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 for metal 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 heat- treated 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 machineability, 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.
Steel
Steel is defined as an alloy of iron with not more than 2.0 weight percent carbon. The most common method of producing steel is to refine pig iron by oxidation of impurities and excess carbon, which have a greater affinity for oxygen than iron.
The principal reduction methods used are the basic oxygen process (BOP) and the electric furnace process, each representing a type of furnace in which the refining takes place. The BOP primarily uses molten pig iron as the initial furnace charge; the electric furnace can use a charge of selected steel scrap. Another process, called the basic open-hearth process, is no longer in use in the United States. Although it constituted the major steel producing process for decades, it has succumbed to the more advanced and economical BOP and electric furnaces.
The pig iron is reduced to the desired steel composition through use of acid and/or basic reactions with fluxing agents, heat, oxygen, and time. Excess carbon is oxidized and lost as gas; impurities float to the surface. Often desired alloying elements are added to the molten pool. The steel can be further refined by using one of various methods of vacuum degassing. As the name suggests, the molten steel is passed through a vacuum chamber with the purpose of removing entrained gases such as oxygen, hydrogen, and carbon dioxide. This operation is performed when extra steel purity is desired, and it results in improved and more uniform properties in the final product form.
The molten steel is then cast into molded ingots, which are then further reduced by hot working in rolling and drawing operations. Alternately, the molten steel may be directly cast into continuous smaller billet or hollow products. The latter process is called continuous casting and has become the preferred method of making steel since it avoids the costly ingot reduction operations.
Alloying additions are made, if required, to the molten steel either while in the reducing furnace as already noted, in the ladle into which the steel is put, or in the ingot into which steel is poured from the ladle.While the steel is molten in the furnace, oxygen is forcibly injected into it to refine the charge. The oxygen combines with excess carbon and is released as a gas. Excess oxygen is, however, unavoidably left in the molten steel. This results in the formation of oxide inclusions in the steel, or porosity, which appear upon solidification. The process of removing the oxygen is known as deoxidizing practice. Deoxidation is achieved by adding silicon, aluminum, or other deoxidizing agents to the molten steel, the amount of which determines the degree of deoxidation and the type of steel seated. The common names given to these various steel types are killed steel, semikilled steel, and rimmed steel.Steel of the killed type is deoxidized almost completely; that is, sufficient deoxi- dizing agent is added to the molten pool to combine with all the excess entrained oxygen. The result is a large number of tiny oxides in the melt. The lack of gas in the molten pool gives the effect of ‘‘killing’’ any visible bubbling activity of the steel, thus the name. Killed steel has a more uniform composition than any of the other types, and usually possesses the best formability at room temperature. A fine- grained structure results from this practice because the many oxides formed act as initiation sites of new grains upon solidification and subsequent recrystallization. This fine-grained character offers toughness superior to the other types of steel.Rimmed steel employs no purposeful addition of deoxidizing agents, and is characterized by relatively violent bubbling and stirring action in the ingot mold. This type exhibits a marked variation in composition across and from top to bottom of the ingot. The outer rim or outer edge of the solidified ingot is relatively pure and ductile material. The amounts of carbon, phosphorous, sulfur, and nonmetallic inclusions in this rim are lower than the average composition of the whole ingot. The amount of these constituents in the inner portion or core is higher than the ingot average. This type of steel costs less to make than the other types and is widely used for structural applications, where good surface appearance of the final product is desired.
Semikilled steel is only partially deoxidized with silicon, aluminum, or both, taking advantage of the positive attributes of killed and rimmed steel.
After casting, or teaming into the ingot molds, the steel is normally further reduced in size and modified in shape by mechanical working. The majority of the reduction process is done hot. During hot working, sufficient heat is maintained to ameliorate the working effects and maintain a structure that is relatively soft and ductile throughout the reduction process.
The steel in the form of ingot, slab, bar, or billet is first brought to the proper temperature throughout and is then passed through rolls or dies. The flow of metal is continuous and preferentially in one (longitudinal) direction. The cross-sectional area is reduced, and the metal is shaped the desired form. The internal structure of the steel is also favorably affected. The working reduces the grain size of the material, and tends to homogenize the overall structure, compared with cast or unworked steel. #Little_PEng