basically squashed flat and bent over with little ears to either side.

On Sunday my job (with wet gloves) was to unfold the ears, press on the side of the envelope to open it up, then he pulled out the blade and between the plates it went.....

Haven't got the HRC figures yet but last time was 62 before tempering, no decarb and very easy cleanup sanding.

[Gerhard Gerber]

On 11/21/2019 at 1:47 AM, Joël Mercier said:

Here's a more recent chart from the same source. We can see S35VN isn't a particularly tough stainless steel.  I have a benchmade in s30v and I'm not impressed by it's edge holding either. 

 

Several manufacturers we "busted" running these super steels too soft, perhaps for ease of sharpening, but you loose a lot of performance.

 

It also seems many companies mess up the steel sharpening on belts, I've watched several videos (different steels and manufacturers ) where the improved performance considerably by sharpening on stones, and several repeated sharpenings to remove some of the damaged steel.

 

Personally I was gifted a Spyderco Mule in Maxamet with very small chips on the edge.  It wasn't difficult as such to fix the edge, but it did completely kill my Lansky rough diamond stone. 

[Alan Longmire]

9 hours ago, Chris Christenberry said:

Man, do I ever have a lot to learn about this process................no, a lot more than a lot!  If the blade was "press quenched", then why does one waste time with quenching in oil/water in the first place?????

These super steels are slow hardening, so we have lots of time.  With 1095 you must drop the temperature from critical to below 1000F in under one second. With 5160 you have around five seconds. With 0-1 it's more like ten.  With these air- hardening stainless steels you have nearly a minute, in some cases even more.  The plates are more for preventing warps than anything else.

[Chris Christenberry]

10 hours ago, Gary Mulkey said:

 

I wanted to remove the heat from the foil so that the press quench would be that much quicker.

 

 

I just wrote out a lengthy dissertation here that only exposed my complete ignorance.  I deleted it and am just going to sit here and listen.  Maybe I can do a "soak" on my brain.

[Jerrod Miller]

12 hours ago, Gerhard Gerber said:

As he put it, whatever is going to happen inside the steel has happened already.

 

Not quite.  Everything you want to happen has happened.  Holding longer will grow grains and possibly affect the carbides.  This is why it is nice to know what things look like prior to the final heat treat.  If the carbides look great, meaning they are appropriately sized and dispersed, you may not need to soak at all as you just have to convert the matrix (everything except the carbides) to austenite and then to martensite.  Soaking is only to deal with carbides (or to grow grains if you ever want to for some reason).  That isn't to say this situation is very common.  If the metal was in a state that was workable for shaping, it probably wasn't an ideal final carbide distribution.  But let's say you accidentally over tempered it.  Then it is possible that a no soak heat and re-quench might be just fine.  

[Gerhard Gerber]

Hi Jerrod

 

I suspect these concerns are more relevant to me heat treating 5160.  What I've been doing is putting the blades in the kiln when it's passed 600C.  The blade is then in the kiln while it gains the last 200C. 

Results have been good but I do worry about this, and I need to (and do) adjust according to blade size and thickness......but largely by "feel" 

 

As far as my mentor's comments are concerned, he's mostly standardized on Elmax and Damasteel, and his treatment is slightly different from the official specifications.....based on what he was told at the factory, and specifically for his application.