Metal seals are divided into ferrous and non-ferrous metals. Ferrous metals refer to alloys containing iron, chromium and manganese, such as cast iron and its ferroalloys. Compared with rubber and plastic, metal sealing ring has higher hardness, higher mechanical strength, lower elongation and wider temperature range. Metal sealing ring is compatible with hydraulic pressure. Oil or hydraulic fluid medium is compatible, but it has poor compatibility with water and is easy to rust. The other is non-ferrous metals, some of which are pure non-ferrous metals, such as copper washer, and some are non-ferrous metal alloys. Their strength, elongation, temperature range, elasticity and other properties are usually between ferrous metal and synthetic rubber plastic. At high temperature and high pressure, metal materials are superior to synthetic rubber and plastic, but they are usually inferior to rubber and plastic in elasticity and flexibility.