Chromium allows for the stainless steel to be hardened and toughened. Steel that is fully hardened is very resistant to galling. Only steel that is high in carbon can be hardened and tempered. Springs must be made of hardened steel. Another difference is that steel can be hardened by heat treating. The ultimate ironclad armor was case hardened nickel-steel. Carbon steels are not very hardenable meaning they can not be hardened throughout thick sections. Depending on the temperature and composition of the steel, it can be hardened or softened. The dangerous game projectiles were often hardened lead alloy. High phosphorus iron can also be hardened by cold hammering. BLU-109 penetrating warhead for use against hardened targets. Tank armor and other removable vehicle armor can also be hardened with depleted uranium plates. It is most effective in protecting against shaped charges and specially hardened kinetic energy penetrators. Armor- or concrete-piercing fuzes are specially hardened. The city itself is chiefly built upon terraces made up of hardened volcanic sand.