Microstructure and Oxidation Behavior of C‐HRA‐5 Austenitic Heat‐Resistant Steel in Air at the Temperature Range of 650–750 °C

This study investigates the oxidation behavior and microstructure characterization of C‐HRA‐5 (i.e., a new austenitic heat‐resistant steel) in the air at temperatures ranging from 650 to 750 °C over a 1000‐hour duration. The oxidation behavior and mechanism are analyzed using gravimetric evaluation, thermodynamic analysis, microscopic morphology, and microstructure characterization. The results indicate that the oxidation behavior follows a parabolic law at each temperature. With increasing temperature, the oxide film gradually grows and transforms from small lump particles to strips and needles, eventually covering the entire substrate surface over time. Moreover, long‐term oxidation exposure promotes the formation of various phases, including M23C6, σ, MX, Z, nanosized Cu‐rich, and Laves phases, within the metallic substrate. Considering potential applications in new‐generation power plants, this study provides a solid foundation to disclose the possible oxidation of C‐HRA‐5 austenitic heat‐resistant steel at high temperatures.


Introduction
Despite the growing interest in alternative energy sources, [1] coal-fired power generation is still required in some countries like China due to its abundant coal reserves and limited oil and gas resources. [2]owever, the current energy shortages in the country have become increasingly severe.On the other side, although renewal energy units like hydrogen, [3] batteries, [4] supercapacitors, [5] solar cells, [6] fuel cells, [7] hydrocarbon fuel, [8] etc. are becoming popular especially for hybrid vehicles, they have difficulty for large-scale usage.Meanwhile, the combustion of coal can be used for large-scale supply of electricity; however, it emits substantial amounts of pollutants, including carbon dioxide, [9] leading to a range of associated issues. [10]1a,5f,11] One of the most effective strategies involves optimizing the thermal efficiency of the coal-fired power plants by optimizing their operational parameters. [12]10a,13] However, with the increase of temperature and pressure, the ultrasupercritical unit with higher working parameters and more harsh working conditions required the material to have great performance. [14]Superheater and reheater tubes are essential in the ultrasupercritical unit. [15]10a,14-16] Therefore, studying the high-temperature oxidation resistance properties of heat-resistant steel materials for the ultrasupercritical unit boiler tubes in-service environment is of great practical significance.
16a,21a,22] In addition, precipitation strengthening is the main strengthening method for these advanced austenitic heat-resistant steels to meet hightemperature strength requirements and long-term operational safety and stability at high temperatures. [23][24] For example, Chi et al. [24a] found that the Cu-rich phase, in combination with nanosize Nb-rich MX phase and grain-boundary M 23 C 6 carbide in the austenitic matrix, contributed to excellent strengthening effect to 18Cr9NiCuNb austenitic heat-resistant steel.Yan et al. [24d] found that chromium depletion is formed because of the formed large amount of discontinuous chromium carbide along the grain boundary, which promoted crack nucleation and propagation at the grain boundary, decreasing the grain strength.
C-HRA-5 austenitic heat-resistant steel is based on the traditional Fe-22Cr-25Ni stainless steel, in which about 3.0 wt% Cu and a small amount of W, Co, Nb, Mo, N are added.21d] However, C-HRA-5 austenitic heat-resistant steel is a potential candidate material for application in the new generation power plants with outstanding mechanical properties and high-temperature oxidation resistance.Nonetheless, very little information about this new steel's microstructural evolution and oxidation behavior at high temperatures is available.Therefore, an in-depth understanding of the behavior of C-HRA-5 at high temperatures is needed considering both oxidation aspects and microstructural features.
In this study, the microstructure, oxidation behavior, and oxidation mechanism of C-HRA-5 in the temperature range of 650-750 °C are investigated for a duration of up to 1000 h in an air environment.The investigation utilizes various analytical techniques, including gravimetric evaluation, X-ray diffraction (XRD) analyses, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermodynamic analysis.Using these methods, a comprehensive understanding of the material's response to oxidation is achieved.

Material and Heat Treatment
The material used in this investigation was C-HRA-5 austenitic heat-resistant steel and its chemical composition is given in Table 1.The original samples were solution treated at 1200 °C for one hour, followed by water quenching.The steels were cut into flat-like samples with 20 mm Â 15 mm Â 3 mm dimensions.Before the oxidation test, these specimens were mechanically ground up to 2000 grit silicon carbide papers followed by ultrasonic cleaning in ethanol, degreased with acetone, and then quickly dried using hot air.

High-Temperature Oxidation Procedure
High-temperature oxidation test was carried out in static air at 650, 700, and 750 °C.The alumina crucible used in oxidation was heat treated in a horizontal tube furnace at 1000 °C, following the period allotted for heat treatment to ensure that its weight remained unchanged.The alloy's oxidation kinetics curves were determined using the static weight gain method.The specimens were removed from the furnace at specific time intervals of 100 and 1000 h, cooled in air, and then weighed with an electronic balance with a resolution of 0.1 mg.In order to ensure result accuracy, three parallel samples were utilized to obtain the average value of mass change.

Characterizations and Analysis
The phases of the oxidized surface of the samples were subjected to XRD (TD3500), which operated at 40 kV and 200 mA with Cu-Kα radiation (λ = 1.54 Å), and the 2θ range was set from 20°to 80°at a step size of 0.02°and at a scan speed of 5°min À1 .MDI Jade 6.0 software was used to identify the XRD patterns.SEM, TESCAN VEGA3, was performed to measure oxide layer thickness and identify the layer structure's morphology.Energydispersive X-ray spectroscopy (EDS) was used to analyze the chemical composition of oxide layers.Furthermore, TEM (JEM-F200) was applied to identify the residues formed in the matrix.

Oxidation Kinetics
Figure 1 shows the oxidation kinetics of the C-HRA-5 during oxidation in air at 650-750 °C.It can be seen that C-HRA-5 samples showed weight gain after oxidation.As shown in Figure 1a, the oxidation weight gain of the sample became much higher with the increase of temperature and the oxidation weight gain is about two orders at 750 °C compared to 650 °C.
In this experiment's initial stage (0-200 h), the mass gain of all samples increased rapidly, which gradually formed oxide particles on the matrix surface.This is because the oxygen molecules were adsorbed onto the surface of specimens, where oxygen directly reacted with the alloying elements on the surface to form oxides.When reaching 200 h, the oxidation weight gain gradually slowed down, and the weight gain curve gradually became smooth.
The oxidation weight gain approximately conformed to the parabolic law and continuously decreased in its rate as time extended.Figure 1b shows the relationship between the square of mass gain and oxidation time.The parabolic law can be expressed by Equation (1).
where Δm (mg) is the mass change of the specimens after oxidation, A (cm 2 ) is the surface area of the specimens, k p (mg 2 cm À4 s À1 ) represents the oxidation rate, t (s) is the oxidation time, and C is a constant.The k p values for all samples oxidized in air at 650-750 °C were determined by the slope of the square of mass gain versus oxidation time (Figure 1b) and calculated by Equation (1), which are listed in Table 2.   hardly changes with the increase in temperature.But the spinel oxide FeCr 2 O 4 is obviously found at higher temperatures.With the increase of temperature, the oxidation products gradually increase, while at 700 and 750 °C, the oxidation products are mainly FeCr 2 O 4 and Cr 2 O 3 .Although the Cr 2 O 3 peak is enhanced, the strong matrix peak can still be detected, indicating that it produces a very thin oxide film.The obvious difference is that matrix austenite peak strength is weakened at 750 °C.

Phase Analysis
The above results proved that C-HRA-5 presents good hightemperature oxidation resistance properties.

Surface Analysis
In order to better illustrate the morphology of oxides after hightemperature oxidation of C-HRA-5 in static air at 650, 700, and 750 °C for 1000 h, the morphologies and composition of the scale formed on the surface were characterized and analyzed systematically.The surface morphology of the C-HRA-5 sample after high-temperature oxidation in air at 650 °C for 1000 h is shown in Figure 3.It can be seen that oxidation is incomplete in the whole experiment process.The scratches polished by sandpaper are still clearly visible, and fine small oxide particles are locally gathered on the surface, while oxidation products do not cover most areas.After magnifying the gathered oxide particles areas, it can be seen that many particles accumulate and aggregate, and many uniformly distributed fine oxide particles are locally covered on the surfaces, as shown in Figure 3b.According to EDS results, the large oxide particles mainly comprise Fe, Cr, Mn, O, etc.While some dense fine particles are mainly Cr-rich oxides, the scattered particles are mainly composed of Fe, Cr, and O.This is because of the preferential oxidation of Cr in the initial oxidation stage. [25]In addition, Mn was observed on the C-HRA-5 sample in addition to the dominant elements.Combined with XRD phase analysis, it is inferred that the surface oxidation products of C-HRA-5 steel after high-temperature oxidation at 650 °C may be composed of Fe, Cr, Mn composite oxides and Cr 2 O 3 as well as some spinel structure oxide FeCr 2 O 4 , Table 3.
Figure 4 shows the surface morphology of the C-HRA-5 exposed to 700 °C in air for 1000 h.It can be seen that the oxide film exhibits greater completeness compared to 650 °C.There are some large particles that are gathered outside and the fine particles are attached to the inside.The magnified images in Figure 4b show the layer composed of larger oxides with a granular structure and the fine oxides composed of spinel oxides.Table 4 compares the results of EDS of points A, B, and C.
The large particles with spinel structure oxides near the substrate surface are rich in Mn and contain a relatively high amount of Cr, but the Fe content in the fine particles is comparatively high.From the atomic ratio of each element, it can be inferred that the oxide is mainly Fe-rich oxides (Fe, Cr) 3 O 4 .Combined with XRD analysis, it is speculated that the relatively dense spinel oxide FeCr 2 O 4 is formed in the oxidation process and covered on the substrate surface.This is because in the initial stage, Cr is preferentially oxidized to Cr 2 O 3 , but with the continuation of oxidation, the generated oxides will interact with each other and further produce stable spinel structure composite oxide FeCr 2 O 4 .
The surface morphology of the C-HRA-5 sample after being exposed to 750 °C in air for 1000 h is shown in Figure 5.It can be seen that the oxide film is relatively complete, uniformly    covered the entire matrix surface, indicating an accelerated formation of the oxide layer at this temperature.In addition, some oxide particles are extruded and piled together to form small aggregates, as shown in Figure 5b,c.This is because as the structure of these oxides changes, there are more gaps between the oxides, which provide a convenient channel for the diffusion of oxygen atoms and other alloying elements and accelerate the oxidation reaction.These oxides particles have formed a continuous oxides layer covered on the top of the surface.EDS results show that the content of Cr and O is higher, meaning that the oxides film layer is mainly composed of Cr oxides.Furthermore, there is only a slight difference in the proportion of Cr and O compared to different shapes of oxide particles, while the inner oxide layer majorly contains O, Cr, and Fe according to the EDS results and small different Mn and Cu is observed.Combined with the XRD analysis results, it can be inferred that there may be Cr-rich oxides and spinel oxide FeCr 2 O 4 Table 5.

Cross-Section Analysis
Cross-sectional morphology and elemental distribution of the austenitic heat-resistant steel C-HRA-5 after being exposed in air at different temperatures for 1000 h are shown in Figure 6.As shown in Figure 6, there are different morphology features.It can be seen that the oxide film structure became rather thick and some void (cracks) flaws appeared with increasing temperature.According to the EDS mapping results, a thin oxide layer is developed at the matrix interface and mainly consists of Fe-Cr oxide.Other elements do not show any significant boundaries, and there is no noticeable enrichment of elements in the oxide film layer.In addition, a small amount of white block precipitates was formed in the matrix.

Microstructure Analysis
Figure 7 shows the microstructure of the austenitic heat-resistant steel C-HRA-5 after being solution treated at 1200 °C.There is homogeneous austenite organization.Some small particles are obviously scattered within the grain and on the grain boundaries and are detected by optical microscope (OM), TEM, as well as the selected area electron diffraction (SAED) patterns.It can be assumed that these precipitates are Nb-rich phases with a diameter of about 0.5 μm and have not been dissolved during the solution-treated process.At high temperatures, C-HRA-5 with high contents of alloying elements gives rise to an easy precipitation, and these precipitate phases could form in a very short time.Therefore, it can be concluded that some precipitates would form and grow gradually in C-HRA-5 at the stage of the oxidation test.Figure 8 shows a TEM micrograph of precipitations in the matrix and the corresponding SAED patterns of C-HRA-5 after exposure to air at 650 °C for 1000 h.Two different types of precipitates can be observed inside the grains.Cr 23 C 6 is mainly distributed at grain boundaries and the Z phase with rounded spheres or rectangular blocks within the crystal.The formation of the Z phase is related to the transformation of the MX phase containing Nb, which provides a nucleation position for the Z-phase in the matrix. [26]21b,26] The main types of precipitation phases in the matrix at 700 °C for 1000 h are shown in Figure 9.In addition to the Cr 23 C 6 phase and Z phase, which appeared at 650 °C, σ phase was also detected.The Cr 23 C 6 phase is mainly distributed at the grain boundary and is a rectangular block or irregular triangle.The Z and MX phases are distributed at the grain boundary and crystal.The Z phase at the grain boundary is an irregular rectangular block, while the Z phase in the crystal is a square block.The MX phase is spherical at the grain boundary and has a small square block in the crystal.A long rod-like σ phase was detected around M 23 C 6 , with a small amount of σ phase, and the size tended to grow with the increase of aging time.
Previous research has shown that the preferential precipitation of the M 23 C 6 phase promoted the precipitation of the σ phase during high-temperature aging, which can nucleate at the interface between M 23 C 6 and austenitic matrix and grow by consuming M 23 C 6 . [27]Moreover, σ phase is a kind of brittle phase.Its precipitation will seriously deteriorate the plastic toughness of  the material. [28]The σ phase that precipitated in the crystal could greatly improve the durable strength of the material, but the σ phase that precipitated at the grain boundary is not conducive to the durable strength of the material. [28,29]Hu et al. [30] found the precipitation of fine diffuse distribution of MX phase in 25Cr-20Ni austenitic heat-resistant steel at the 750 °C lasting experiment, which caused the C, N depletion areas and triggered a large number of σ-phase-dependent MX phase precipitation.Therefore, if the uniform dispersion of the MX phase in the crystal is the core of the σ phase, making the σ phase in the crystal have equally uniform dispersion precipitation.It may be a method to achieve a strengthening effect on the heatresistant steel material.
TEM micrographs of precipitations in the matrix and the corresponding SAED patterns of C-HRA-5 after exposure in air at 750 °C for 1000 h are shown in Figure 10.It can be seen that in addition to Cr 23 C 6 , Z phase and σ phase appearing at 650 and 700 °C and MX and Laves phase are also detected.The Z phase shows an irregular rectangular shape at the grain boundary, while the σ phase and the MX phase serve as the nucleation sites and the Laves phase with the bamboo leaf shape is in the crystal form.21a,24c,d,31] In addition, Laves phase and σ phase are brittle, and the cracks easily occur around these phases during the service. [32]In addition, Cr-, Ni-, and Mo enriched in Laves phase and σ phase will lead to the depletion of these elements in the austenitic matrix, which will reduce mechanical properties and the aggravation corrosion of the material. [33]n addition, some nanoscale precipitation phases were detected in the matrix, as shown in Figure 11. Figure 11a shows a Cu-rich phase with spherical shape according to the EDS mapping analysis.24a,31b] The nucleation growth of the Cu-rich phase is a diffusion phase transition, and the rate of phase transition is affected by the content of copper elements in the austenite matrix and the diffusion rate of copper atoms.The high strain energy and low interfacial energy and the special coherent relationship with the austenite matrix make the Cu-rich phase's growth rate slow. [34]Besides, the low diffusion coefficient of Cu atoms is another reason for the low growth of Cu-rich phase. [35]he nanosize Cu-rich phase is uniformly dispersed and densely distributed in the matrix.This Cu-rich phase can effectively hinder the dislocation movement and is conducive to improving the high-temperature creep strength of the steel. [35,36]31b,37] Therefore, adding Cu to   austenitic heat-resistant steels can cause a high-temperature strengthening effect by inducing nanosize Cu-rich phase precipitation. [36,38]igure 11b shows the special morphology of the precipitation phase with the form of a short rod or cross star shape, which is rich in Nb, Cr, and N elements according to EDS mapping.24c,39] Okada et al. [34] also reported the presence of the Z phase in NF709 steel.When the Z phase with a small size is evenly distributed in the crystal, it can harden and delay recrystallization. [39,40]During the precipitation of the Z phase, the coarsening rate is very slow because of the low content of Nb in C-HRA-5, which leads to Nb's relatively long diffusion distance, making it difficult for the Z phase to grow up.On the other hand, the Z phase tends to nucleate on dislocation, which results in a large amount of Z phase existing in the whole matrix. [39,41]24a,39,41] 4. Discussion

Oxidation Behavior
The standard Gibbs free energy of the oxidation reaction of Fe, Cr, Ni, and Mn at high temperature to produce the corresponding oxides is shown in Table 6, [20,25] and it can be concluded that under the high temperature of 650, 700, and 750 °C, these alloying elements have the thermodynamic conditions for the oxidation reaction.Under experimental conditions, the oxidation reaction of the alloying elements to generate the corresponding oxides can be carried out spontaneously, and the factors that affect the degree of high-temperature oxidation include the interface reaction rate and diffusion rate of reaction elements.
31b] Therefore, Cr 2 O 3 is preferred to be generated on the sample surface in the initial stage of the high-temperature reaction due to the high oxygen activity of Cr for C-HRA-5 austenitic heatresistant steel.
At high temperatures, metal elements react with oxygen in the ambient medium to form oxides with a low-equilibrium partial pressure.With further extension of the experiment time, more oxide particles are produced on the surface of the sample, which gradually cover the surface of the substrate with the extension of time.On the outside of the generated oxide particles, it directly combines with oxygen and continues to grow, while the interaction between the oxides in contact with the matrix interface will result in a solid phase to further generate the dense spinel structure composite oxide FeCr 2 O 4 , [20a,31b] following the reaction shown by the following equation. (2) The corresponding results are shown in Table 6.FeCr 2 O 4 has a lower standard Gibbs free energy generation at three experimental temperatures, which is because the oxide formed first will further serve as the nucleation site for spinel oxide, covering the substrate surface and promoting the formation of new oxides.Furthermore, the inadequate supply of Cr makes the Cr 2 O 3 thermodynamically unstable and converts to stable oxide FeCr 2 O 4 .Therefore, the bilayer oxide film formed at 700 and 750 °C may be Cr 2 O 3 and spinel structure FeCr 2 O 4 .

Oxidation Mechanism
Based on the results mentioned above, the possible oxidation mechanisms of C-HRA-5 austenitic heat-resistant steel are schematically revealed in Figure 12, which show the oxidation scale film and the microstructure characterization at different temperatures.At the initial reaction stage, the oxidation reaction rate is relatively fast.20a,37,42] These oxides nucleate at the matrix/scale interface and gradually grow to the matrix surface, at which point the oxides are relatively dispersed oxide particles.
With the progress of the reaction, the oxide particles will gradually expand the growth area and grow up.16a,24d,31b] However, the oxide generated on the surface prevents the alloying elements being involved in the  reaction in the matrix from contacting oxygen molecules directly, which slows down oxidation.20b,42a,43] At this time, the oxidation process is relatively slow, which is consistent with the analysis of oxidation kinetics.
In addition, the structure of oxide film changes with the increase in temperature because the diffusion rate of alloying elements is accelerated at high temperatures.16a,21a,31b] The oxide scale film is gradually thick with the extension of oxidation time.20a,44] The generated oxide scale film thus has a better protective effect on the matrix.

Conclusion
This article investigated the microstructure characterization, oxidation kinetics, thermodynamic analysis, and microscopic morphology of the surface oxide layer of austenitic heat-resistant steel C-HRA-5 under static air at 650-750 °C for 1000 h.At 650, 700, and 750 °C, the oxidation weight gain after 1000 h is very small, showing good high-temperature oxidation resistance.The oxidation rate increased with increasing temperature and the oxidation behavior of C-HRA-5 approximately follows parabolic rate law.With the increase in temperature, the morphology of the oxides gradually grows from the initial fine lumpy particles to long, needle-like structures, and the oxidation products almost completely cover the surface of the substrate, which presents an outstanding oxidation resistance.After a long period of hightemperature exposure, M 23 C 6 , MX, σ phase, and Laves phase are formed inside the metallic matrix.Furthermore, there are also small diffuse cross-star or rod-shaped Z phases and uniformly diffuse distributions of spherical nanosized Cu-rich phases.This investigation discloses the oxidation and structural evolution of C-HRA-5 austenitic heat-resistant steel at high temperatures to understand their potential high-temperature applications such as new-generation power plants.

Figure 2
Figure2shows the phase constitutions of the C-HRA-5 steel surface oxides detected by XRD after 1000 h oxidation in air at 650-750 °C.It can be seen that the main diffraction peaks are from austenite matrix, indicating that the oxide layer formed at temperatures of 650, 700, and 750 °C after 1000 h does not completely cover the surface of the specimen or the oxide layer formed is very thin.As shown in Figure2, the XRD patterns prove, compared with the standard PDF card, that the oxides forming on the surfaces primarily consist of Cr 2 O 3 and spinel oxides FeCr 2 O 4 .At 650 °C, the oxidation products are mainly FeCr 2 O 4 , the oxide layer is thin, and the matrix peak is obvious.Furthermore, Cr 2 O 3 and the spinel oxide FeCr 2 O 4 are detected at different temperatures.The composition of the surface oxides

Figure 3 .
Figure 3. Surface morphology of C-HRA-5 exposed to 650 °C in air for 1000 h: a) low-magnification SEM; b) high-magnification SEM of the area labeled A in Fig. (a); and c) high magnifications of the area labeled B in Fig. (a), respectively.

Figure 4 .
Figure 4. Surface morphology of the C-HRA-5 exposed to 700 °C in air for 1000 h: a) low magnification and b) high magnification.

Figure 5 .
Figure 5. Surface morphology of C-HRA-5 exposed to 700 °C in air for 1000 h: a) low magnification; b) high magnification; and c) high magnifications of the area labeled A in Fig. (b).

Figure 7 .
Figure 7. Microstructure of the austenitic heat-resistant steel C-HRA-5 samples: a) OM patterns; b) TEM micrographs of the primary Nb-rich phase; and c) corresponding SAED patterns.

Figure 8 .
Figure 8. TEM micrograph of precipitations in the matrix, corresponding SAED patterns of C-HRA-5 after exposure in air at 650 °C for 1000 h a) the existence of Cr 23 C 6 in the boundary and b) the presence of Z phase.

Figure 6 .
Figure 6.Cross-section morphology and elemental distribution of the austenitic heat-resistant steel C-HRA-5 after being exposed in air at a) 650 °C; b) 700 °C, and c) 750 °C for 1000 h.

Figure 10 .
Figure 10.TEM micrograph of precipitations in the matrix, corresponding SAED patterns of C-HRA-5 after exposed in air at 750 °C for 1000 h a) the existence of σ phase and MX phase; b) the presence of Z phase; and c) the existence of Laves phase.

Figure 11 .
Figure 11.TEM micrograph of nanoscale precipitations in the matrix and corresponding mapping of C-HRA-5 after exposure in air at 650 °C for 1000 h: a) nanosize Cu-rich phase; b) special morphological precipitation phase.

Figure 9 .
Figure 9. TEM micrograph of precipitations in matrix and corresponding SAED patterns of C-HRA-5 after exposure in air at 700 °C for 1000 h a) the presence of Z phase; b) the existence of Z phas and Cr 23 C 6 ; and c) the existence of Cr 23 C 6 and σ phase.

Figure 12 .
Figure 12.Schematic diagram for the formation mechanisms of the oxide scale film in the C-HRA-5 austenitic heat-resistant steel.

Table 3 .
EDS point results of element concentrations of the points marked by 1-3 in Figure3(at%).

Table 4 .
EDS point results of element concentrations of the points marked by A-C in Figure 4 (at%).

Table 5 .
EDS point results of element concentrations of the points marked by B-E in Figure 5 (at%).

Table 6 .
Standard Gibbs free energy of the oxides.