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Physical Metallurgy of Steel

Like all other metals, iron and steel are crystalline in structure, composed of atoms in a fixed lattice. As noted earlier, iron may exist in one of two cubic forms, body- centered (BCC) or face-centered (FCC).

At room temperature, pure iron is composed of a body-centered cubic lattice. In this form it is known as alpha iron, also called ferrite, which is soft, ductile, and magnetic. When heated above about 1415°F (768°C), alpha iron loses its magnetism but retains its body-centered crystalline structure. This temperature is called the Fermi temperature. The crystal structure changes to face-centered cubic at about 1670°F (910°C), at which temperature alpha iron is transformed to gamma iron, the FCC form, and remains nonmagnetic. As temperature rises further, another phase change occurs at 2570°F (1410°C), when delta iron is formed. This phase is again body-centered like that of the low-temperature alpha iron. It is stable to the melting temperature. In cooling very slowly from the liquid state, the phases reappear in reverse order.

The solid-state transformations of atomic structure, which occur in pure iron during heating to and cooling from the melting point, are called allotropic changes. The temperatures at which these changes take place are known as transformation or critical temperatures.

When carbon is added to iron and steel is produced, the same changes in phase occur, but a more complex relationship with temperature occurs. The effects of varying amounts of carbon content in iron on phase stability as temperature varies is represented in Fig. A3.18.

This diagram is called an equilibrium phase diagram, and in this case is the very familiar iron-carbon (Fe-C) phase diagram. With this diagram, one can determine which stable phase the steel will assume at a given composition and temperature. Likewise, the effect of increasing or decreasing the amount of carbon content in iron on these critical temperatures can be predicted.

Phase diagrams are plotted in weight or atomic percent (horizontal axis) versus temperature (vertical axis). A single-phase region usually represents an area of high concentration of a single element, or an intermetallic single phase stable over a range of composition and temperature. Between these single-phase regions are regions where multiple phases coexist, in relative amounts at any given temperature approximated by the proximity of the specific composition to the single-phase regions. On the Fe-C diagram, single-phase regions are represented by those marked as alpha, gamma, and delta, and Fe3C or cementite, which is a stable intermetallic phase.

The critical transformation temperatures in steel are the A1, corresponding to about 1335°F (724°C), and A3 referred to as the lower and upper critical temperatures of steel. The A3 constitutes the boundary with the gamma phase, and its temperature varies with carbon content. The lower critical temperature, on the other hand, stays constant over the entire range of steel compositions.

These critical temperatures, as well as the entire phase diagram, represent transformations that occur under controlled, very slow cooling and heating (i.e., equilibrium) conditions. More rapid heating and cooling rates, like those encountered in normal steel processing, change these critical temperatures upward and downward, respectively. Additions of other alloying elements also will shift the critical transformation points.

It is the effective use by the metallurgist of the knowledge contained on this and similar phase diagrams that allows for the manipulation of properties of engineering materials by varying their chemistry and heat treatment. For steel, the principal phases and their properties are briefly summarized in the following list:

  • Austenite: A single-phase solid solution of carbon in gamma iron (FCC). It exists in ordinary steels only at elevated temperatures, but it is also found at room temperatures, but it is also found at room temperature in certain stainless steels (e.g., 18 Cr–8 Ni type) classified as austenitic stainless steels. This structure has high ductility and toughness.

  • Ferrite: Alpha iron (BCC), containing a small amount of carbon (0.04–0.05 percent) in solid solution. This phase is soft, ductile, and relatively weak.

  • Cementite: Iron carbide, Fe3C, a compound containing 6.67 percent carbon, which is very hard and extremely brittle. Cementite appears as part of most steel structures, the form of which depends on the specifics of the heat treatment which the steel has received (see pearlite).

  • Pearlite: A mixture of alternating plates of iron carbide (cementite) and ferrite (lamellar structure), which form on slow cooling from within the gamma range. This condition generally represents a good blend of strength, ductility, and fair machineability. It is the equilibrium structure in steel.

  • Bainite: A mixture of ferrite and cementite, which is harder and stronger than pearlite. It forms by the transformation of austenite in many steels during fairly rapid cooling, but not fast enought to cause martensite formation. The structure consists of ferrite and iron carbide, but unlike pearlite, the aggregate is nonla- mellar.

  • Martensite: The hardest constituent achievable by heat-treating of steels, it is formed by the rapid cooling of austenite to a temperature below the martensite start or Ms temperature. Martensite consists of a distorted cubic unit cell (body-centered tetragonal) which contains substantial quantities of carbon in interstitial solution in the lattice. The Ms temperature varies with steel composition.

These latter two microstructural constituents, bainite and martensite, will not be found on the Fe-C phase diagram because they are the direct result of cooling steel at an accelerated rate, which prevents atomic diffusion required to maintain equilibrium conditions. The effects of nonequilibrium cooling of a steel are represented on an isothermal transformation diagram, or a time-temperature transformation (T-T-T) diagram. An example of each is shown in Fig. A3.19.

The horizontal axis of the diagram is time, usually log scale; the vertical axis is temperature. A single diagram represents a given steel alloy composition and depicts the various equilibrium and nonequilib-rium phases that will be formed, and their mix, with a given cooling rate from astarting temperature in the austenitic phase region. The diagram is used by enteringit at the alloys temperature at time = 0, represented as a point of the vertical axis.The cooling rate describes the time/temperature path taken by the material fromthe starting point, through the field of transformation phases, to the final point ofsample cooling. The metallurgical phases or constituents in the final state can thusbe predicted. The continuous path followed between the two points also has abearing on final microstructure. The T-T-T diagram is similar to the equilibriumphase diagram in that single and multiple phase fields are depicted. However, it differs from the equilibrium diagram in that it is a dynamic representation of phaseformation with time. Thus quickly cooling to a given temperature above the Mswill result, for example, in coexistence of austenite, ferrite and, cementite (A, F,and C on the figure). However, as time progresses at that temperature, the austenitecontinues to decompose into more ferrite and cementite, until complete transforma-tion is achieved. Cooling to below the Ms temperature causes tranformation tomartensite. If the path of cooling had intersected the ‘‘nose’’ of the T-T-T curve,some ferrite will form and be combined with the martensite in the final microstructure, since martensite can only be formed by quenching austenite. The ferrite thatformed on cooling is stable and unaffected by further cooling. #Little_PEng

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