Grain size refers to the size of the scale called grain size, usually unit volume (or unit area) in the grain number or grain average line length (or diameter) said. Grain size grade is used to express grain size in industrial production. The standard grain size is divided into 12 grades, 1 ~ 4 grades are coarse grain, 5 ~ 8 grades are fine grain, and 9 ~ 12 grades are ultra-fine grain.

When a metal crystallizes, each grain is grown from a single nucleus, so the size of the grain depends on the number of nuclei and the relative size of the grain growth rate. The number of Nuclei is expressed in terms of nucleation rate. The larger the nucleation rate, the more the number of nuclei per unit volume, and the smaller the grain size. The smaller the growth rate is, the more batches and the more nuclei are formed during the growth, and the smaller the grains are. On the contrary, the smaller the nucleation rate and the faster the growth rate, the coarser the grain size. Therefore, the grain size depends on the ratio of nucleation rate N and Growth Rate G, the larger the ratio of N/G, the smaller the grain size.

1.Concept of grain size

(1)Grain size

A measure of grain size is called grain size. Grain self-size is usually evaluated or measured by different methods such as length, area, volume, or number of grain size grades. The grain size expressed by the number of grain size grades is independent of the measurement method and the unit of measurement.

(2)Actual grain size

The actual grain size refers to the austenite grain size obtained under specific heat treatment or hot working conditions. The actual grain size basically reflects the grain size obtained during the actual heat treatment or under the hot working conditions of the steel, and directly affects the grain size of the products obtained after the cooling of the steel, generally refers to the actual grain size.

(3) Intrinsic Grain size

The essential grain size is used to indicate the tendency of AUSTENITE grain growth. It is a kind of property, not a specific grain. According to the difference of austenite grain growth tendency, steel can be divided into two kinds: essential coarse grain steel and essential fine grain steel.

The Standard Method for determining the essential grain size is heating the steel to 930 °C ± 10 °c, holding the steel for 3 H ~ 8 H and determining the austenite grain size. The steel with grain size of 1 ~ 4 grade is essentially coarse grain steel, and the steel with grain size of 5 ~ 8 grade is essentially fine grain steel. The effect of heating temperature on austenite grain size is shown in the figure below. In general, the grain growth tendency of the essentially fine grain steel is small, the actual grain is fine after normal heat treatment, the quenching temperature range is wide, the production is easy to master, high quality carbon steel and alloy steel are essentially fine grain steel. The grain growth tendency of essentially coarse grain steel is large, so the heating temperature must be strictly controlled in production. To prevent overheating and grain coarsening. It is worth noting that the heating temperature exceeds 930 °C. It is also possible to obtain very coarse austenite grains in essentially fine grained steels. It is even coarser than that of a steel with essentially coarse grain at the same temperature.

(4)Average grain size and dual grain size

In fact, the grain size distribution in the metal matrix can not be exactly the same, but the distribution of the grain size in most cases is similar to a single log-normal distribution, which is usually expressed as "average grain size" . For some metals, the distribution of grain size under certain hot working conditions. Because grain size is related to properties, it is necessary to reflect grain size and distribution correctly.

2.The grain size of copper (copper plate, copper strip) is related to the following factors

(1) chemical composition. Generally speaking, a small amount of added elements (such as iron) and impurities will become the crystal nucleus during solidification. The more the crystal nucleus, the smaller the grain size. The effect of chemical composition on grain size of different alloys is different. In general, the purer the metal, the larger the grain size; the grain size of the single-phase alloy is larger than that of the multiphase alloy;.

(2) cooling intensity. When casting, the greater the cooling intensity, the grain did not have time to grow stool to complete the solidification process, the grain is also smaller.

(3) annealing temperature and time. Grain growth depends on atomic diffusion, and the process of atomic diffusion depends on the diffusion rate and time. The longer the time is, the more complete the diffusion is, and the higher the temperature is, the higher the diffusion rate is. Therefore, the grain size increases with the increase of temperature. The time is longer than the time needed to finish crystallization, and the effect of temperature on the grain size is more significant.

(4) return heating rate. The faster the heating speed, the faster the recovery and diffusion, the higher the initial recrystallization temperature and the smaller the grain size. The grain size can be refined by rapid heating in production.

(5) total processing rate before return. When the total cold working rate before annealing is less than the critical deformation degree of the alloy (generally refers to the processing rate of 10% ~ 20%) , the uneven deformation causes the individual broken grains to grow up by rapidly devouring the surrounding grains and the coarse grains appear, there is no nucleation of new nuclei. When the critical deformation is over, the new nuclei form and grow, except the original grains merge and grow. With the increase of deformation degree, grain breakage and new nucleus increase, the recrystallization temperature and grain size decrease. When working out the production process, the total processing rate before annealing should avoid near the critical processing rate, especially the total processing rate before final annealing should be greater than the critical deformation degree.

(6) original grain size. The higher the original grain size is, the coarser the grain size will be. The finishing temperature of hot rolling and the intermediate annealing have some influence on the grain size of the finished product, but decrease with the increase of the deformation degree. 

Source: Web Collation

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