View Full Version : Inert Furnaces
08-30-2010, 11:54 AM
I'm getting way ahead of myself as usual, but where do makers get vacuum inert furnances at for Hting stainless and other high alloy steels? I've been looking and pretty much all I seem to be able to find is industrial models which range between 78-100 thousand+ which is not including the freight and setup expenses. So where can smaller units more practical for the home shop be gotten from, if there is any? Are they custom built or what?
If the makers that do Ht their own stainless use salt baths instead, what are the advantages and disadvantages of that method over inert furnaces?
Pardon my ignorance; but Hting my own steels is something I really want to be able to do later on.
08-30-2010, 12:05 PM
I don't know of many individual makers who use inert vaccume furnaces. Most makers use a kiln along the lines of an Evenheat, Sugar Creek, or Paragon oven ($400-$2000 depending on size and controller options). These are not irnert atmosphere kilns. However, I know of some people who have used argon flooding to remove oxygen. I don't believe it is very cost effective, and success of removing oxygen from the atmosphere is limited in many cases.
For stainless steels, most makers use stainless foil. The blade is wrapped in a pouch and the ends are sealed by crimping. This grealy limits the amount of air in contact with the blade to just the small amount in the packet. Because most stainless steels are air or plate quenched, you can quench while the blade is still in the packet.
Slat baths are a great means of heat treating steel, but they are VERY dangerous if you don't know what you're doing (and even if you do). Having a salt bath solution at 2000F is an explosion of molten salt, 3rd degree burns, burnt down shop, and even death waiting to happen for the uninitiated. The slightest amount of moisture coming in contact with the salt can cause them to explode out violently (think of a turkey frier on EXTREME steroids). They can also be very oxidizing to equipment in your shop.
They are great about protecting blades and for marquenching, and many makers use them. But just do your homework and be VERY cautious if you decide to go this route.
08-30-2010, 12:16 PM
So basically; salt baths should be in a separate, humidity controlled, room pretty much isolated from other equipment if possible? And how do you go about air quenching the stainless steels if you just use the stainless foil since they are air hardening?
08-30-2010, 12:25 PM
I'm going to defer the salt bath question to those more familiar with them than myself. I studied up on it at one point when I was looking at building some salt tanks, but I decided I really had no need with the size of my hobby knife production. You can keep them in the shop, but just have to be careful about storage and maintenance of the salts as well as avoiding moisture contact. I'm not trying to steer you away from salts by any means as they are great in heat treating, but just make sure you do your homework. Kevin Cashen is a great guy to talk to about salt tanks.
As for the stainless steels, they are typically referred to as air quench steels as they will harden just with cooling in still air. However, many makers use a compressed air quench or large aluminum plates to speed the quench some. I personally use the plates. I have two 1"x6"x12" plates that are mounted in a wood vise. After my soak at temperature, I remove the packet from the oven (blade wrapped in SS foil pouch), and I place the packet between the plates. I lightly clamp the plates down to make good contact, and the aluminum acts as a large heat sink, drawing the heat out of the steel and cooling them faster than you would in room air. Most makers I know are using plates with stainless steels, maybe combining with compressed air. I'm not sure of many who just use compressed air, so I'm not sure I could answer how effective it is to leave the packet on and simply air quench. The great thing about stainless is that you have a much longer amount of time to drop your temperature below the pearlite "nose" than with, say, 1084 (where you have roughly 1 second).
08-31-2010, 07:57 AM
That was my misunderstanding then, I was under the impression that stainless was quenched by gases being purged away from it. Thanks for the info.
08-31-2010, 08:57 AM
It is my understanding that there are some commercial heat treaters who use nitrogen injection equiped ovens that can control the cooling rate as fast or slow as desired within the oven, but those are well beyond the means of this miserly knifemaker :).
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