Crystallization of iron and steel

As a rule all metals crystallize in solidifying. Space crystal lattices appear in a metal when it converts from the liquid to the solid state. This process is called crystallization. Crystallization takes place in the following way. Atoms continuously move in a liquid metal and, as the temperature decreases, the motion of atoms slows down, the atoms come closer together and form crystals called the centers of crystallization. Then newly formed crystals group round these centers and new centers appear at the same time. So the process consists of two stages: the formation of the centers of crystallization and the growth of crystals round these centers. Iron also crystallizes in solidifying. If crys­tallization could be uninterrupted, it would be found that the iron particles formed almost perfect cubes. Practically it is not the case as crystallization starts at numerous centers and.proceeds by deposition of solid particles of iron from the melt. These particles grow and come into collision preventing each other from taking a regular cubic form.

The small shiny crystalline particles evident in a fractured surface of low carbon steel are fa­miliar to every steel worker. These imperfect crystals are commonly called grains. The appearance and size of crystalline grains seen on a freshly fractured surface is responsible for the terms: "coarse' grained", "fine grained", "shiny", "dull", "fibrous" and many others.

The atoms (or molecules) making up the crystalline structure are grouped according to definite geometrical patterns. This pattern is known as a space lattice and consists of a series of points in space so located that each point is situated similarly to every other point. There are two modifications of this system: one is known as the body-centered cubic lattice, the other is the face-centered cubic lattice. The crystal particles or atoms are held together in this spacing by strong attractive forces which are very strong.

By means of X-rays it has been found that at ordinary temperatures iron crystallizes as a body-centered cube while gamma-iron, the high-temperature allotropic modification, crys­tallizes as a face-centered cube.

All metals and alloys are entirely crystalline in the sol­id state. If the whole melt, considered to be homogeneous, is kept at the same temperature, solidification will begin at certain positions in the body of the melt, called centers of crystallization, or "nuclei". These nuclei are very small to be visible. The building up of the crys­tals around these various centers occurs by the atoms arranging themselves in a definite pattern which repeats itself. Each crystal grows by causing atoms to change from the random arrangement in the liquid and become a part of the atomic pattern of the crystal itself. Since steel does not solidify at one definite temperature but over the range of temperatures, metal rich in iron solidifies first in a skel­eton structure extending in three directions. As the metal cools, this skeleton is extended in all directions and the open spaces between the crystals are gradually filled in with metal slightly less rich in iron which trans­forms from the liquid state and becomes a part of the cry­stal skeleton.

The gradual solidification of the body of metal is progressing from many nuclei with the formation of many crystals. These crystals, of course, meet if solidification has proceeded far enough. When all of the liquid metal has transformed to the solid and crystal­line state, the steel is made up of a very large number of these crystals.

1. Find in the text English equivalents for the following words and word-combinations:

За допомогою рентгенівських променів, поступове затвердіння, кристалічний стан, при звичайній температурі, безладне розташування, безперервний, тільки що утворені, зростання кристалів, кристалічні частки, одночасно, сили тяжіння, однорідний, видимий, у відповідності з, принаймні охолодження металу.

1. Answer the questions:

1. What happens to metals in solidifying?

2. What are the two stages of crystallization?

3. What can be seen in a fractured surface of low carbon steel?

4. Are the appearance and size of crystalline grains always the same?

5. What modifications of a space lattice do you know?

6. What is the centre of crystallization?

7. What do atoms do around the centers of crystallization?

8. What does a metal in the solid state consist of?