As indicated in Fig. 1.14, when a low-carbon steel is heated above the A1 temperature line, austenite, a solid solution of carbon in gamma iron, begins to appear in the ferrite matrix.
Each island of austenite grows until it intersects its neighbor. With further increase in temperature, these grains grow larger. The final grain size depends on the temperature above the A3 line to which the metal is heated. When the steel cools, the relative coarseness of the grains passes to the ferrite-plus-pearlite phase.
At rolling and forging temperatures, therefore, many steels grow coarse grains. Hot working, however, refines the grain size. The temperature at the final stage of the hot-working process determines the final grain size. When the finishing temperature is relatively high, the grains may be rather coarse when the steel is air-cooled. In that case, the grain size can be reduced if the steel is normalized (reheated to just above the A3 line and again air-cooled).
(See Art. 1.22.)
Fine grains improve many properties of steels. Other factors being the same, steels with finer grain size have better notch toughness because of lower transition temperatures (see Art. 1.14) than coarser-grained steels. Also, decreasing grain size improves bendability and ductility. Furthermore fine grain size in quenched and tempered steel improves yield strength.
And there is less distortion, less quench cracking, and lower internal stress in heat-treated products.
On the other hand, for some applications, coarse-grained steels are desirable. They permit deeper hardening. If the steels should be used in elevated-temperature service, they offer higher load-carrying capacity and higher creep strength than fine-grained steels.
Austenitic-grain growth may be inhibited by carbides that dissolve slowly or remain undissolved in the austenite or by a suitable dispersion of nonmetallic inclusions. Steels produced this way are called fine-grained. Steels not made with grain-growth inhibitors are called coarse-grained.
When heated above the critical temperature, 1340F, grains in coarse-grained steels grow gradually. The grains in fine-grained steels grow only slightly, if at all, until a certain temperature, the coarsening temperature, is reached. Above this, abrupt coarsening occurs. The resulting grain size may be larger than that of coarse-grained steel at the same temperature.
Note further that either fine-grained or coarse-grained steels can be heat-treated to be either fine-grained or coarse-grained (see Art. 1.22).
The usual method of making fine-grained steels involves controlled aluminum deoxidation (see also Art. 1.24). The inhibiting agent in such steels may be a submicroscopic dispersion of aluminum nitride or aluminum oxide.
(W. T. Lankford, Jr., ed., The Making, Shaping and Treating of Steel, Association of Iron and Steel Engineers, Pittsburgh, Pa.)
