Solidification (metallurgy)
Metals are usually used as an alloy , which causes them to solidify in the temperature interval between solidus and liquidus . Exceptions are eutectic alloys, which have a single solidification temperature. Segregations occur during solidification and voids and non-metallic inclusions can arise in the solid , which negatively affect the properties of the metal. This is influenced by the targeted solidification of the metal through shape and solidification speed.
Solidification structure
In general, steel solidifies through crystal formation. The shape of the crystals that are formed depends on the specific solidification conditions and leads to different properties.
- Small equiaxed grains arise primarily at the contact with the mold. The cooling rate is very high, so that a great deal of hypothermia develops very quickly. This creates a large number of condensation nuclei. These grow until adjacent grains are reached and a common grain boundary is formed.
- Dendritic structure (English columnar grains) arise when the speed of crystal growth is comparable to the speed of the liquidus front. The melt is not sufficiently supercooled to produce a significant amount of condensation nuclei.
- Large equiaxed grains are formed preferentially in the center of the mold. The solidification rate increases, so that the melt is severely supercooled and condensation nuclei develop, which grow into large grains.
Solidification process
The real solidification process is approached through various stages in which more and more phenomena can be taken into account. The first and simplest description of the solidification process takes place using a phase diagram , which strictly speaking only describes the adiabatic case: If the melt falls below the liquidus temperature, a solid phase is created in addition to the liquid phase. During the cooling process, the composition of the liquid phase changes so that it remains on the liquidus line. The concentration in the solid phase increases according to the solidus line. In this description the composition of both the liquid and the solid phase is completely homogeneous at any temperature. This is known as the rule of conodes . The homogeneous distribution in the solid phase is only possible in very long periods of time due to the significantly lower diffusion rate in the solid.
Another description of the Scheil equation ignores diffusion in the solid. The concentration in the solid changes in the course of solidification and creates a concentration profile which explains the micro-segregation. The description using the Scheil equation describes a very rapid solidification over time. In this description, the residual melt reaches significantly higher concentrations, which in some cases leads to a significant decrease in the solidus temperature (see graphic).
If the solidification rate is in a range in which diffusion can take place in the solid, intermediate models are used, such as the back-diffusion model. Part of the concentration gradient in the solid is broken down by diffusion processes during solidification. The concentration of the residual melt does not increase as clearly as according to the Scheil equation; accordingly, the solidus temperature lies between that according to the Lever rule and the Scheil equation. To calculate the solidification according to the back diffusion model, the solidification rate is required. At extremely low solidification speeds, the curve approaches that according to the level rule for adiabatic solidification. For extremely high solidification speeds, the curve changes into that according to the Scheil equation.
These simple models are based on the assumption that only one solidification process takes place during solidification. However, the observation shows that the solidification runs roughly in three sections and is visible in the cross section as in the sketch:
- non-directional solidification with fine grain in the outer area (outer equiaxed zone)
- dendritic solidification with very coarse grain in the middle area (columnar zone)
- non-directional solidification with medium grain in the interior (equiaxed zone)
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- E. Kozeschnik: A scheil-gulliver model with back-diffusion applied to the microsegregation of chromium in Fe-Cr-C alloys. In: Metallurgical and Materials Transactions A. Volume 31, No. 66, 2000, pp. 1682-1684. (link.springer.com)
- JA Dantzi, M. Rappaz: Solidification. EPFL Press, Lausanne 2009, ISBN 978-2-940222-17-9 .