Oxygen State Forms in Cast Iron and their Effect on Graphite Crystallization

Abstract

Oxygen influence on the structure and properties of cast iron is contradictory and depends on its state. It is supposed that the chemically combined oxygen exists into molten metal as Non-metallic inclusion, and it acts as a Graphite crystallization substrate and also increases graphitization degree of the cast iron. Dissolved oxygen deactivates the potential graphitization centers that are why cast iron chill tendency is increased. At the same time, according to experimental results, the more cast iron is saturated by oxygen, the higher its tendency for graphitization modification. In this study, the influence of the oxygen state on graphite forming process during the cast iron crystallization is considered and the details of its action on the graphite crystallization at different stages of casting solidification process are also represented. It is shown that oxygen, initially present in molten melt in chemically combined state, does not have a significant effect on the Cast Iron crystallization. At the same time, the dissolved oxygen in non-modified cast iron causes chill, but in modified Cast Iron the over-equilibrium dissolved Oxygen content promotes graphitizing crystallization by forming dispersed pre-crystallizing (“fresh”) non-metallic inclusions in liquid cast iron. On the basis of these concepts, the original interpretation of several well-known facts concerning graphitization modifying is presented. These facts have been widely discussed at the present time, and there is a great number of contraversal opinions on this account. Particularly, the new aspects of the damping effect of the graphitization modifying of the cast iron have been discussed. The causes of ferrosilicon graphitizing power increase by addition of the aluminum, calcium, barium, rare-earth metals and other powerful elements-deoxidizers to it are shown. The additional factors of high effectiveness of the in-mold graphitization modifying of cast iron are presented also

Influence of boron- and barium-containing modifiers on the structure of low-chromium cast iron

“…Being one of the strongest deoxidizers of steel and cast iron, when introduced into liquid metal, it forms refractory compounds with sulfur and oxygen (t melt BaO = 2113 °C, t melt BaS = 2200°С) [ 26 , 27 , 28 , 29 , 30 ]. According to the opinion of the authors [ 26 , 30 ], it can be assumed that the resulting finely dispersed non-metallic inclusions ( Figure 2 ) serve as additional crystallization centers during solidification of the melt and contribute to obtaining a fine-grained dense structure and increased purity of the metal surface.

Figure 2 Distribution of chemical elements in sample 2 with finely-dispersed inclusions: а) inside the pearlite colony, b) at the external boundary of the colony.

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“…It is manifested by increasing precipitation of carbides mainly inside the grains, and not along their boundaries, since boron significantly lowers the surface tension of the grain boundaries, and most of the “active” boron in the solid solution concentrating along the grain boundaries, fills the existing vacancies. This in turn prevents diffusion and reduces carbon segregation along the grain boundaries [ 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 ]. Thus, boron atoms in cast iron, being adsorbed on the surface of the solid phase, deactivate graphite nuclei during crystallization and have a stabilizing (deinoculating) effect on the structure.…”

“…The conducted literature review reveals that most of the available works describe the effect of barium-containing additives on the structure and properties of steels [ 26 , 27 , 28 , 29 ], as well as gray and high-strength cast irons [ 30 , 31 , 32 ].…”

Preparation Vermicular Graphite in Thin and Thick Wall Iron Castings

“…Oxygen in graphitic cast irons substantially influences crystallization in that actively participates in nuclei formation [11]. The experimental works were aimed at evaluation of oxygen influence during manufacture of cast irons on structure.…”

Modifying Effect of a New Boron-Barium Ferroalloy on the Wear Resistance of Low-Chromium Cast Iron

“…At the same time, it is a well-known fact that barium, along with calcium and magnesium, is one of the most effective deoxidizers, desulfurizers, and modifiers of cast iron and steel [16,18,19]. Barium in the composition of modifying additives leads to the grinding of non-metallic inclusions in the structure of the processed alloy, the homogenization of the liquid metal, a decrease in the liquidus temperature, and an increase in technological plasticity [20].…”