Samurai swords are made with soft steel in the core and hard steel on the outside. Hard steels are produced when a relatively high amount of carbon is dissolved in molten iron during smelting. Low carbon content results in softer steel.
Bladesmiths beat the layers of steel together and fold them up to 20 times to remove impurities and spread the carbon more evenly through the layers. At the atomic level, carbon atoms fill gaps in the iron’s crystal lattice, strengthening fracture points. This produces a strong, flexible sword with an edge that’s hard enough for sharpening to a fine point.
Answered by Michael Simpson