GLASS CERAMIC COATINGS FOR FERROUS METALS PROTECTION
Lyudmyla L. Bragina
National Technical University, Kharkov Polytechnic Institute, Frunse Str., 21, Kharkov 61002, Ukraine

Introduction
The contemporary level of development in metallurgy and mechanical engineering is characterized by the great labour expenditure, power consumption and material consumption required for the processing of metals into products. Beside, technological processes are foreseed a considerable number of process stages, during which the semi finished item is heated under high temperature to achieve as a rule the required degree of ductility in a oxidizing atmosphere before hot deformation and under heat treatment. This leads inevitably to the irrevocable losses of metal not only due to the gas corrosion, but also due to the formation of defective layer on the surface of blanks caused by the processes of oxidation, decarburisation, scale embedding or pressing in, gas saturation and so on. The scaling and decarburisation sharply increase with increasing of work piece surface and mass and reach for example 6-8 % at hot rolling of steel strip [1] and 7.7 % during forging of rotor shafts for slow speed turbine from 24-faced ingot from vacuum degassed steel with mass equal to 190 tons [2].
Therefore, the protection of ferrous metal from oxidation under high temperature technological heating is considered to be one of the most important problems of the contemporary material science, the solution of which refers to the scientific directions of the first priority. It is especially actual conformably to the metallurgical and engineering work pieces and details.
So the solution of the problem considered was performed in the working conditions in the following manner: decreasing of temperature and heating duration, use of low-oxidising heating, salt melts and glass melts as a heat transfer agent.
The methods mentioned above which only partially solve the considered problem are difficult for realization in case of multitonnage or in-line production of large-sized products from carbon steel and low-alloy easily oxidisable steels.
Use of heat-proof coatings on the basis of different glasses, enamels, refractory oxides, silicates and other materials is effective and economical method of protection the metals from oxidation at high temperatures.
These coatings have been effective for protection of titanium, its alloys and high-alloy steels [3, 4].
The protection of easily oxidizable steel and alloys, in particular, carbon and low-alloy steels on the industrial scale during the long high-temperature technological heating in metallurgy and engineering industry through the use of heatproof coatings appeared to be a highly complicated problem. Its solution discussed in this paper.

Experimental
Degree of coatings protective function has been evaluated by the comparison of metal losses in scale during heating on specimens with and without coatings: Δg, kg/m²·our, at isothermal heating and Δg, kg/m² ?cycle at complicated temperature–time schedule. For estimation the coatings protective action on manufacturing probe and work piece value of metal burnout used G, %.
Complex estimation method of coatings protective action include also the investigation their influence on degree of decarbonisation, steel surface structure change, on the intergrain oxidation, the micro hardness and strength properties of covered metal.
The glasses density was defined by pycnometer method, electrical conductivity was studied by compensative method with use of quartz U-similar cells and platinum plate electrodes.
Porosity, deformation under load, relative density, kinetics sintering, properties of ceramics and glass-ceramic coatings were defined by standard methods. The new phases in coatings and contact layers was diagnosed by rentgenography, petrography analysis, rentgenofluorescence spectrography and by use of laser microanalyzer.

Results and discussions
It is established that the abovementioned problem can be solved taking into consideration the multitonnage of blanks and details and in-line production only if the composition and the technology of the coating application satisfy the following requirements:

Systematic study of protective action of coatings prepared on the basis of aluminoborosilicate, boroaluminosilicate and aluminosilicate glasses used for production of lubricants, protective-andlubricative and protective coatings for steels and alloys, heatproof enamels, optical glasses, glass forming slag and non-alkaline eutectic alloys showed following. These coatings not reduce losses into scale (Δg) for the samples from the carbon steel during heating of metallurgical workplaces– slabs before rolling (600–1250 °C, isothermal stand at 1250 °C, Δg =l.56 kg/m²·cyole), but even increase them up to 2.8 kg/m²·cycle (Table 1).

The greatest scaling has taken place under easily melting alkaline-silicate glasses and was accompanied by a formation of considerable quantity of iron in coatings. This might be due to the processes of electrochemical corrosion at the contact coating–metal. The main preconditions besides of the thermodynamic instability of iron in melts of glasses [5] mentioned above appeared to be: an electrolytic nature of these melts, which are characterized by the ionic conduction which sharply increases with rising of temperature, macro-and-microninhomogeneity of the steel surface [6], the presence in furnace atmosphere and in melts of compounds, which can perform functions of depolarises, oxygen and H₂O in particular.

The revealing of factors the most responsible for the processes development, which lead to the losses of ferrous metal if the glasses protective technological coatings are used, showed the following.
The kinetics of low-alloy steel at the temperature interval 600-1000 °C both at availability of coatings and without them is subjected to the parabolic law (Fig.1):
Δg = kτ (1)
where k – constant of oxidation rate;
τ – time;
Δg – losses into scale.
As the temperature of isothermal stand rises the n values decrease and k values increase. This was accompanied by the increasing of Δg.

1 *) Are investigated 47 different glasses and glass-forming materials and 18 eutectic alloys in SiO₂– Al₂O₃–CaO, SiO₂–Al₂O₃–MnO, SiO₂–Al₂O₃–FeO (Fe₂O₃) systems with eutectic forming temperature 1060-1400 °C.

This process was characterized by the relative similarity of n values whether the coatings were present or not and by a considerable difference of k values especially in comparison with the samples protected by the most effective compositions.

Nevertheless, the approximation of k values to 1 at temperature rising up to 1000 °C, i.e. transition (Eq.1) from parabolic dependence to linear and at availability of coatings (Table 2) testifies to tendency of worsening glass-coatings protective action with increasing of heating duration as a function of temperature.
This is confirmed by the curves run g=ƒ(T), subjection to the equation:
g = A⋅ e^{−Q / RT}, (2)
where Q – activation energy, kC/mole;
R – gas constant;
T – temperature, K
At the initial stages of heating at 600 °C, when the coating has not get fused, the Δg values of all studied glasses compositions and Δg value of unprotected steel similar to each other and mode up 10-4 kg/m2² As the coatings fusing and their continuity increased the difference between unprotected and protected steel also increased up to 0.44 kg/m² at 1000 °C.

studied glasses compositions and Δg value of unprotected steel similar to each other and mode up 10-4 kg/m2. ²s the coatings fusing and their continuity increased the difference between unprotected and protected steel also increased up to 0.44 kg/m² at 1000 °C.
But the tendency to the sharp increase in temperature coefficient Q values with rising of temperature above 300 °C (Table 2) due to the change of processes character at the contact of glass melt with metal testifies to the lack of any properties for using of studied glasses as a reliable protection from oxidation of ferrous metals at temperature 1000 °C even those which were characterized by the low values of pre-exponential factor A.
The study of the temperature dependence of melts density (ρ) and specific electric conductivity (æ) of investigated glasses and the correlation of the obtained data with corrosive activity and coatings protective action through the use of coefficients of paired correlation i i Δg ρ and i i Δg æ showed the following. When the service temperature of coating is equal to 900 °C increases the significance of glass density as a factor, which determines micro- and submicroporosity and responsibility for oxygen diffusion and metal losses at this period, i.e. at this stage of heating if the coating is available the chemical corrosion plays a predominant role.

Fig.1 The oxidation kinetics of the 09?2C steel at 600 °C (curves 1), 700 °C (curves 2), 800 °C (curves 3), 900 °C (curves 4), 1000 °C (curves 5): a – uncoated steel; b – glass-coatings 7– 10; c – glass-coatings 7–9.

With rising of temperature above 800 °C a quantity of melts in coatings is increased and the values of their æ sharply increased of corrosive activity of melts. Also electromotive force values of element Fe-α?melt?Fe₃C, which simulates local microelements at the surface of low-alloy steel under glass-melt, are increased from 2-8 mV at 20 °C to 8-22 mV at 1000 °C. Therefore the processes of electrochemical corrosion became dominating. This is confirmed also by practically linear dependence between Δg and æ at 800, 900 and 1000 °C (Fig.2).
The values æ of the investigated melts have been sharply increased from 0.20 to 0.54 Ω^-1⋅cm^-1 with solution of Iron oxides in them corresponding to the increase Fe total from 8 to 17 wt.%. This may be connected with transition of ionic conduction into ionic-electron conduction, and the increase of æ led to the further increase of Δg. That is why during the formation and the service of glass-coatings on the work pieces from easily oxidizing steels with scale already existing on their surface it is highly difficult to avoid the processes of electrochemical corrosion even if the lowalkaline and non-alkaline compositions with low initial values æ are used.
The widening of the temperature range and extension of the service life of the glass ceramic coatings requires the reducing of the amount of the liquid phase in them with the decreased electric conduction or application of the refractory sub layer with the maximum degree of stoichiometry and low electric conduction on the surface of the metal. The realization of this condition served as a basis for the creation of the principally new thermo mobile heat resistant coatings for ferrous metals [7].

Fig. 2 The dependence of steel 09?2? losses under glass-coatings from æ of their melts: a – at 700 °C; b – at 800 °C; c – at 900 °C; d – at 1000 °C.

The sintering mechanism for the compositions “refractory filler–glass binder” where the liquid phase reacts with the solid one was recognized as the most acceptable for the compositions. The conditions required for the realization of the known stages of this type of sintering with reference to the developed compositions included the following: maintenance of the sufficient amount of melt through the introduction of glasses or other materials which form liquid phase at the operation temperature of coatings; selection of such a combination of the refractory fillers and glassformation materials that provides the required degree of wetting of the solid phase by the arising melt and its dissolving in it; selection of the optimal granulometric composition for the refractory materials, which provides the dense compaction of their grains; the use of the components whose sintering results in the formation of the compounds that significantly improve the protective properties of the formed coatings; the use of the materials which undergo different changes in the process of heating accompanied by the increase in volume that excludes the formation of cracks in the coatings caused by the shrinkage during their sintering; the strengthening of these effects that may be reached through the crystallization of the new formations at the coating-metal substrate contact as a result of its interaction with the iron oxides.
The result of the scientific and experimental developments was the creation of heatproof silicate thermo mobile coatings of different modifications for protection of easily oxidizing steels of wide range including, which form difficult to remove-and-liquid scale; and also a technologies for application of these coatings on cold and hot ingots and slabs as well as on the large capacity engineering blanks [8].
The high efficiency of these coatings is confirmed both by the reduced oxidation of metal, the identical ferrito-pearlite microstructure and also by the micro hardness values of the surface and the core of the steel details after heating (Fig. 3).

Fig. 3 The influence of coatings on content of alloying elements and micro hardness of surface layer: a – 1, 2, 3 – change of content under coatings respectively C, Ni, Cr; 1a, 2a, 3a – the same – without coatings; b – change of micro hardness under coatings 5 (1) and without it (2).

The continuous protection of metal in a wide temperature range is possible due to the formation of the double-layer coatings in the initial state and the processes that occur at the boundary of the upper vitreous layer and the lower ceramic layer directly in the sub layer and at the area of contact
with the metal during heating. This led to the formation of the several barrier zones directed from
the upper layer to the metal.
Fig. 4 shows the diagram that illustrates the formation of the protective coatings intended for the protection of ingots of low -alloy steel covered with the scale that are operated at a very severe conditions during heating prior to the rolling. In its initial state the coating consists of a powerful ceramic sub layer represented by the composition of the refractory aluminosilicate filler, alkaline – silicate glass and the clay binder and the upper layer consisting of alkaline- silicate glass. During heating the upper layer was vitrified and interacted with the components of the sub layer forming the β-alumina at the boundary of their contact. In the sub layer itself the partial dissolving of the fireclay and alumina grains was observed in the melted glass binder with the increase in temperature alongside with the mullitization within the volume of the clay component and on the surface of the fireclay grains; the growth of the quantity and size of the mullite crystals was accompanied by the formation of the continuous mullite aggregations already at 1100 °C, thereby the length of the acicular crystals reached 30 μm . Simultaneously the transition of γ-Al₂O₃ into α-Al₂O₃ took place and the aggregates of alumina particles cemented by the films of sodium aluminate and the sperelites of the alkaline β-alumina having the size up to 60 μm were formed. The pores of the sub layer were filled with the melt of the alkaline aluminosilicate glass and the cracks formed in the quartz grains that were contained in the fireclay and clay were filled with the isotropic crystobalite.

Fig. 4 The formation of protective glass-ceramic coatings for ferrous metals: 1 – metal, 2 – scale, 3 – γ-alumina, 4 – pores, 5 – clay, 6 – fireclay, 7 – glass, 8 – mullite, 9 – α-alumina, 10 – changing grain of fireclay, 11 – hercynite.

And finally the most important thing is that the fireclay grains were vitrified while contacting the steel. It was accompanied by the dissolving of the iron oxides of scale in the melt, thereat the glass refraction index Ng was increased from 1.535 to 1.620; the reduction in amount of mullite and the formation of FeAl₂O₄- hercynite was observed with further growth of its crystals.
Fig. 5 shows the fragment of aluminosilicate coating which alkaline-phosphate binder microstructure (a) and microstructure of one coatings contact zone (b) after heating on slab at 600 - 1250 °C during 5 hour. These microphotographs are indicated feature of coating mineral composition in middle part it and at contact with steel.

Thus, the continuous compaction of the coating, its sufficient heat proofness with the increase of the service temperature and the ensuring of protection of metal within the whole interval of firing resulted in the liquid phase sintering at the boundary of the coating layers and at the boundary with the scale.
The wide range of control over the composition and structure of the created thermo mobile coating in the direction of extension of the transition zones and creation of the one-layer coatings allowed for the getting of compositions which can protect plain carbon steel, mean-alloy and high-alloy steel during their heating prior to the hot deformation and thermal treatment of a different kind.

Fig. 5 Microstructure of aluminosilicate coating after heating on slabs: a – middle part of coating (1 – fireclay grain; 2 – alumina aggregation; 3 – film of alkaline phosphate, 100x ); b – contact zone (1 – crystals and aggregates of hercynite; 2 – mullite needles, 200x).

As an example of the created coating is following. Especially dangerous is diffusion annealing heating of the easily oxydizable steels in gas furnaces. Here, besides the formation of considerable layer of scaling and decarburising of surface, the depth of which reaches 4000 μm for the meanalloy steel and 4500 μm for the low-alloy steel a burn of the grains boundaries takes place, which spreads to the depth up to 650 μm at heating the mean-alloy, steel up to 1150 °C during 14 hours isothermal stand (Fig. 6).

Fig.6 The micro-relief of the cast mean-alloy steel surface after homoge nizative heating: steel without coating: in gas furnace (a); in electric furnace (b); steel under coating in electric furnace (c).

The effectiveness of the coatings is confirmed also by the presence of wustite, magnetite andtraces of hematite in scale under it and by the absence of alloying elements, in particular, chrome and nickel in its layers, which are in contact with a blank, according to the data of emission and spectral analysis, as for the scale, which was removed from the blank without coating it contained up to 20-25 % of perfectly developed scalenohedron crystals of hematite an a considerable quantity of Mn, Cr, Ni and Si. The depth of decarburisation under coating was equal 100 μm, the difference of thermal expansion linear coefficient values of coating and steel at 1000 °C was 8-10 grad. This provides its separation from 85-100 % of blanks surface during cooling.
The use of this coatings in heavy engineering allowed to reduce the metal losses into scale, the decarburisation, dealloying of surface of blanks and parts by 3-5 times and reduce essentially the strain force. Adaptability to manufacture and simplicity of preparation and application of the coatings on the non-precleaned surface, self-separation during cooling together with scale or burnt on sand and improvement of labour conditions contribute to the wide use.

Conclusions
Fundamental basis of glass ceramic coatings synthesis for scale formation and decarbonisation protection of easily oxidizable steel and alloys has been developed.
The use of these coatings in heavy engineering and metallurgy (technological heating at 600-1350 °C) allowed to reduce the metal losses into scale, the decarbonisation and dealloying of blanks surface by 3-5 times and to reduce essentially the strain force.

Summary
The importance and necessity of ferrous metal protection from oxidation at high temperature technological heating were examined. The possibility of coatings use based on aluminoborosilicate, boroaluminosilicate and aluminosilicate glasses, heatproof enamels, optical glasses, glass-forming slag and non-alkaline eutectic alloys for protection of carbon and low-alloy steels from oxidation at heating before 1250 °? was studied. The most importance factors, which are responsible for the losses under glasscoatings at indicated heating, were established.
Scientific principles of thermo mobile glass ceramic coatings synthesis for scale formation and decarbonisation, protection of easily oxidizable steels and alloys has been developed.
Different modifications of these coatings for protection of the wide range indicated metals including those that form difficult-to-remove and liquid scale have been created.
The use of these coatings in heavy engineering and metallurgy (technological heating at 600-1350 °C) allowed to reduce the metal losses into scale, the decarbonisation and alloying of blanks surface by 3-5 times and to reduce the strain force at heat deformation.

References
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