salt bath vs oven

Theoretically, the rapid heating associated with salt pots could be a problem and cause or contribute to cracking, but how much of a problem that might be would depend on a number of variables. It‘s related to quench cracking. They are also dangerous.

Cracking:
http://www.industrialheating.com/articles/90314-quench-cracking

The atmosphere in an electric oven can oxidize the steel, but there are ways around it and/or to minimize it. There are also concerns with temperature fluctuations. Both problems can be minimized or dealt with by using a muffle inside the furnace chamber.
 
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On a much less technical level than Tai offers, I have scars from a pot of molten lead that 'blew up' from a drop of water back when I was making lead shot from old tire balancing weights. That pot was probably something like 500F. I hope I never actually experience molten salt going off, though I nearly did once. So safety is a BIG concern.

Obviously, I've never personally used salt for HT, but I'm told it can corrode other things in your shop. There was a thread on this a while back and some suggested the corrosion is manageable.

Salt offers some tempting capabilities. FWIW, I do have a number of customers that come to me for HT because they dislike the clean-up they have to do when they send their blades for salt HT. That surprised me a bit.

Now, if anyone has a nice vacuum furnace for sale....... :biggrin:

My 2 cents (which we're not supposed to use in Canada anymore)
 
badgerknives said:
which is better? i'm looking at heat treating 52100 mostly.
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Badger, there are some concepts that are not being entirely understood or heavily minsinterpreted in this thread, I have quite a few years of personal experience with salts and would be happy to discuss them with you if you woud like to e-mail me (kevin@cashenblades.com) although I must warn you that my experince has made me even more of a worrywart than knifemaker.ca about the safety aspects.
 
If there are any doubts about the possibility of cracking or crack initiation/propagation from rapid heating, it’s really just kindergarten physics and applies to steel, but I recommend researching it on your own,… rapid thermal cycling, rapid thermal expansion, thermal stress, quench cracking,… failures due to heating treating etc. You decide.

The danger factor of salt pots should be obvious.
 
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From the article you posted Tai

Cracks occurring during heating are most often related to material imperfections, seams and inclusions.
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So if you're seeing cracks from the rapid heating, I'd suspect bad material over the rapid heating. Once again another reason to know your steel and if there are problems suspected, contact the supplier.


Zeb
 
With all these failures you keep bringing up about heat treating something as "complicated" as a knife blade I'm supprised that anyone can do it, or why we even try.

Zeb
 
And how many years of experience do you have with salts Tai?


One said:
... but I recommend researching it on your own...
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I would say the same as my research includes a bit more than misinterpretation of 30 second Google searches. It is entirely within my rights to have whatever private conversations I wish so I will do exactly that to keep the peace here, and allow you all the latitude you like to hand out your pearls of wisdom with no interference from me. I suggest you take that very generous compromise and be happy.
 
Zeb, no it's not that. It's just being aware of the possible problems and complications, so we can deal with them.

Virtually all steel has flaws, inclusions etc., to some degree. It's safer to assume that they are present and exist in the worst possible locations than to assume that they don't... With something as problematic as knife blades,... thermal gradients, long thin parts, prone to cracking and distortion like knife blades, it's especially important to play it as safe as you can, without completly sacrificing your knife or knifemaking concept and/or philosophy.

… This all makes any definitive answers about which is "better or best" very hard, if not impossible, to come by and is part of the reason heat treating is not considered an exact science, but rather an art that involves lots of science.
 
Kevin, let’s try and look at this from an emotionally detached scientific perspective and not make it personal.

I think it’s important to understand some of the basic principals involved and how they might apply to knifemaking. I think folks are fully capable of "interpreting" the "facts" and "principals" for themselves.
 
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I see the two main advantages of salt pots as being, shielded from oxygen and very uniform heating in the final outcome,... aside from thermal gradients etc.
 
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I once had a pot of molten lead explode in to my face, because we were worried about it exploding we had set it up out side, luckly it was winter and there was a mud puddle near by:). thanks for the in put guys.
 
Does not the industry use salt pots for many of their applications because it does heat up faster, holds that heat more consistently, allows for better efficiency of the resources needed to heat the steel and manipulation of the steels structure in both austenizing and tempering? They are about speed time and money so if something as simple as a knife shaped object would crack apart all the time I would not think they would pursue them the way they have. Same goes for fluidised beds which are using alumina and other mediums with an inert gas blown up through the media to heat as quickly but minimize the risk of having a molten bath but not the steel. Induction is even faster I believe and it too is used.

I have not used a salt pot so the above are educated conclusions and from reading a couple industry books on the matter and their application including the last one on fluidised beds as well as talking to some that use them on a regular basis for knife making.

I have been researching and tetering back and forth on building a set for the past two years. I have the SS 316 1'4" seamless pipe, venturi burners and am just deciding on the Houghton salts but have decided to really focus on my oven once more before going to the salt pots. I built another open shop for them so there is nothing to burn down in the event my personal pool of lava were to erupt or a weld to fail. I have been over and over again the safety of the salts expansion and see how to manage it by making a chimney using a tapered rod before it solidifies,as well keeping the contaminates in the salts managed for the correct PH and many more issues.

I think that having a pot or bed setup is a tool to be respected and is not a tool for everyone nor is it a magic bullet and in fact I am convinced it is most likely the opposite if acute attention to detail at all times are not met with its construction, operation and continued use. For this reason I began asking myself real questions and took stock of what I want in a blade and what can I really accomplish with an oven. In the end I have decided to focus on my oven for a bit more till I can no longer achieve what I want. If I want to marquech I can buy the proper oils for that but again there are dangers with it as well. I check the calibration of my thermocouples once a month now with an ice bath, insure I am using proper quenching oils for a steel and that they are at the correct temp, That my forging and grinding are as even as possible and proper care of the steel structure has been managed though out the entire process, Using IR thermometers for determining steel temps on the interrupted quench so I know when its at 400* or less as well as proper cooling then tempering as soon as possible to convert any RA.

All of this can be done with an accurate PID controlled oven and a few other simple tools and is much safer than molten salt but it just takes a bit longer and in the end for me and those that care about me I am willing to take the extra time and added safety.

The reason I teter is that I like cpm 3v a lot and I want to do my own heat treats and sub zero treatments but I am concerned because I have been told that taking my oven to 1950* stresses it too much. Is this true? I dont know but I do know the oven is designed for temps up too 2300 and I never plan to go above 1975*. Having a high temp post would aid me greatly in working with this steel but as mentioned above I am going to forgo the pots for now till I know for certain I can no longer reach my goals with the oven.

I have responded here to post my thoughts and reasoning for the discussion of oven vs salt as one that has studied and has an acedemic understanding of the principles and risks vs befits. For now I choose the oven and dont believe with my current skills in using it and my current understanding of how steel reacts that I am at a disadvantage. I do believe that salts provide the maker with the very best in heat treating and the broadest range of artistic expression when choosing how to manipulate the crystalline structure of the steel but it always comes at risk and a cost. Again I am a nobody in this world but I am someone willing to search out the truth before moving forward and applaud you for asking.-shane
 
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Shane, no one said a "knife shaped object would crack apart all the time". Salt pots are an acceptable alternative for knife blades, but not the end all of and alls. I was just pointing out an inherent difference between salt pots and ovens and the possible risks involved with rapid heating. It’s purely theoretic, just something to consider and be aware of.

A programmable vacuum and atmosphere laboratory tube/muffle furnace, would probably be the “best” for what you are talking about,… but the price makes them impractical and out of reach for your average knife maker. They aren’t an absolute necessity,… although if I had the money to spare I’d get one.

What I use now is an old fashioned, DIY, gas or wood/charcoal version of a tube/muffle furnace. I used a standard electric furnace for over 10 years, and just got tired of it, so I switched over to the tube furnace. It's not for everyone, has advantages and disadvantages like anything else... but I like it and have been very happy with the results.
 
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One said:
Shane, no one said a "knife shaped object would crack apart all the time". Salt pots are an acceptable alternative for knife blades, but not the end all of and alls. I was just pointing out an inherent difference between salt pots and ovens and the possible risks involved with rapid heating. It’s purely theoretic, just something to consider and be aware of.....
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I am sorry I misunderstood what you were saying Tai and please never read my posts wondering if it was typed in a sarcastic or unprofessional manner as I have nothing but respect for all you guys.
 
No worries Shane. It happens all the time, though I was trying to word it as gently and politely as possible. I Hope no one else took it the wrong way.

I have a lot of respect for newbies , old dogs and anyone who is open minded, interested in learning new things, improving and growing in the craft.
 
Let's face it. A lot of the equipment that we use can injure, maim, or even kill. Last year there was a post on one of these boards from someone who came very close to doing himself in by using a gas forge without adequate ventilation. I ran into a man at an archery shoot who's neighbor managed to take the end of a finger off with his band saw. May dad shortened three of his fingers with a shaper. The point is that we need to know the dangers of the tools that we use and observe the proper safety precautions each and every time that we use them. If you think that your process would benefit from a high or low temperature salt pot then learn the safety precautions for using one and observe them.

Doug
 
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