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The decarburisation of steels containing up to 3wt.%Si in wet hydrogen atmospheres is an important industrial process. Since these atmospheres have an oxidising potential, oxidation of the steel occurs, often with a considerable influence on the decarburisation, with numerous investigations67,69,73,78,82-85 undertaken into the oxidation and decarburisation of silicon steels. Various factors influence the diffusion of oxygen and silicon in iron, and therefore the oxidation characteristics. These factors include the86:

 Heat treatment temperature;

 Oxidising potential of the furnace atmosphere;

 Duration of the heat treatment;

 Time for the specimen to heat to the furnace temperature;

 Initial silicon content;

 Isomorphic iron structure;

 Available diffusion paths in the oxidation zone.

Tuck82 found that two forms of scaling occurred during the oxidation of a low carbon 1.74wt.%Si steel, namely:

1) Protective scale formation; 2) Non-protective scale formation.

Logani84 investigated the formation of a FeO-Fe2SiO4 scale on a ferritic 1.5wt.%Si steel exposed

to a CO/CO2 atmosphere at 1000°C. The oxidation of this alloy exhibitted an initial region of

variable reaction rate, followed by a region of linear reaction behaviour. The initial growth of an oxide scale on a Fe-Si alloy often did not proceed uniformly over the surfaces. Instead, the nuclei of the more stable oxides appear in the metastable oxide films of the steel, and grew into well defined nodules during the early oxidation stages. This behaviour was pronounced in atmospheres of low oxygen potential at elevated temperatures.

Nucleation of the nodules occurred in the oxide film on the alloy grain boundaries, on threefold grain boundary intersections, and within the film covering the alloy grains. These nodules, which

comprised FeO, with fayalite, Fe2SiO4, as a minor constituent, had circular bases due to isotropic

lateral growth. Only those nodules formed at boundaries and grain boundary intersections grew rapidly when exposed to a 50 vol.% CO2 atmosphere. Their areal coverage increased rapidly,

leading to complete coverage after 8hr. The nodules on the specimens exposed to the pure CO2

atmosphere grew at a much faster rate than in the lower oxygen potential atmospheres, with complete coverage being obtained after about 30min84.

Internal oxidation was exhibited in the alloy beneath the nodules. Oxide precipitates occurred within the alloy matrix and boundaries, with their size decreasing at an increasing distance from the alloy surface. Figure 3.12 illustrates that the external oxide grew at a linear rate, while the internal oxidation zone grew at a continuously decreasing rate, attaining a maximum penetration depth of approximately 50µm after 40hr. By then the external scale thickness was approximately 300µm.

Figure 3.12 Growth kinetics of the external and internal oxidation zones of a 0.006wt.%C/1.74wt.%Si steel with a 50 vol.% CO2 atmosphere at atmospheric

pressure (from Ref. 84).

The oxidation rate of carbon and low alloy steels at higher temperatures is predominantly faster in steam than in air, and faster in air than in CO2.. Tuck82 found that a Fe/1.74wt.%Si

steel oxidised many times faster in steam than in air. The oxide scale formed in air, Figure 3.13(a), comprised an outer part of silicon free alpha-Fe2O3. The inner portion of the scale,

hematite/magnetite interface. The absence of wustite formation suggests that the silica-rich layer formed in the early stages of oxidation effectively impeded the outward diffusion of iron ions.

(a) (b)

Figure 3.13 Scale structures for (a) 20hr at 980°C in still air, and (b) for 5hr at 934°C in CO2

1,000x. (from Ref. 82).

Silicon was present in a wustite matrix as large fayalite particles after exposure to CO2, Figure

3.13(b), instead of the fine silica or fayalite dispersion formed in air82. This silicon rich layer was incapable of preventing the outward diffusion of iron ions, which resulted in magnetite and wustite being maintained in the scale. Silicon, which is much less noble than iron, reacted with oxygen at suitable nucleation sites to form SiO2. Concurrently, the iron in the surface

layer, which is now denuded of silicon, also reacted with the oxide ions adsorbed on the surface. This resulted in the iron forming a layer of iron oxide or oxides outside the silica. As the reaction proceeded, silicon diffused to the surface of the metal and reacted with the oxygen dissolving into the alloy from the iron oxide. This allowed the silica layer to increase in thickness and extent, therefore hindering the further outward flow of iron ions. This process continued until the silica layer was sufficiently thick to prevent the outward diffusion of iron ions.

Consequently, the type of scale formed governs the subsequent oxidation of the alloy82. With CO2 or steam atmospheres, the gas/scale phase boundary is rate determining, which results

initially in linear scaling. As the supply of oxygen ions, by the decomposition of either CO2 or

steam, is outweighed by the supply through the outward diffusion of iron ions, only wustite forms in the early stages. Thus, wustite and silica readily react to form fayalite. The fayalite formed presumably has vacancies in part of its lattice since the wustite from which it is formed has a cation-deficient structure. Consequently, iron ions can diffuse outwards through the fayalite layer. Furthermore, all the fayalite formation subsequent to the initial layer corresponding to the original metal surface is as discrete particles, rather than as a continuous layer. Therefore the wustite in which the fayalite particles are imbedded will provide ready diffusion paths for iron. In oxygen, the outer scale/gas phase boundary reaction is not rate-determining, with this reaction being much faster than the diffusion of iron through the silicon layer as it is formed. It has been suggested that either:

a) No wustite is formed outside the silica layer to allow the formation of fayalite; or

b) If wustite is formed, its existence is so short lived that it is converted to higher iron oxides before any fayalite-forming reaction can occur.

When wustite is present, it can readily react with the silica to produce fayalite, and also with the small quantity of alumina present in the steel. In the absence of wustite, the silica and alumina cannot react readily. For magnetite to react with silica, it would first have to be reduced to wustite, which is unlikely in the scale formed on Fe-Si alloys in oxygen.

Various oxidation reactions are possible for Fe-Si alloys. Yamazaki69 considered the potential oxidation reactions (3.5) to (3.7) and constructed a relationship between p_{H O} p_H

2 / 2 ratio and temperature, Figure 3.14.

[Si] + 2H2O(g) = SiO2(s) + 2H2(g) (3.5)

[Fe] + H2O(g) = FeO(s) + H2(g) (3.6)

[2Fe] + SiO2(s) + 2H2O(g) = Fe2SiO4(s) + 2H2(g) (3.7)

External oxidation involves oxides of reactive solutes such as silicon, manganese and aluminium in atmospheres reducing to iron, and mixed iron-solute oxides for atmospheres oxidising to iron. Internal oxidation arises when oxygen diffuses into the alloy and reacts with solute atoms, precipitating second phase oxides beneath the steel surface. The formation of external or internal

temperature and oxygen potential of the atmosphere87. Factors which increase the oxygen flux into the alloy relative to the counter diffusion flux of reacting solute to the surface will favour internal oxidation. Factors having the opposite effect will favour external oxidation.

Figure 3.14 Relationships between p_{H O} p_H

2 / 2 ratio and temperature for the potential oxidation reactions of Fe-Si alloys (from Ref. 69).

Geiger87 demonstrated that the transition from internal to external oxidation of a 0.024wt.%C/1.80wt.%Si steel occurred with increasing temperature. Figure 3.15(a) illustrates the influence of pH O₂ / pH₂ ratio on the weight gain of a Fe-3wt.%Si steel

85

. Below ratios of 0.02, the weight gain was approximately 5µg/cm2. However, the weight gain increased rapidly with increasing pH O₂ / pH₂ ratios above this. The influence of pH O₂ / pH₂ ratio on the

thickness of the SiO2 layer forming on this steel is illustrated in Figure 3.15(b). Morito85

concluded that external oxidation occurred below a p_{H O} p_H

2 / 2 ratio of 0.02, and internal oxidation above 0.025.

The silicon concentration across the oxidised layer of a Fe-3wt.%Si steel, Figure 3.16, demonstrates inner and outer zones79, irrespective of the p_{H O} p_H

2 / 2 ratio. The outer zone, typically less than 50nm thick, was very rich in silicon and oxygen, with silicon mostly present as silica. The inner zone, typically 1-2µm thick, had much lower silicon and oxygen contents,

with silica particles precipitated in an iron matrix. The silicon enrichment of the outer zone was found to increase as the p_{H O} p_H

2 / 2 ratio decreased

79

.

(a) (b)

Figure 3.15 Oxidation of a 0.021wt.%C/2.96wt.%Si steel for 1hr at 850°C illustrating the influence of p_{H O} p_H

2 / 2 ratio on (a) weight gain, and (b) thickness of SiO2 layer (from Ref.79).

Figure 3.16 Alloy element concentration profile of oxidised layer for 10min decarburisation of a Fe-3wt.%Si steel at 801°C, pH O₂ / pH₂=0.20 (from Ref. 79).