higher temperature forging will make steel harder, sharper, but more brittle, so if you have the whole blade like that it could break easier
slower forging makes the steel more flexible but softer
so what they can do is make the blade portion the harder sharper kind of steel and the core more flexible and softer
I’m typing this from memory from some thread on swords I read a long time ago, but it’s pretty cool
Materials Engineering is sorta my wheelhouse and I can confirm what you said. If you look at the different blade compositions, you’ll notice that in every case, the actual edge is made from hard steel. As you said, the reason for this is because it is harder and can be made sharper. However, making the entire blade out of it isn’t the best idea because it will not be flexible at all and very prone to chipping or breaking. The medium steel adds strength to the backbone of the blade. The iron is heavier than the steel, adding a bit of weight to the blade. It is also easier to make than steel, so it can be used for a large volume of the blade. The folded steel is the toughest part of the blade, being made from a very elaborate process which removes impurities from the steel and makes it incredibly tough. The Soshu Kitae blade type, also known as the “seven layers method” is the design used by the famous swordsmith Masamune. More information can be found here.