Carburization of austenitic and ferritic alloys in hydrocarbon environnnents at high tennperature

The technical and industrial aspects of high temperature corrosion of materials exposed to a variety of aggressive environments have significant importance. These environments include combustion product gases and hydrocarbon gases with low oxygen potentials and high carbon potentials. In the refinery and petrochemical industries, austenitic and ferritic alloys are usually used for tubes in fired furnaces. The temperature range for exposure of austenitic alloys is 800-1100 °C, and for ferritic alloys 500-700 °C, with carbon activities â > 1 in many cases. In both applications, the carburization process involves carbon (coke) deposition on the inner diameter, carbon absorption at the metal surface, diffusion of carbon inside the alloy, and precipitation and transformation of carbides to a depth increasing with service. The overall kinetics of the internal carburization are approximately parabolic, controlled by carbon diffusion and carbide precipitation. Ferritic alloys exhibit gross but uniform carburization while non-uniform intragranular and grain-boundary carburization is observed in austenitic alloys.


INTRODUCTION
In industrial environments such as in oil refineries and petrochemical plants, engineering alloys such as Fe-Cr-Mo and Fe-Cr-Ni often come into contact with corrosive gases containing oxygen, sulfur, carbon, and nitrogen at a relatively low oxygen activities and high total pressure.The corrosive gases may participate in scale formation, or alternatively, may dissolve or diffuse internally and react with alloy elements and precipitate as second phases within the grain or along the grain boundaries.Typical examples are thermal and steam cracking tubes in ethylene, propylene, synthesis gas production and fire heated tubes in crude oil and visbreaking furnaces^ ' .
Crude oil distillation and visbreaking are thermal cracking processes through which, at a moderate temperature of 500 to 700 °C, heavy hydrocarbon molecules are converted into lighter ones having a lower boiling point and greater economic value than the initial load^'^l The fireheated tubes are Fe-Cr-Mo alloys (Fe'2V4Cr-lMo, Fe-SCr-i/zMo, Fe-9Cr4Mo).On the other hand, in ethylene steam crackers and syngas reformer furnaces, hydrocarbons and steam pass through tubes that are heated to temperatures above 900 °C in many cases exceeding 1100 °C.The alloys used in these furnaces are typically wrought stainless steels or Fe-Cr-Ni centrifugal cast alloys (HK, HP).In both cases, it is important to select the alloy composition or control the environment to minimize the damage produced by interactions with aggressive oxidants.These materials are selected because a scale rich in Cr203 is formed on the surface of these structural components which protects the material against high temperature corrosion.Strong oxidation, carburization, sulfidation or nitriding only occurs if the environment does not give to chromium oxide formation or if the protectivity of the scale is destroyed by other sch mecnanisms^ [1][2][3] Metals or alloys are generally susceptible to carburization when exposed to an environment containing CO, CH4 or other hydrocarbon gases such as ethylene (C2H4) or propane (C3H8) at elevated temperatures.During thermal and steam cracking operation, carbon is deposited in the form of coke on the internal surfaces of the tubes.This has many deleterious effects.The efficiency of heat transfer is reduced and the metal skin temperature increased to maintain the process temperature.The presence of coke will eventually lead to carburization of the tubes when it is periodically removed by oxidation in water vapor and air.Carburization attack generally results in the formation of internal carbides that often cause the alloy to suffer embrittlement as well as other mechanical degradation'^', especially at low temperature.

THERMODYNAMICS
Petrochemical and refinery environments contain gas mixtures of CO, CO2, H2, H2O, CH4, H^Cy (hydrocarbons), and organic compounds.The alloys are likely carburized if (ac)environment ^ (ac)alloy This carburization can proceed by one of the following reactions: In equilibrium, the carbon activity in the environment can be calculated by: ^Q, where, Figuro L Varioción de la actividad de carbono con la temperatura y la composición del CH4 en la mezcla.variation of carbon activity, (a^.)) in the environment with temperature and gas composition, vol.(%), of a mixture H2'CH4 at 600 and 750 °C.For this reaction at all temperatures: AGf = 14669.52-1.987T (5.32 InT --0.00183T -58832.41/ 2T^ -25.16) (7) In ferritic and austenitic alloys, ingress of carbon into the alloy results in the formation of chromium carbides, principally.There are three forms of chromium carbides: Cr23C5, Cr7C3, and Cr3C2.During carburization, a stability diagram such as the one shown in figure 2 can best describe the relative stability of these carbides.At very low oxygen partial pressure and low carbon activity in the alloy, the most stable carbide is Cr23C6.Considering the following equilibrium^ ^" .

ÜQ = Carbon activity
Where cicrj^Q, the activity of the solid carbide precipitated, is assumed to be unity.Rearranging the equation (9), it becomes:  I.

FIELD CARBURIZATION
The main object of this paper is to compare the morphological differences in carburization occurred in austenitic and ferritic alloys exposed for a long time in environments with carbon activity over one in many cases.The carbon activity inside the alloys is too low and thermodynamic calculations show a high potential for carbon diffusivity.Carburization proceeds both along grain boundaries (grain boundary carburization) as well as within the grains (bulk carburization).Samples Figura 3. Energía libre estándar de formación para algi unos carburos [7] first case, in a sample of ferritic alloy 9Cr-lMo extracted from a tube closed to the outlet of the radiation zone shows the evident bulk carburization through all the cross section along the inside diameter.This tube had 102000 h of continuous exposition at 600 °C, average.In the second example, a the tube of alloy HP'40 extracted from a coil of the radiation zone of the furnace had 88000 h of continuous exposition over 900 °C, but in this case carburization only occurred along the austenitic grain boundary, see figure 4(a) for ferritic alloy and figure 4(b) for an austenitic alloy.This performance suggests two different mechanisms of carburization in this particular application of alloys in the oil refinery and the petrochemical industry.
Although the alloy 9Cr-lMo had an oxide layer over the internal surface, the carburization was homogeneous along the internal diameter.Conversely, the presence of a pre-existing oxide film over the internal surface in the alloy HK'40, formed in air before exposure to the carburization environment, reduced or inhibited carburization.Porosity, cracks, spallation and possible reduction of chromium oxides to carbides can occur and the underlying metal matrix will consequently be exposed to the carburizing environment^ .

CONCLUSIONS
extracted from tubes of fired furnaces, visbreaking and pyrolysis furnaces, show this difference.In the In industrial visbreaking and pyrolysis furnaces, the carbon environment activity is hardly dependent    (HP'40) alloys observed for samples extracted from ethylene furnace tubes.In this case the simple unidirectional mathematical model used for explaining internal oxidation or describing carburization needs a different and additional arrangement.In this particular case, was seen internal carburization only in areas where the oxide film was absent.
The different behavior mechanism for ferritic and austenitic alloys involves several factors: for ferrite, carbon solubility is much less but carbon diffusivity is much higher; chromium diffusivity is much higher.For austenite, carbon solubility is high, but carbon diffusivity is low, and chromium diffusivity is low.The grain boundary precipitation for austenitic alloys does not mean that grain boundary carbon diffusion was important, because carbon is quite soluble in austenite and does not need the grain boundary for diffusion.Rather, probably the Cr23C6 lattice does not match well to the austenite lattice (perhaps better to the ferrite lattice).Therefore, the precipitation fits better in the austenite grain boundary than in the austenite bulk.Besides precipitation in a lattice also requires lattice diffusion of solvent atoms.This is faster in ferrite, but in austenite, grain boundary diffusion for iron may be needed.

REFERENCES
of temperature and blend gas composition.At low temperature the CH4 concentration in the mixture H2'CH4 is too high resulting in a high carbon potential.
Bulk carbon diffusion is the main mechanism of carburization in ferritic alloys Fe-9Cr-lMo observed in samples extracted from visbreaking fired furnace tubes.A mathematical model similar to the unidirectional internal oxidation that considers the interaction between carbon and active elements is applicable for describing this phenomenon.

Figure 1 .
Figure 1.Variation of carbon activity with temperature and CH4 composition in the mixture.

Figure 3 .
Figure 3. Standard free energies of formation for some carbides^^l

Figure 4 .
Figure 4. (a) Cross section of a steel 9Cr-l Mo carburized in the radiation zone of a visbreaking furnace after 102 000 h of continuous exposition lOOX.(b) View of the internal cross section of austenitic steel HP-40 used in the production of ethylene 1OOX.

Table I .
Composition of 9Cr-1Mo modified ferritic steel and calculation of iron molar fraction Tabla I. Composición química del acero terrífico Fe-9Cr-lMo modificado y cálculo de la fracción molar del hierro