PHYSICAL METALLURGY OF STEEL Chapter II

Like I said before, that Piping & Fabrication will continue this post of Physical Metallurgy of Steel Chapter II, and here we getting more and more of information of piping materials.

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.

FIGURE A3.18 Iron-carbon equilibrium diagram

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 nonlamellar.

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.

FIGURE A3.19 Isotherm transformation diagrams for
AISI 1050 (a) and AISI 4340 (b)

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 nonequilibrium phases that will be formed, and their mix, with a given cooling rate from a starting temperature in the austenitic phase region. The diagram is used by entering it 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 from the starting point, through the field of transformation phases, to the final point of sample cooling. The metallurgical phases or constituents in the final state can thus be predicted. The continuous path followed between the two points also has a bearing on final microstructure. The T-T-T diagram is similar to the equilibrium phase 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 phase formation with time. Thus quickly cooling to a given temperature above the Ms will 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 austenite continues to decompose into more ferrite and cementite, until complete transformation is achieved. Cooling to below the Ms temperature causes tranformation to martensite. 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 that formed on cooling is stable and unaffected by further cooling.