Cast steel structure

All steel products undergo a solidification stage from a liquid to a solid state. Even those of them, which were manufactured by powder metallurgy methods, so all steel powders are also made from liquid metal. Therefore, the hardening process affects the properties of all steel products..

Cast structure three factors

The solidification process of the steel ingot has a strong influence on the following three important factors in the quality of the cast ingot.:

  • Microsegregation of alloying elements, carbides and inclusions.
  • Microstructure (grain size, grain shape and phase types.
  • Porosity level in cast metal.

Strong microsegregation is common in the steel casting process, large grain size and significant porosity. All this leads to a decrease in the mechanical properties of steel castings.. All of the above three properties of cast metal are greatly improved by intense hot plastic working., such, like forging or rolling. Therefore, rolled steel and steel forgings usually have higher mechanical properties., than cast steel.

Solidification front

When the material solidifies from the liquid phase, there is a solid front, which moves into the liquid. The shape of this solid front plays an important role in controlling the three critical quality factors of the steel ingot., the above.

The process of solidification of an ingot in a mold is schematically shown in the figure. 1. The mold is shown on the left., which was filled with liquid steel and allowed to cool. The figure shows the moment, when the solid front has passed inward about a third of the way to the center of the ingot. As the heat is taken away from the bottom, and from the walls of the mold, the solid front grows in about the same way as from the bottom, and from the walls of the mold. Since the solid phase is denser, than liquid, the hardened steel shrinks. This leads to a decrease in the height of the ingot as the solidification front moves towards the center of the mold..

zatverdevanie-stalnogo-slitka+Picture 1 - Solidification front in steel ingot

Dendrides in steel

On the right of the picture 1 the leaders of the section of the solidification front are shown. One would expect, that the shape of the solidification front is flat, as shown in callout A of the figure 1. For very pure metals, this is, basically, right, but for steels and almost all metal alloys - no. The solidification front actually consists of many small branched structures, which, as many believe, look like pine trees, growing into liquid. Moreover, their branches grow perpendicular to the trunk., as shown in callout B in figure 1. Each such small tree-like structure is called a "dendrid", which in Greek means "tree". If liquid metal were transparent, then the view when viewed perpendicular to the solidification front, would be as shown in callout C. This view would be like that, seen flying over a pine plantation. but, sure, individual dendrides are very small and to see them, need, least, magnifying glass.

Interdendride distance

Interdendridic distance - the distance between dendrides – depends on, how fast the solidification front moves. And the speed of its progress depends on, how quickly heat is removed from liquid metal. On the picture 2 distance interdendride distance - distance between main stems – denoted by latin letter d. In large steel ingots, which cool slowly, interdendritic distance can be 1 mm (1000 μm). Continuous casting of steel, when heat is taken away very intensively, the interdendritic distance in steel is about 300 μm. When welding steel, when there is a small liquid bath of liquid steel, cooling is even faster. Therefore, the interdendride distance in this case can be about 100 μm.

dendridy-stali+Picture 2- Three iron dendrids,
growing vertically into a liquid during solidification

Dendrid stems

The diameter of the main dendridic trunks is about ten times less than the distance between them.. The diameter of a human hair is approximately 50 μm. Therefore, most dendrides have trunks of size, which is equal to or even less than a human hair. These tiny dendrides play a huge role in the first and third quality factors of steel ingots - microsegregation and porosity., but almost do not affect the second factor - the microstructure of the steel.

Source: John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007