Elimination in steel

Liquid iron is able to dissolve in itself much more than any elements. – alloying and impurities, than solid iron. Sulfur provides the most striking example of this.. If liquid iron at a temperature 1000 ºC can dissolve 31 % sulfur, then solid iron at the same temperature - only 0,01 %. It means, that during the crystallization of steel, all sulfur will dissolve in the liquid phase of the steel and almost none of it will dissolve in the solid phase.

Elimination of sulfur in steel

Sulfur is present in solid iron as small particles, which are called inclusions. Sulfide inclusions are chemical compounds of iron sulfide, having the formula (MnFe)S. Manganese, which is present in all steels, is included in these sulfides in the amount, which depends on the total manganese content in the steel and the crystallization rate. After the end of crystallization, sulfide particles are distributed over the volume of the steel ingot very heterogeneously.. This phenomenon is called liquation in the Russian-language technical literature., and in English-microsegregation (microsegregation).

Dendrides are the cause of segregation., therefore it is often called "dendridic liquation". The scheme for the formation of liquation is shown in the figure. 1.

dendridy-stali+Picture 1 - Scheme of the movement of impurities in the interdendride regions
during crystallization of steel

Solid dendrides practically do not contain sulfur atoms. Therefore, these atoms are pushed out of the growing dendridic branches into the liquid regions between the two dendrides. This is shown by the arrows in the figure. 1.

Formation of sulfides in steel

The highest sulfur content is in the interdendridic regions at the base of the dendrides - at the bottom of the figure. When the concentration of sulfur in iron becomes high enough, sulfide particles are formed. Depending on the chemical composition of the steel, sulfide inclusions are formed in the liquid in the form of solid particles or liquid droplets. These droplets harden later at temperatures, which are below the solidification temperature of the dendrid base. In this way, all sulfide particles are located in the interdendride regions, which are indicated in the figure as zones with a high concentration of impurities.

Effect of liquation on steel structure

Atoms other than iron, which are deliberately added to iron when making steel, such as manganese, carbon and alloying elements, also segregate in the area between dendrides cast steel. The phenomenon of liquation-microsegregation can lead to the formation of particles in austenite, which, it would seem, should not be according to the iron-carbon phase diagram.

To understand, how can it be, consider an iron-carbon alloy Fe-1,8C, whose composition is shown in the iron-carbon phase diagram in the figure 2.

fazovaya-diagramma-zhelezo-uglerodPicture 2 - Transformations of Fe-1.8% C alloy
on the iron-carbon phase diagram

Dendride growth kinetics

The phase diagram shows, that when this alloy containing 1,8 % carbon is at temperature 1100 ºS, it should consist of single-phase austenite without any particles. However, due to the phenomenon of carbon segregation between dendrides, the structure of the alloy may turn out to be slightly different.

The phase diagram in the figure 2 predicts, that this alloy begins to form a solid phase, when the temperature of the liquid drops to 1400 ºS. It means, that the tips of the primary dendrides in the figure 1 were the very first solid phase, which formed in the liquid phase at a temperature of about 1400 ºS. The phase diagram also predicts, that the carbon content of these first solid crystals will be only 0,7 %. In this way, during the formation of the first crystals of dendrides, the carbon content in them should fall from 1,8 to 0,7 %, and the difference is 1,1 % must be "pushed" into the liquid phase. This is shown schematically by arrows between the dendrides in the figure. 2.

When moving to the base of the dendrides in the figure 2 the carbon content in the interdendrite regions increases, and the temperature is dropping. When the temperature of the liquid drops to 1148 ºS, then according to the picture 2 it is seen, that the carbon content in the interdendride fluid will reach the eutectic composition 4,3 % carbon. Therefore, all remaining liquid will solidify at a temperature 1148 ºС as a eutectic mixture of austenite, containing cementite particles. Then there are two different options, which can occur with a further decrease in temperature from 1148 to 1100 ºS.

Equilibrium crystallization of steel

The dendride bases were the first to form from the liquid and they have a carbon content of about 0,7 %, which is significantly below the nominal content 1,8 %. This creates the conditions for, so that carbon from freshly formed cementite particles diffuses into the “older” dendride bases, carbon-depleted. This will lead to, that these cementite particles just dissolve. This is the first option, as a result of which at a temperature 1100 ºС single-phase austenite will form. Besides, if the diffusion coefficient of carbon is high enough, and the steel will cool slowly, then carbon will diffuse into the bases of austenitic dendrides. As a result, homogeneous austenite will form at a temperature 1100 ºС with carbon content 1,8 %, as predicted by the phase diagram in the figure 2. This type of crystallization of steel is called equilibrium crystallization.

Non-equilibrium crystallization of steel

In the second variant, the diffusion rate is not enough, so that when cooling steel from 1148 to 1100 ºС dissolve all cementite particles, which formed in a solid eutectic at a temperature 1148 ºS. In this case, the result at a temperature 1100 ºС will be a mixture of austenite and cementite particles Fe3C. Since the equilibrium phase diagram predicts, that alloy with 1,8 % should not contain carbon at temperature 1100 ºС no cementite particles, then this crystallization variant is called nonequilibrium crystallization.

The role of diffusion in segregation

This is the second - nonequilibrium - variant shows, how is it possible, that particles are formed between dendrides in steel – liquation occurs, although, according to the phase diagram, a single-phase austenite without any particles should be formed. The thing is, that in pure iron-carbon alloys, the diffusion coefficient of tiny carbon atoms is very large. Therefore, when they solidify, equilibrium crystallization occurs. Real steel, However, contain manganese and alloying elements. Manganese and some of the alloying elements, such as molybdenum and chromium, are strong carbide-forming elements. Cementite carbide particles in this case take the form (FeX)3C, where X is a combination of manganese, molybdenum and chromium. To dissolve these carbide particles, atoms of element X should diffuse, and also carbon atoms. However, these elements diffuse thousands of times slower., than carbon. These are the reasons why, that usually crystallization of low-alloyed, tool and stainless steels occurs as non-equilibrium crystallization.

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