Wiki-Forge welding

It is a process in the manufacturing of Japanese knives where the jigane (soft iron) and hagane (hard steel) are joined together by heat and pressure from hammering the metals. By adding a layer of powdered refractory material such as feldspar onto the heated jigane to prevent the formation of oxide, the hagane is then placed on top and hammered to join the two metals together.

Related websites
The Art of Making Japanese Knives: Detailed Process | KIREAJI