Observation of Inclusion in Aluminum Deoxidized Iron.

Abstract

Aluminum-deoxidized iron at 1 873 K was solidified at 3 different cooling speed; (1) the ultra-rapid cooling of iron using twin rollers, (2) the quenching of iron into copper mold, and (3) the quenching of the iron-bearing crucible in a water bath; the most rapid cooling rate achieved with (1), which was probably about 10 5 K/s, followed (2) and (3). Dendritic, maple-like, polygonal, network-like, coral-like and spherical inclusions were observed in the samples. The dendritic, maple-like and polygonal inclusions varied in size from several tens to a few mm and were classified as primary inclusions since their sizes were independent of cooling speed. However, the network-like and coral-like inclusions (in the sample cooled ultra-rapidly and quenched into copper mold), and spherical inclusions were classified as secondary inclusions since they decreased in size with increased cooling speed. A few large spherical inclusions, which would be primary inclusions, were also present.The analysis of the electronic diffraction of the inclusions established that the a, g, d-alumina were present as secondary inclusions. An amorphous silica spherical inclusion was also observed.

Thermodynamic Analysis on Metastable Alumina Formation in Aluminum Deoxidized Iron Based on Ostwald's Step Rule and Classical Homogeneous Nucleation Theories.

“…[1][2][3][4][5][6][7] As was reported in the latest report, 2) the d and g-alumina were observed as secondary inclusions formed during cooling of iron. However, the d, g, k, and qaluminas are not stable at steelmaking temperatures and are metastable at ordinary temperatures, whilst the a-alumina is stable.…”

“…Therefore, many of the primary inclusions were found to be composed of the unstable alumina. 2) From the results of the previous calculation, 13) the nucleation of a-alumina is scarce at any oxygen content below the critical point. The nucleation, with respect to the liquid and the unstable aluminas, will be more difficult below the critical point.…”

“…The network-like inclusions in the same sample seemed to be formed when several coral-like inclusions were superimposed upon one another. 2) Thus, in a sample quenched in water, the spherical liquid inclusions formed during cooling would have time to float upward, collide and coalesce with each other in liquid iron, since the cooling speed is rather slow, and finally, they could solidify as the network-like or coral-like inclusions. The above process for the network-like or coral-like inclusion in the sample quenched in water might occur during the early stage of aluminum deoxidation process in liquid iron with a composition which exceeded the critical point of supersaturation.…”

“…) 3 }/{(a (e) Al ) 2 The term DG B is the Gibbs free energy change of the parent liquid iron phase excluding the aluminum and oxygen consumed during nucleation. This term is expressed by the activities of aluminum, oxygen and iron before and after nucleation.…”

Effect of Impurity Te on the Morphology of Alumina Particles in Molten Iron

“…the morphology of alumina particles show that: (1) faceted particles are formed at low supersaturation degree, whereas spherical and dendritic particles are formed at high supersaturation degree; [8][9][10][11][12][13][14][15] (2) stirring can modify the form of dendritic particles by changing the directions of supersaturation gradient, making them coral-like; 16) (3) stirring and long holding time favor the clustering of alumina particles; 8) (4) sintering and densification of clusters proceed with holding time, forming large polyhedral particles. 8,16) The formation mechanisms of different morphologies of alumina particles in liquid steel have been reviewed by Dekkers et al 16) However, the origin of some morphologies, such as octahedral and plate-like, is still not fully understood.…”