What is the grain size in steel

Grain size

Grain size, average size of the crystallites of a polycrystalline material, especially in the case of metals, in which the crystallites are also called Grains are designated. The grain size depends to a large extent on the subcooling of the metal melt and on its content of impurities. With only slight hypothermia, the Crystallization rate can be considerable while the number of germs is still small. Then large grains arise, a so-called coarse grain. Admixtures can act as nuclei and also inhibit crystal growth, so generally promote the formation of small grains, one fine grain. For this reason, pure iron, for example, tends to form coarse grains more than steel with a - albeit low - content of carbon.

A fine grain is usually desired because it ensures a more even distribution of the impurities, more perfect isotropy, greater strength, hardness, toughness and impact energy. Rarely, coarse grain has advantages that are better machinability of low-carbon steel, greater through-hardenability and lower coercive force. The Grain size distribution analysis, the determination of the grain size of a material, can be done in Micrograph measured and in units of area (μm2) must be specified. However, it is common to specify the ASTM grain size (ASTM: American Society for Testing Materials): the microsection is compared with the ASTM standard series of stylized grain boundary networks of size 1 (16 grains / mm2) up to 8 (around 200 grains / mm2) magnified a hundred times. The grain sizes 1 ... 3 count towards the coarse grain, the sizes 6 ... 8 towards the fine grain.