Why does forging metal make a better knife than casting one?
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Casted steel (iron and carbon) solidifies into malleable crystals. Shared planes between crystals shear easily. Folding and flattening (forging) cast steel misaligns planes between the malleable crystals, which in turn strengthens the steel.
The hammer strikes of forging actually rearrange the molecules of the steel to make it harder than if it was simply cast.
Harder steel keeps an edge longer and it less likely to bend.
The downside to forging is that the steel becomes more brittle, so snapping becomes a concern.
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It depends on the metal. You’re fine casting a bronze knife, although it’s good to then hammer it a bit, it makes it harder and tougher (which keeps a better edge). It just behaves “nicely”, because it’s structure isn’t very complex.
You could even cast elementally pure iron and it’d behave “nicely” (i.e. forging wouldn’t make a massive difference), although you won’t find elementally pure iron outside of a specialist environment, like a lab. It really likes to alloy with stuff.
And you can cast, well, cast iron. It’s right there an the name. But it’s really brittle and just overall kind of crap for anything but simple things, like pots or frying pans.
To cast steel, you need stupid high temperatures to melt it, so you won’t find molten steel outside of a steelworks. All steel starts molten, and technically is cast.
But iron alloys, whether it’s steel, cast iron or wrought iron, are… Complex. It takes several university modules to understand their behaviour. It suffices to say that when you cast them, you not only get porosity, but you also get non-uniform structures that drastically impact the mechanical properties, mostly negatively (except for edge cases). You can control those structures, and with that the properties, through various heat treatments and mechanical deformation. And deforming a metal at various temperatures is… Forging. Although yiu still have to heat treat it after shaping, to get the desired final properties.
People started out forging iron simply because no forge could get hot enough to melt it, like bronze, so it was straight up impossible to cast it. But even when we learned how, it turns out that through the peculiar nature of the iron-carbon alloy, forging and heat treating it is STILL the best way to manufacture things out of it.
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