Known, that the grain size of the steel should be kept as small as possible. This gives it a high viscosity, and also improves strength. Hot rolled or cast steel comes from manufacturer with some grain size. This grain size can change significantly during subsequent heat treatment or hot forming operations., eg, forging. It is important to understand, what controls these changes.
Steel grain shape
The shape and some properties of the grain of steel and other metals can be surprisingly well modeled using soap bubbles.. A simple experiment in the kitchen can give a fairly deep understanding of grain geometry in metals..
In a transparent glass, you need to drop a few drops of liquid soap and add warm water to the thickness of the layer at the bottom 3 mm. With a straw or a thin cocktail tube, lightly mix the contents at the bottom of the glass, and then strongly blow into it through a straw or tube.
The surface tension forces in liquid soap are similar to the surface tension forces on the surfaces of metal grain boundaries.. Therefore, the shape and nature of the growth of soap bubbles very well model the same for metal grains.. Soap grains provide an excellent model of the three-dimensional shape of metal grains. for instance, such a seven-sided soap grain on the surface of a glass with numbered faces in the figure 1. The shape of the grains is clearly visible at the bottom of the glass.
Conventional metallographic photographs of the grain structure show only a two-dimensional cross section of the grains., and soap grains make it possible to visualize, how these grains are located below the plane of the photo.
Steel grain growth
Metal grain growth can be modeled, placing the tip of the straw in the center of the glass and lightly blow inward: the bubble at the tip of the straw will expand.
maybe, a more pleasant way to model the essence of the grain shape of metals is another way: quickly empty a bottle of beer beer mug. The whole empty bottle will be filled with wonderful bubbles, whose physics is the same, as in metal grains (picture 2).
Source: John D. Verhoeven, Steel Metallurgy for the Non-metallurgist, ASM International, 2007