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I am going to make a spoon out of 80% iron and 20% chromium (by weight).

If I slowly lower the temperature from about 1700C (at which all is melted) to room temperature, I believe it will finish as a "zebra mixture" with grains of iron and grains of chromium (to be accurate, each grain will have about 1% of the other element too). For example, suppose I see 50um grains. I probably will get a little segregation of the two elements at the nanometer level.

On the other hand, if I rapidly lower the temperature with a water quench, the atoms will be somewhat more mixed which might affect the spoon's hardness, so it seems worthwhile to try this also.

Since spoons usually have carbon, manganese, and nickel added too, I believe there will be a problem with both of my new spoons. What is it?

I understand already that carbon is added to make the steel harder, but would my spoons be so soft that, if made with normal spoon dimensions, their necks will bend too easily when I scoop ice cream? I'd like to get a feeling for how soft these spoons will be.

bobuhito
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  • do you think that carbon is somehow more expensive than Cr? – Tiger Guy Nov 17 '21 at 21:01
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    FYI: It's not steel if there is no carbon. – DKNguyen Nov 17 '21 at 21:09
  • @TigerGuy No, but I just assumed I would at least need Cr because I don't want my spoon to rust within a year. – bobuhito Nov 17 '21 at 21:18
  • Question...how is this cheaper than just purchasing a stainless steel? I know you say cheapest, but mass production in this case would drive down costs past fewest ingredients, since 303/304 stainless steel is very common but iron-chromium alloy is a specialty item. You might also be interested to know that although 303 contains less chromium than 440, 303 is more rust resistant. So it may not be as straightforward as just having 20% chromium. The lack of nickel or increased carbon in 440 may play a role. – DKNguyen Nov 17 '21 at 21:33
  • @DKNguyen It is debatable as to whether this is "cheapest". If you need to, replace that word with "simplest". Anyway, my question is more about the physics and less about the cost...it's easiest to think about just 2 ingredients, so please answer disregarding the cost if you can. – bobuhito Nov 17 '21 at 21:45
  • General purpose stainless for household goods is often 18% Cr for whatever reason. "18-10" Cr-Ni is especially common ... I believe when the nickel is eliminated, it is usually because there is some other objective for the alloy besides corrosion resistance. – Pete W Nov 18 '21 at 01:30
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    I believe you should research ferritic steel. High chromium, low nickel and little carbon. – DKNguyen Nov 18 '21 at 04:20

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The "zebra" structure you imagine does not occur. Iron and typical stainless elements are mutually soluble as liquids, so freeze homogeneously. There can be chrome depletion around carbides ( sensitization) but something like an electron microscope with EDAX is needed to "see" it. The leanest, and likely cheapest, stainless is 5% Cr in AISI listings, it resists oxidation well enough that it can't be cut with oxyacetylene torch. The general public likely accepts 13% Cr as a stainless ( auto exhaust pipes, knives, and many other applications). I suspect the zebralike pattern you may have seen is "Damascus" steel which is layers of high and low carbon content carbon steels that can be seen by eye. Most logical would be to make spoons with 410 SS (13% Cr) . Not "S" grade ( low carbon ) and not 420 SS ( high carbon cutlery grade). Air cool will harden it, temper at about 800F.

blacksmith37
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  • The Fe-Cr phase diagram shows that segregation ("zebra") must eventually come, but thank you, I now see that segregation is a slow solid-state diffusion process which would require years of slow cooling through the critical 600C to 400C window. So, I edited my question - Can you answer for the two specific questions at the end? Instead of my chosen 20% chromium, an answer for 5% or 13% is fine. I really just want to understand why I still need carbon or manganese or nickel or more processing. – bobuhito Nov 18 '21 at 16:58