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Heat Treating and Metallurgy Discussion of heat treatment and metallurgy in knife making.

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Old 08-07-2017, 05:40 PM
KevBooth KevBooth is offline
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Salt vs. Oven

From a completely metallurgical stand point, safety issues aside, why would a guy buy an oven over a salt bath? I get the tempering thing that could be done in a conventional oven, but besides that? Is there any other advantages of an oven over a salt bath?
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Old 08-07-2017, 09:06 PM
jimmontg jimmontg is offline
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Not an expert Kevin, but salt baths are used for very high heats and high temperature tempering to mainly keep oxygen off the metal. There are others who can tell you much more. Salt melts somewhere around 500 degrees so you get the idea. It will protect the steel from decarburization which is a BIG concern for many steels. Imagine a 2200 degree austenizing heat with a 1000 degree temper. Either use salt baths or vacuum oven/controlled atmosphere and oxygen must never be allowed to hit the surface if it is a called for requirement. It is not just for knives and has many industrial uses as well. For some tooling it is part of the price when you wonder why that end mill cost 4x more than this one.

Don't ask me how they quench a knife in a salt bath as I have no idea. lol


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Old 08-07-2017, 11:45 PM
Doug Lester Doug Lester is offline
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Quenching would be done in a low temperature molten salt bath that will melt at a lower temperature as opposed to a high temperature salt bath. You can marquench (also known as martempering) with it by quenching the austenetic steel in it a holding it in the low temperature bath until the austenite converts to bainite or a mixture of bainite and martensite, depending on the alloy and the temperature and time selected. It's safer than using oil for the process but making a blade with all bainite or a mixture of bainite and martensite is a questionable process so I'd skip it.

You could also use a low temperature salt bath in place of an oven for regular tempering if you have a salt that will melt in the range that you want. The only advantage that I could see is that the molten salts would transfer heat better the the blade.

Doug


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Old 08-08-2017, 08:18 AM
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Ed Caffrey Ed Caffrey is offline
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I have/use both, and each has their place in the shop, and each has it's own specific strengths and weaknesses.

A digitally controlled heat treat oven has the advantage of accurate, multi-segment heat treat operations, which is a requirement when heat treating most stainless steels.

These sort of operations are at best very tedious with salt tanks.

Salt tanks, particularly high temp ones, must be built with careful consideration in mind to achieve accuracy. If the actual tank is too large (too much volume) overshoot/undershoot becomes major issue. Generally the size of the salt tank should be ONLY large enough to fit the widest blade you ever intend to place in it.

The "salts" that are used in a high temp tank are generally a "neutral" type salt..... if, after a number uses, that "balance" of the salts depletes, it can literally suck the carbon out of blades. this is why salt tanks always have a layer of carbon/charcoal, etc. floating on their surfaces.... the salts "consume" the carbon from these materials to re-balance itself.

For me the biggest advantage to salts is the fact that I can take a blade very close to finished, and then heat treat it, and afterward only minimal clean up is required.... I particularly like salt tanks for carbon steels/ folder blades.

In the the end, both are essential pieces of equipment in my shop. There are many other differences, but those are just some that come immediately to my mind.


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Old 08-08-2017, 08:42 AM
Kevin R. Cashen Kevin R. Cashen is offline
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Quote:
Originally Posted by KevBooth View Post
From a completely metallurgical stand point, safety issues aside, why would a guy buy an oven over a salt bath? I get the tempering thing that could be done in a conventional oven, but besides that? Is there any other advantages of an oven over a salt bath?
Thanks

I had many differences immediately come to mind, but when I include your qualifier of from a completely metallurgical standpoint, the list of differences narrows quite a bit. The steel only knows heat, it doesnt care what the source is. Atmospheric shielding is one metallurgical factor, but it can be taken off the table if the oven has an argon purge. The main factor left on the table is speed and evenness of heating. Liquid salts are highly conductive and will transfer heat much more quickly than an oven, this will have various metallurgical effects. And, of course, evenness of heating, accomplished by the convective and conductive salts can be a distinct advantage.

Setting the metallurgical effects aside, for the average knifemaker the cost, expense, upkeep and safety considerations makes ovens very attractive compared to salts. With the aforementioned argon purge, a good oven could hold its own against salts at typical knifemaking tasks, when compared to the hassles and complications of salts.
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