Generally, "stainless" and "steel" cannot be joined easily. Therefore, by using the technique of "forge welding" used by blacksmith, "stainless" and "steel" are joined firmly.
First, the cutout material is formed into a 3-layer structure, in the order of "stainless" + "steel" + "stainless."
A chemical called "forge welding material (borax)" is placed between each material, and the cutout material is placed into the kiln (850-900 degrees) to make it red. Hammering right after makes each material joined firmly.