PHYSICAL METALLURGY OF STEEL (chapter I)

After trying to make this mood back, I mean good mood, now I'll try to make one step ahead, even still I don't where to start but with Piping & Fabrication, I will try to be patient until that chance will comes to me. And now we discuss about Physical Metallurgy of Steel chapter I.

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, bodycentered (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), andA3 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.