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Brian Dougherty

S35VN Positive Pressure Quench?

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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)

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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.  

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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%.

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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.  

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