eg, If I were to quench a sheet of steel vs a bar, how and where would martensite form for each one?
I would post this on say, /r/metallurgy, but I’m afraid it won’t pick up enough attention for reliable answers. Sorry.
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For a given alloy of steel, there will be what we call a critical cooling rate, the rate that the temperature needs to drop in degrees per second for martensite to form in a quench. The important thing to remember is that that cooling rate applies to each distinct point in the steel. For example, the very surface a a piece of steel which is actually in contact with the quenching medium will have a very high cooling rate and will almost definitely form martensite as long as you’re doing things correctly. The very core of the same piece however is, effectively speaking, insulated from the quenching medium by all the steel around it which may drop the cooling rate at that point enough that you won’t fully transform to martensite.
This effect is most significant in plain carbon or very low alloy grades like 1080 or W1 tool steel; you’ll sometimes hear these grades called “surface hardening” because even under perfect conditions in a very harsh quench you will only fully harden a fraction of an inch into the surface. For applications where a thick part needs to be through hardened, you use an alloy grade with a lower critical cooling rate, which allows you to tolerate the lower cooling rates in the center of your part and still get a fully martensitic structure.
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