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aging (hardening) for stainless


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I have been working on some stainless knives & kitchen items and want to understand the hardening process for stainless better - I have read some specifications for this process, but, wondered if anyone had any experience. I am forging the material, after an annealing fire. From what I've read, I have to "solution treat (which I interpret as annealing) the pieces after I've worked them, before they can be hardened, then heating the pieces to 850-900 degrees F and holding from 1 to 4 hours, then air cooled (have access to a small kiln for this).

I've read some of the related topics that recommend not working in stainless for knives, but, these pieces will left in kitchen sinks or proximity to water, so stainless element is important. I understand that the hardness achievable isn't what can be achieved in high carbon steel, but, for the application the stainless hardening treatment should be fine. Also, my understanding is that the precipitation hardening after work, restores some of the corrosion resistance. So the treatment seems critical to finishing the pieces. Thoughts?

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Many stainless steels can get very hard. A 200 or 300 series isn't a good choice, some 400 series will work. Also many other SS to choose from.

There are many things that effect it. The term "stainless steel" refers to a steel with over 12.5% free chromium, this forms oxides on contact with air that slow corrosion, and make trouble when attempting forge welding it. Remember that its stain less, not stain never. The rest of the alloy additions effect whether is makes good blades or not.

So back to your opening question, what steel did you use?

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I have read some spec's on 400 that make it a better candidate for my project.
What is your project? A ladle? (304 would be fine for a ladle.) A serrated bread knife? (Some of the 400s would be just fine for a serrated bread knife.) Are you making an osthyvel? Perhaps an ulu? Or an usuba?
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so I've attached a photo of my project knives thus far. I am calling them cheese knives, don't like the idea of a super sharp knife at my craft show booth or as x-mas gifts...first thing that happened to me at a x-mas sale I did was that a child picked up a knife I had and cut his hand - his mother wasn't happy. I'm not so fixated on a very hard edge, though I'd like to understand the process, and see what's attainable for me at my shop, but, treating the stainless so that it is as corrosion resistant as possible is also important. I don't want to have to continue to put disclaimers on my stuff that you can't leave it in the sink, etc.

If the lesser grade (300) is better for corrosion, then maybe this is my material. I still believe I have to go through the aging process for the material. But, if I can accomplish aging for a 400 type stainless for a hardened edge this too would be desirable, as I am making more than one type of knife.

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What you need to understand is that some grades of stainless steel cannot be hardened by thermal treatment. Period. They are austenite -- the soft, malleable phase of steel, which we normally achieve by heating steel in a forge -- at room temperature. 304 is one of those. In fact it's similar to many non-ferrous metals in sense that heating to around 1850 and cooling it rapidly will anneal it. You can harden 304 some by cold working it. And aging is not a process that would apply to cutlery steels. Stainless steels that are capable of hardening are hardened like non-stainless tool steels, except that they're air hardening, and the temperatures generally are higher and the soaks longer.

There are dozens of high quality stainless steels that are very suitable for cutlery, ranging from 440 to 154CM to S30V to 12C27 -- the list goes on and on. If you want to make serious knives out of stainless, you're going to need to buy one of those. But they are expensive, hard to work (they're usually not forged), and have very particular heat treating requirements that you will not be able to achieve with a simple forge. You'll need a pyrometer of some sort and a heat treating oven, high temp salts, an old kiln, something that'll allow you to achieve and maintain an exact temperature (within say 10 or 15 degrees) for a half hour or more. An alternative would be to send your blades out for professional heat treating.

If all you want to make are more or less edgeless cheese knives and table knives of the variety you can buy cheaply in any big box store, you can use 304. Don't bother heat treating it. As someone else mentioned, if you're serious about corrosion resistance you should look into passivation.

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Thanks, all good advise. I have experimented a bit with different attempts at hardening the 300 stock I have, and observed the what would be hardening quench, anneals the stainless, as well as the cold work hardening that can happen. I think that for my project the 300 probably is fine, though I will work on the passivity/electro polish idea. I would like to try some 400 for better knife edges as well - I do have access to kilns that are pretty superb (one plus to dating a potter) so I do have the ability to accurately take pieces up to high temps and hold them for hours. Also one of you mentioned some stainless that hardens like "non-stainless" - standard hardening & temper? I'd like to know more about this material as well. Sorry for all the questions, just in a developmental state, soaking up as much as I can as I try to develop some products. Every knife in my kitchen drawer is stainless. Are there suitable high carbon steels that can be forged that belong in the "average"(read incredibly lazy & unwilling to care for quality crafted wares) kitchen?

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Different alloys have different forging and heat treating requirements---doesn't matter if it's Stainless Steel or Titanium. You look up the alloy and determine what it needs and provide it.

A book like "The Complete Bladesmith" will provide a lot of information on forging and heat treating of various alloys. I have a 440C campknife that I forged 25 years ago that is still shiny bright where polished and black where left as forged with no maintenance over a lot of campouts and being thrown in the dishwasher afterwards.

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Also one of you mentioned some stainless that hardens like "non-stainless" - standard hardening & temper? I'd like to know more about this material as well.


All I was trying to get at is that most cutlery stainless (at least that I'm familiar with, and since I lack the means to properly HT stainless, this is entirely theory for me) is hardened and quenched, usually by air or by contact with heavy steel or aluminum plates, then tempered, in a manner very much like that used for conventional steels with sufficient carbon to harden significantly. The times and temperatures are different, and the process is much more exacting, but it's basically the same. If you look around for heat treating info on something like 154CM, ATS34 or 440C, it won't look dissimilar to the typical quench-and-temper process that you're probably familiar with (except that tempering stainless often involves a cryo step). But if you look at what's involved in age or "precipitation" hardening, that's a very different process. It increases strength but doesn't give knife-level hardness, and it's only applied to specific steels -- usually very low carbon alloys, which generally isn't what you want for a blade. You seem to be operating on the belief that precipitation hardening is how all stainless steels are hardened, and to my understanding that's not the case.
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Thanks fellows! I think I will stick with the 300 stuff I've got for my cheese knife type items and accept the hardness as is, though I will look into polishing, acid bath, etc. I did find some 440 spec's on working with 440c and this seems workable for some finer edged tools, though, it seems less "forge-able" I'll just have to see how it works. I will check out that book, too, I've seen it come up in a few of the forum topics. Any pictures of the 440 camp knife?

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