METALLIC MATERIALS ON PIPING MATERIALS

Metallic Materials is one of piping materials that Piping & Fabrication will expose today, and with this post I hope we all know what is Metallic Materials on Piping Materials.

Metals are divided into two types: ferrous, which includes iron and iron-base alloys; and nonferrous, covering other metals and alloys. Metallurgy deals with the extraction of metals from ores and also with the combining, treating, and processing of metals into useful engineering materials. This section presents the fundamental metallurgical concepts and practices associated with the most common engineering metals, and outlines metallurgical considerations appropriate in the selection process of metals for piping system construction.

Ferrous Metals
Metallic iron, one of the most common of metals, is very rarely found in nature in its pure form. It occurs in the form of mineral oxides (Fe2O3 or Fe3O4), and as such it comprises about 6 percent of the earth’s crust. The first step in the production of iron and steel is the reduction of the ore with coke and limestone in the blast furnace. In this process, the oxygen is removed from the ore, leaving a mixture of
iron and carbon and small amounts of other elements as impurities. Coke is the reducing element and source of heat. The limestone (CaCO3) acts as a fluxing agent which combines with impurities of the ore in the molten state and floats them to the top of the molten metal pool, where they can be removed as slag. The product removed from the blast furnace is called pig iron and is an impure form of iron containing about 4 percent carbon by weight percent. Liquid pig iron cast from the blast furnace is sometimes used directly formetal castings. More often, however, the iron is remelted in a cupola, or furnace, to further refine it and adjust its composition.

Cast Iron
Pig iron that has been remelted is known as cast iron, a term applicable to iron possessing carbon in excess of 2 weight percent. Compared with steel, cast iron is inferior in malleability, strength, toughness, and ductility. On the other hand, cast iron has better fluidity in the molten state and can be cast satisfactorily into

complicated shapes. It is also less costly than steel. The most important types of cast iron are white and gray cast irons.

White cast iron is so known because of the silvery appearance of its fracture surface when broken. In this alloy, the carbon is present in the form of iron carbide (Fe3C), also known as cementite. This carbide is chiefly responsible for the high hardness, brittleness, and poor machineability characteristic of white cast iron.

Chilled iron, a form of white cast iron, is cast against metal chills that cause rapid cooling, promoting the formation of cementite. Consequently, a structure is obtained which possesses high wear- and abrasion-resistance, the principal attribute of the material, but retains white cast iron’s characteristic brittleness.
Malleable cast iron is the name given to white cast iron that has been heattreated to change its cementite into nodules of graphite. The iron becomes more malleable because, in this condition, the carbon as carbide no longer exists continuously through the metal matrix.

Gray iron is a widely used type of cast iron. In this alloy, the carbon predominantly exists in the form of graphite flakes. The typical appearance of a fracture of this iron is gray since the graphite flakes are exposed. The strength of gray iron depends on the size of the graphite particles and the amount of cementite formed together with the graphite. The strength of the iron increases as the graphite crystal size decreases and the amount of cementite increases. This material is easily machined. A wide range of tensile strengths can be achieved by alloying gray iron with elements, such as nickel, chromium, and molybdenum.

Another member of the cast-iron family is so-called ductile iron. It is a high carbon magnesium-treated product containing graphite in the form of spheroids. Ductile iron is similar to gray cast iron in machine ability, but it possesses superior mechanical properties. This alloy is especially suited for pressure castings. By special procedures (casting against the chill) it is possible to obtain a carbide-containing, abrasion-resistant surface with an interior possessing good ductility.