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Minimum temperature at which steel can be hardened?

What is the lowest temperature steel must reach in a hardening process.

David Sigmon
patent attorney - ex-engineer - Greenville, South Carolina
2004

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It depends on the composition of the steel. The ASM Heat Treater's Guide lists recommended heat treating temperatures for common SAE/AISI grades of steel.

Steve Bizub
- St Louis, Missouri
2004

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As an initial point of reference, A1 is ~ 727 C, with A3 varying depending upon the concentration of C, Ni, Mn, Cr, Si, Mo, Ti, etc. If you provide some additional information on composition, etc., then perhaps we can further refine the answer.

Toby Padfield
- Michigan
2004

Your question is not specific enough. If you are referring to hardening that takes place by martensite formation after austenitizing a hardenable steel and then quenching it in a suitable medium (oil, water, air, etc.), then it is usually taken to be the A3 temperature. This refers to the temperature at which transformation of ferrite to austenite is completed. Technically, some martensite will form if the steel is heated above A1, which is where ferrite begins to transform to austenite, and then quenched. A1 is lower than A3. Either way, these temperatures vary depending on the exact chemical composition, and to a degree, on the heating rate (extremely rapid heating by induction differs from slow furnace heating).

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2004

David, that all depends on the hardening mechanism that you're considering, and therein lies the rub- there's a lot of them in an alloy system as generic as "steel". The most common way to harden steel is through a quench and temper mechanism, wherein you form martensite, a body centered tetragonal structure. To get there, you have to first get into the usually higher temperature austenite (face centered cubic) structure, and the temperature you have to reach depends on the alloy content of the steel. Heck, there are a lot of "steels" that are austenitic at room temperature, like some stainless and Hadfield grades. So, you really should specify your alloy, i.e. which "steel" you mean.

If you're only thinking iron-carbon, you're probably thinking of the 727 C phase diagram boundary line between BCC and FCC. Not unreasonable.

Then you need to specify the hardening mechanism. The Q&T I noted earlier is what the world generally thinks of when you say "harden steel", but it doesn't have to be. You can harden steel by ion implantation, for example. Shoot an energetic beam of nitrogen ions at a steel surface, and you can get a really, really thin hard surface layer. Such might meet the definition, and it can be done at liquid hydrogen temperatures.

So as unsatisfying as it is, we need more information. Good luck!

Lee Gearhart
metallurgist - E. Aurora, New York