eli5 – why is cast metal weaker than forged metal?
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When liquid steel cools it forms crystal structures. But this happens at different places within the steel at the same time. And these crystals that start in different places form in different orientations. Where they meet there will be a much weaker structure as the crystals do not connect well. It is much easier for the steel to break where these different crystal structures meet then in a continuous structure. When you forge the steel you are moving the crystals around so they meet at other orientations throughout the process. When they meet at different orientation they form weak bonds which will get broken in the next hammer hit but when they meet at the correct orientation they form a continuous strong crystal.
There are also a few other crystal defects which can be fixed by forging. Sometimes as the crystals form and solidify there may be a gap in the crystal structure as there happened to not be an atom there at the time. This creates a tiny weak spot and when the hammer hits the crystal the atoms will shift around filling this hole and creating a hole a different place. So you force these holes out to the edge of the crystal. Similarly you might have areas of too much carbon or too much iron which again creates weaker crystals and the hammering process will force these around to form a continuous strong alloy. It is not uncommon to see excess carbon flake off the outside of a steel piece as it is being forged. The same happens with any trapped oxygen or even air pockets.
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