Improvement of oxidation resistance of 9 mass% chromium steel for advanced-ultra supercritical power plant boilers by pre-oxidation treatment

Corrosion Science 114 (2017) 1–9

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Improvement of oxidation resistance of 9 mass% chromium steel for advanced-ultra supercritical power plant boilers by pre-oxidation treatment Fujio Abe a,∗ , H. Kutsumi b , H. Haruyama c , H. Okubo d a

Structural Materials Division, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan Formerly at National Institute for Materials Science (NIMS) and at present with Kamijima Ltd., 2-23-13 Naka-ikegami, Ohta-ku, Tokyo 146-0081, Japan c Formerly at National Institute for Materials Science (NIMS) and at present with Mitsubishi Hitachi Power Systems, Ltd., 3-1-1 Saiwai-cho, Hitachi, 317-8585, Japan d National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan b

a r t i c l e

i n f o

Article history: Received 6 May 2016 Received in revised form 12 September 2016 Accepted 13 October 2016 Available online 18 October 2016 Keywords: A. Low alloy steel, 9 mass% Cr steel B. Stem C. Oxidation, Cr2 O3 scale Pre-oxidation treatment

a b s t r a c t Effect of pre-oxidation treatment on oxidation resistance in steam at 650 ◦ C has been investigated for 9 mass% chromium (Cr) steel for advanced ultra-supercritical power plants. The formation of thin scale of Cr2 O3 -rich oxides is achieved by pre-oxidation in argon gas containing 0.3 ppm oxygen, which significantly improves the oxidation resistance of the steel in steam at 650 ◦ C for a long time exceeding 20,000 h. Effects of pre-oxidation conditions, such as oxygen concentration in argon gas and pre-oxidation temperature and time, and of silicon concentration in the steel on the oxidation resistance in stream are systematically investigated. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction The materials development projects for advanced ultrasupercritical (A-USC) coal-fired power plants with maximum steam temperature of 700 ◦ C or above have been performed to achieve a high efficiency in Europe [1–4], in the United States [5,6], in Japan [7] and recently in other countries, China [8] and India [9]. These projects involve the replacement of martensitic 9 to 12 mass% chromium (Cr) steels with high-strength nickel (Ni)-base alloys for the boiler and turbine components subjected to the highest temperature. In the present paper, the concentration of alloying elements in steels is basically expressed in mass% and 9 to 12 mass% chromium steels are denoted 9 to 12Cr steels. To minimize the requirement of expensive Ni-base alloys, martensitic 9 to 12Cr steels can be used for the components subjected to the next highest temperatures, below 650 ◦ C. Critical issues for the development of martensitic 9 to 12Cr steels for boilers at 650 ◦ C are the improvement of oxidation resistance as well as long-term creep strength, including welded joints.

∗ Corresponding author. E-mail address: [email protected] (F. Abe). http://dx.doi.org/10.1016/j.corsci.2016.10.008 0010-938X/© 2016 Elsevier Ltd. All rights reserved.

In National Institute for Materials Science, several versions of martensitic 9Cr steel strengthened by boron (MARB), by nitrides (MARN) and by both boron and nitrides (MARBN) have been developed for application to boiler components with maximum steam temperature at 650 ◦ C [10,11]. MARBN, having a chemical composition of 9Cr-3 mass% tungsten (W)-3 mass% cobalt (Co)-0.2 mass% vanadium (V)-0.05 mass% niobium (Nb) steel with 120–150 ppm boron and 60–90 ppm nitrogen, exhibits not only much higher creep rupture strength of base metal than conventional creep strength enhanced ferritic (CSEF) steels Grade 91 (9Cr-1Mo-V-Nb), Grade 92 (9Cr-0.5Mo-1.8W-V-Nb) and Grade 122 (11Cr-0.5Mo2W-V-Nb-copper (Cu)) but also substantially no degradation in creep strength due to Type IV fracture in heat-affected-zone of welded joints at 650 ◦ C. In general, the oxidation resistance of steels improves with increasing Cr concentration by forming protective Cr2 O3 scale [12–14]. However, the formation of martensitic microstructure for strengthening steels is restricted below about 12 mass% Cr and the long-term creep strength decreases with increasing Cr concentration in the range of 9 to 12 mass% Cr [15]. Usually, 9 to 12 mass% Cr is not enough for the formation of protective Cr2 O3 scale on the steel surface in steam environment at around 600 ◦ C. The oxidation resistance of 9 to 12Cr steels has been improved by the addition

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tion of protective Cr2 O3 -rich scale on the surface of 9Cr steel by the pre-oxidation treatment in argon gas containing small amount of oxygen [24–26], by the combination of shot-peening of Cr and pre-oxidation treatment in air at 700 ◦ C [27], and by the coating of Ni-(20–50 mass%) Cr thin layers [28–34]. The shot-peening of Cr and coating of Ni-Cr thin layer for the inner surface of main steam pipe involve complicated and expensive processes, while the preoxidation treatment in argon gas is simple as shown by just heating of main steam pipe in argon gas. Although steels are not oxidized in argon gas because of an inert gas, small amount of oxygen contained in argon gas, which corresponds to low partial pressure of oxygen, causes unique oxidation of steels, e.g., selective oxidation of Cr [24–26]. The purpose of the present research is to investigate the influencing factors of pre-oxidation treatment, such as pre-oxidation temperature and time, oxygen concentration in argon gas and minor element Si in 9Cr steel, on the subsequent oxidation resistance of 9Cr steel in steam at 650 ◦ C. Fig. 1. Weight gain of 9Cr-3W-0.2V-0.05Nb steel with different Si concentrations due to oxidation in steam at 650 ◦ C for 100 h after pre-oxidation in argon gas containing 0.3 ppm oxygen at 700 ◦ C, as a function of pre-oxidation time.

Fig. 2. Schematic weight gain curves for 9Cr steel during oxidation in steam at 650 ◦ C after pre-oxidation in argon gas containing small amount of oxygen. (a) effect of pre-oxidation time and (b) effect of Si concentration.

of minor elements, such as silicon (Si) [16,17] and aluminium (Al) [18], and by grain refinement [19], although the major constituent of surface oxides is Fe3 O4 . We think that the formation of protective Cr2 O3 scale is essential for the development of oxidation-resistant 9Cr steel for boilers in A-USC power plants, in which the maximum allowable temperature of ferritic steels should be risen up to 650 ◦ C. At present, the maximum allowable temperature for the CSEF steels Grade 91, Grade 92 and Grade 122 is 600 to 620 ◦ C in USC power plants. The resistance to exfoliation of oxide scale is another issue for the development of oxidation-resistant steels [14,20–22]. We have revealed that the addition of 3 mass% palladium (Pd) significantly improves the oxidation resistance of 9Cr steel in steam at 650 ◦ C by the formation of thin scale of Cr2 O3 [23]. This suggests that the formation of protective Cr2 O3 scale is possible even for 9Cr steel. However, the application of 9Cr steel containing 3 mass% Pd to main steam pipe of A-USC power plants is economically unfeasible, because Pd is very expensive. We have also achieved the forma-

2. Experimental procedure The steels examined were 9Cr-3W-0.2V-0.05Nb (mass%) steel with different Si concentrations of 0 to 0.8 mass% and 9Cr-3W3Co-0.2V-0.05Nb steel MARN, MARB1 and MARB2 with different carbon, nitrogen and boron concentrations. The MARN, MARB1 and MARB2 are martensitic 9Cr steel strengthened by nitrides, by 100 ppm boron and by 200 ppm boron, respectively, which were alloy-designed from a viewpoint of creep strength improvement [10,11]. Tables 1 and 2 give the chemical compositions of the steels. The chemical analysis of the steels was carried out by induction coupled plasma emission spectroscopy. The heat treatment conditions were normalizing at 1100 ◦ C for 0.5 h and then tempering at 800 ◦ C for 1 h for the 9Cr-3W-0.2V-0.05Nb steels with different Si concentrations, while those were normalizing at 1100, 1150 and 1150 ◦ C for MARN, MARB1 and MARB2, respectively, for 0.5 h and then tempering at 800 ◦ C for 1 h for the three steels. The sheet specimens having a size of 10 × 20 × 2 mm were cut from bulk materials, which were already heat treated, ground on a SiC emery paper of 320 grit, rinsed in acetone and then supplied to the pre-oxidation treatment in argon gas containing small amount of oxygen or directly to the oxidation test in steam. The pre-oxidation treatment was carried out in argon gas at 500 to 700 ◦ C, using the furnace used for the oxidation test. The argon gas containing 0.3 vol ppm oxygen was supplied from an argon gas line arrayed in testing rooms of our institute into the furnace for the pre-oxidation treatment, except for the experiments examining the effect of oxygen concentration from 0.1 ppm to 1 vol% in argon gas on the subsequent oxidation resistance in steam. The argon gases containing 0.1 ppm and 1 vol% oxygen were supplied from argon gas cylinders containing specified oxygen concentration of 0.1 ppm and 1 vol%, respectively. The oxidation test was carried out in steam at 650 ◦ C for up to a long time exceeding 20,000 h. The sheet specimens were placed in ceramic crucibles, which were carefully set in the test furnace. At first, the test furnace was filled with argon gas and then evacuated using a rotary pump. This process was repeated 3 times to remove residual air present in the furnace. Next, the furnace was heated to 650 ◦ C in 3 h. When the furnace temperature exceeded 200 ◦ C, steam was introduced into the furnace. The flow rate of steam was 0.21 l/s, which was maintained by controlling the flow rate of the water supplied to the steam generator. The water used to produce the steam was deionized to an electric conductivity of below 8.0 ␮S/m by passing it through an ion-exchange resin, and was degassed to a dissolved oxygen concentration of less than 10 ppb by blowing argon gas into the water tank. After comple-

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Fig. 3. STEM-EDS analysis of cross-section of 9Cr-3W-0.2V-0.05Nb steel with 0.8 mass% Si after pre-oxidation in argon gas containing 0.3 ppm oxygen at 700 ◦ C for 50 h.

Table 1 Chemical compositions of 9Cr-3W-0.2V-0.05Nb steel with different Si concentrations examined. (mass%)

0Si 0.1Si 0.3Si 0.5Si 0.8Si

C

Si

Mn

Cr

W

V

Nb

N

B

0.088 0.093 0.095 0.090 0.095

0.01 0.11 0.26 0.46 0.84

0.39 0.29 0.35 0.30 0.40

8.93 8.90 8.71 8.73 8.70

3.38 3.48 3.66 3.57 3.73

0.22 0.21 0.20 0.21 0.21

0.070 0.062 0.063 0.060 0.065

0.0022 0.0019 0.0018 0.0020 0.0020

0.0025 0.0023 0.0025 0.0017 0.0010

tion of the test duration, the furnace temperature was brought to 200 ◦ C in 1 h and steam purging was stopped at the end of this process. The details of the experimental procedure and apparatus were described elsewhere [23]. The specimens were weighed by a microbalance before and after the oxidation test. The polished section of the specimens was examined by scanning transmission electron microscope with energy dispersive X-ray spectrometer (STEM-EDS), by selected are electron diffraction in transmission electron microscope and by optical microscope. The surface oxide constituents were identified by Xray diffraction using filtered Cr K␣ (␭ = 2.29100 × 10−10 m). The diffraction studies were carried out on the as-oxidized specimens at room temperature. The diffraction patterns were compared with

power diffraction files by American Society for Testing and Materials. 3. Results and discussion 3.1. Effect of pre-oxidation treatment and Si concentration Fig. 1 shows the weight gain of 9Cr-3W-0.2V-0.05Nb steel with different Si concentrations of 0 to 0.8 mass% after oxidation in steam at 650 ◦ C for 100 h as a function of pre-oxidation time. The pre-oxidation treatment was carried out in argon gas containing 0.3 ppm oxygen at 700 ◦ C. In the condition of no pre-oxidation treatment, corresponding to the pre-oxidation time of 0 h in Fig. 1, the

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oxidation treatment shown in Fig. 1 is correlated with the formation of very thin scale of Cr2 O3 -rich oxides by selective oxidation [12] during the pre-oxidation treatment, which is enhanced by the formation of Si oxides at the interface between the Cr2 O3 -rich oxides and alloy matrix. Guo and co-workers observed the SiO2 -rich layer, which was continuously formed along the interface between the Cr2 O3 -rich scale and alloy matrix of an Fe-5 mass% Cr-5 mass% Si steel after oxidation in a gas of 1 atm O2 at 700 ◦ C for 24 h [17]. The formation of SiO2 -rich layer stabilizes the Cr2 O3 -rich scale. They stated that the oxide scale consisting of Cr2 O3 and SiO2 acted as barriers against the outer diffusion of metal atoms, resulting in improvement of oxidation resistance. Lowe reported that the addition of 3 mass% Si improved the oxidation resistance of a Ni-20 mass% Cr alloy in air at 1100 to 1200 ◦ C by the formation of SiO2 layer beneath the Cr2 O3 layer, although the SiO2 layer was not continuous one [35]. In the present steel, while the Si-enriched layer at the interface between the Cr2 O3 -rich oxide scale and alloy matrix consists of discontinuous Si oxides but not continuous layer even in the steel containing 0.8 mass% Si as shown in Fig. 3, the Cr2 O3 rich oxide scale acts as protective scale during oxidation in steam, resulting in the improvement of oxidation resistance of the 9Cr steel in steam. The formation of very thin scale of Cr2 O3 -rich oxides, having a thickness of less than 0.1 ␮m, is achieved by the combination of Si addition higher than 0.5% and pre-oxidation treatment in argon gas at 700 ◦ C for 25 h or more. On the other hand, the poor resistance to oxidation or the large weight gain at low Si concentrations and short pre-oxidation times in Fig. 1 is correlated with the formation of thick scale of (Fe, Cr, Mn)3 O4 magnetite. Fig. 4. Selected area electron diffraction in TEM for 9Cr-3W-0.2V-0.05Nb steel with 0.8 mass% Si after pre-oxidation in argon gas containing 0.3 ppm oxygen at 700 ◦ C for 50 h.

addition of Si decreases the weight gain of the 9Cr steel in steam but the effect of Si for the improvement of oxidation resistance is not so drastic. The weight gain of the 9Cr steel after oxidation in steam at 650 ◦ C for 100 h becomes reduced with increasing pre-oxidation time and with increasing Si concentration in the steel. This can be understood by the schematic weight gain curves, shown in Fig. 2, of the steel in steam at 650 ◦ C for different pre-oxidation times and for different Si concentrations in the steel. Although the weight gain of the steel increases in steam with increasing oxidation time, the weight gain of the 9Cr steel after oxidation in steam at 650 ◦ C for 100 h is reduced with increasing pre-oxidation time and with increasing Si concentration in the steel. Figs. 3 and 4 show the STEM-EDS analysis and selected area electron diffraction patterns, respectively, of the cross-section of surface oxides formed on 9Cr-3W-0.2V-0.05Nb steel with 0.8 mass% Si after pre-oxidation treatment in argon gas containing 0.3 ppm oxygen at 700 ◦ C for 50 h. The thickness of surface oxide scale formed by the pre-oxidation treatment is about 550 to 600 nm and the major component of the surface oxide is Cr-rich oxides identified as Cr2 O3 containing small amount of Fe. The Mn-rich layer identified as (Fe, Cr, Mn)3 O4 is located close to the scale/steam interface. It should be noted that Si ions are enriched at the interface between the Cr2 O3 -rich oxide scale and alloy matrix. In Fig. 3, the particles that seem to contain high Si and having a size of about 0.1 ␮m are the precipitates of Fe2 W Laves phase but not particles containing Si. The characteristic X-ray images from Si and W cannot be distinguished each other, because the characteristic X-ray peaks from Si and W appear at the same energy. Although the presence of any Si oxides was not confirmed by X-ray diffraction experiments, the enriched Si at the interface suggests the formation of Si oxide layer, presumably SiO2 layer. The present results suggest that the reduction of weight gain of the 9Cr steel after oxidation in steam at 650 ◦ C for 100 h by the pre-

3.2. Effect of oxygen concentration in argon gas supplied for pre-oxidation treatment The effect of oxygen concentration in argon gas supplied to the pre-oxidation treatment on the subsequent oxidation resistance in steam at 650 ◦ C was examined for 9Cr-3W-0.2V-0.05Nb steel containing different Si concentrations of 0 to 0.8 mass%. The pre-oxidation treatment was carried out in argon gas argon gas containing 0.1 ppm, 0.3 ppm and 1 vol% oxygen at 700 ◦ C for 50 h. The weight gain due to oxidation in steam at 650 ◦ C after the preoxidation treatment are shown in Fig. 5 as a function of duration of oxidation test in steam. The scale of the ordinate for the 9Cr-3W0.2V-0.05Nb steel containing 0.8 mass% Si, Fig. 5(d), is magnified, because the weight gain of this steel is much smaller than the other steels. At 0 mass% Si, the weight gain of the 9Cr steel after oxidation in steam at 650 ◦ C is significantly reduced by reducing oxygen concentration in argon gas, Fig. 5(a). At intermediate Si concentrations of 0.3 and 0.5 mass%, the effect of oxygen concentration in argon gas on the weight gain of the 9Cr steel after oxidation in steam at 650 ◦ C is not simple, although the weight gain is smaller at higher Si concentration. At a high Si concentration of 0.8 mass%, the weight gain of the 9Cr steel after oxidation in steam at 650 ◦ C is very small, independent of oxygen concentrations in argon gas. This indicates that at a high Si concentration of 0.8 mass%, very thin scale of Cr2 O3 -rich oxides is formed on the surface of 9Cr steel during the pre-oxidation treatment, independent of oxygen concentrations in argon gas. Fig. 6 shows the X-ray diffraction patterns taken from the 9Cr3W-0.2V-0.05Nb steel containing 0.3 mass% Si after pre-oxidation treatment at 700 ◦ C for 50 h in argon gas containing 0.3 ppm and 1 vol% oxygen. Both Cr2 O3 and (Fe,Cr)3 O4 formed on the specimen surface during the pre-oxidation treatment. Comparing the relative intensities between the Cr2 O3 (012) and (Fe,Cr)3 O4 (022) peaks and that between the Cr2 O3 (113) and (Fe,Cr)3 O4 (400) peaks, the diffraction peaks due to Cr2 O3 are relatively larger than those due to (Fe,Cr)3 O4 in argon gas containing low oxygen concentration of 0.3 ppm, while the diffraction peaks due to (Fe,Cr)3 O4 are relatively

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Fig. 6. X-ray diffraction patterns taken from 9Cr-3W-0.2V-0.05Nb steel with 0.3 mass% Si after pre-oxidation at 700 ◦ C for 50 h in argon gas containing (a) 0.3 ppm and (b) 1 vol% oxygen. Cr K␣ (␭ = 2.29100 × 10−10 m).

Fig. 7. Effect of oxygen concentration in argon gas used for pre-oxidation at 700 ◦ C for 50 h on weight gain of 9Cr-3W-0.2V-0.05Nb steel with different Si concentrations in steam at 650 ◦ C for 1000 h.

Fig. 5. Effect of oxygen concentration in argon gas used for pre-oxidation at 700 ◦ C for 50 h on weight gain of 9Cr-3W-0.2V-0.05Nb steel with different Si concentrations in steam at 650 ◦ C. (a) 0, (b) 0.3, (c) 0.5 and (d) 0.8 mass% Si.

larger than those due to Cr2 O3 in argon gas containing high oxygen concentration of 1 vol% oxygen. This indicates that lower oxygen concentration in argon gas used for the pre-oxidation treatment enhances the formation of Cr2 O3 scale. Fig. 7 shows the weight gain of the 9Cr-3W-0.2V-0.05Nb steel containing 0, 0.3, 0.5 and 0.8 mass% Si due to oxidation in steam at 650 ◦ C for 1000 h after the pre-oxidation treatment, as a function

of oxygen concentration in argon gas supplied to the pre-oxidation treatment. The weight gain of the steel containing 0, 0.3 and 0.5 mass% Si significantly reduces with reducing oxygen concentration in argon gas below 0.3 ppm but it is rather insensitive to the oxygen concentration above 0.3 ppm O2 . The steel containing 0.8 mass% Si exhibits the very small weight gain independent of oxygen concentration in argon gas. Although the lower oxygen concentration in argon gas enhances the formation of Cr2 O3 -rich scale during pre-oxidation treatment as described above, the application of pre-oxidation treatment using argon gas containing 0.1 ppm oxygen to boiler components in AUSC power plants is economically unfeasible, because argon gas containing 0.1 ppm oxygen is more expensive than that containing

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Fig. 8. Effect of pre-oxidation time in argon gas containing 0.3 ppm oxygen at 650 ◦ C on weight gain of 9Cr-3W-0.2V-0.05Nb steel with different Si concentrations in steam at 650 ◦ C for 1000 h.

0.3 ppm oxygen. Therefore, in the experiments examining the effect of pre-oxidation time and temperature on subsequent oxidation of 9Cr steel in steam and the long-term stability of Cr2 O3 -rich scale in steam, which will be described in the next sections 3.3 and 3.4, respectively, argon gas containing 0.3 ppm oxygen is used for the pre-oxidation treatment from an economical point of view. 3.3. Effect of pre-oxidation time and temperature on subsequent oxidation in steam The effect of pre-oxidation time on the subsequent oxidation resistance of 9Cr-3W-0.2V-0.05Nb steel with different Si concentrations of 0 to 0.8 mass% in steam at 650 ◦ C is shown in Fig. 8. The oxidation test was carried out in steam at 650 ◦ C for 1000 h after pre-oxidation in argon gas containing 0.3 ppm oxygen at 650 ◦ C for 0 to 1000 h. The weight gain of the steel after oxidation in steam at 650 ◦ C for 1000 h becomes reduced with increasing pre-oxidation time and with increasing Si concentration in the 9Cr steel. We think that candidate steels for boiler components operating at 650 ◦ C should exhibit oxidation resistance in steam at 650 ◦ C better than the oxidation resistance of T/P91 in steam at 600 ◦ C, where T/P91 means tubes and pipes of Grade 91 steel, because T/P91 are being now used for long duration in USC power plants operating at around 600 ◦ C. The weight gain of T91 in steam at 600 ◦ C for 1000 h [25] is also shown by the dotted line in Fig. 8 for comparison. In the condition of no pre-oxidation treatment, corresponding to the pre-oxidation time of 0 h in Fig. 8, the weight gain of 9Cr-3W-0.2V0.05Nb steel with 0 to 0.8 mass% Si after oxidation in steam at 650 ◦ C for 1000 h is larger than that of T91 in steam at 600 ◦ C for 1000 h. The weight gain of 9Cr-3W-0.2V-0.05Nb steel after oxidation in steam at 650 ◦ C for 1000 h becomes reduced with increasing preoxidation time and with increasing Si concentration in the steel. For the 9Cr-3W-0.2V-0.05Nb steel with 0.3 mass% Si, the weight gain of the steel after oxidation in steam at 650 ◦ C for 1000 h is smaller than that of T91 in steam at 600 ◦ C for 1000 h, if the present steel is subjected to the pre-oxidation treatment for 50 h or more at 650 ◦ C. The boiler steels, such as Grade 91, Grade 92 and Grade 122, contain 0.3 mass% Si. The effect of pre-oxidation temperature on the subsequent oxidation resistance of 9Cr-3W-0.2V-0.05Nb steel with different Si concentrations of 0 to 0.8 mass% in steam at 650 ◦ C is shown in Fig. 9. The oxidation test was carried out in steam at 650 ◦ C for 100 h after pre-oxidation in argon gas containing 0.3 ppm oxygen at 500

Fig. 9. Effect of pre-oxidation temperature in argon gas containing 0.3 ppm oxygen on weight gain of 9Cr-3W-0.2V-0.05Nb steel with different Si concentrations in steam at 650 ◦ C for 100 h.

to 650 ◦ C for 100 h. The weight gain of the steel after oxidation in steam at 650 ◦ C for 100 h becomes reduced with increasing preoxidation temperature and with increasing Si concentration in the 9Cr steel. For the 9Cr-3W-0.2V-0.05Nb steel with 0.3 mass% Si, the weight gain of the steel after oxidation in steam at 650 ◦ C for 100 h is smaller than that of T91 in steam at 600 ◦ C for 100 h, if the present steel is subjected to the pre-oxidation treatment at 500 to 650 ◦ C for 100 h or more. The present results that the weight gain of the steel after oxidation in steam at 650 ◦ C becomes reduced with increasing pre-oxidation time and temperature suggest that the growth of protective Cr2 O3 -rich scale during the pre-oxidation treatment is more significant with increasing pre-oxidation time and temperature, presumably indicating a diffusion-controlled process. 3.4. Long-term stability of Cr2 O3 -rich oxide scale in steam The long-term stability of Cr2 O3 -rich scale formed by the preoxidation treatment is examined for the three steels MARB1, MARB2 and MARN in Table 2 in steam at 650 ◦ C. Fig. 10(a) shows the weight gain of the steels in steam at 650 ◦ C after pre-oxidation in Ar gas containing 0.3 ppm oxygen at 700 ◦ C for 50 h, while Fig. 10(b) shows the weight gain of the steels in steam at 650 ◦ C after no pre-oxidation. The scale of the ordinate of Fig. 10(a) is magnified, because the weight gain of the steels after the pre-oxidation treatment is much smaller than that after no pre-oxidation treatment shown in Fig. 10(b). For the three steels, the oxidation resistance in steam is significantly improved by the pre-oxidation treatment, which produced the thin scale of Cr2 O3 -rich oxides as will be shown later. No evidence was found for the breakaway in the weight gain curves for the three steels with pre-oxidation treatment. The present results indicate that the thin scale of Cr2 O3 -rich oxides formed by the pre-oxidation treatment is stable during oxidation in steam at 650 ◦ C for a long time exceeding 20,000 h. This indicates that the thin scale of Cr2 O3 -rich oxides is highly resistant to exfoliation. The availability of the present pre-oxidation treatment for the improvement of oxidation resistance in steam is also confirmed by the Task 3 of DOE Vision 21 project in the United States [36].

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Table 2 Chemical compositions of MARB1, MARB2 and MARN examined. MARN, MARB1 and MARB2 mean martensitic 9Cr steel strengthened by nitrides, by 100 ppm boron and by 200 ppm boron, respectively. (mass%)

MARN MARB1 MARB2

C

Si

Mn

P

S

Cr

W

Co

V

Nb

N

B

0.005 0.082 0.082

0.78 0.74 0.73

0.49 0.50 0.49

0.001 0.001 0.001

0.004 0.006 0.005

8.95 9.23 9.16

2.50 2.50 2.47

3.3 3.3 3.3

0.20 0.20 0.20

0.048 0.049 0.048

0.05 0.001 0.002

0.007 0.01 0.019

Fig. 11. Optical micrographs of cross section of MARN, MARB1 and MARB2 with and without pre-oxidation treatment in argon gas containing 0.3 ppm oxygen at 700 ◦ C for 50 h, and then oxidation in steam at 650 ◦ C for 500 h.

Fig. 10. Weight gain of MARB1, MARB2 and MARN in steam at 650 ◦ C, after (a) pre-oxidation in argon gas containing 0.3 ppm oxygen at 700 ◦ C for 50 h and (b) no pre-oxidation.

Fig. 11 compares the cross-section of the MARB1, MARB2 and MARN specimens with and without the pre-oxidation treatment, after the oxidation test in steam at 650 ◦ C for 500 h. In the condition of without pre-oxidation, the thick scale having a thickness of 50 to 100 ␮m forms during oxidation in steam. The major component of the thick scale is (Fe, Cr, Mn)3 O4 . The scale is thicker in MARN than in MARB1 and MARB2, reflecting the larger weight gain in MARN than in MARB1 and MARB2 as shown in Fig. 11. The mechanisms for the larger weight gain and thicker scale in MARN than in MARB1 and MARB2 are not clear at present. Fig. 12 shows the STEM-EDS analysis of cross-section of surface oxides on MARB2 after pre-oxidation in argon gas containing 0.3 ppm oxygen at 700 ◦ C for 50 h and then oxidation in steam at 650 ◦ C for 9000 h. The scale structure, such as the Cr2 O3 -rich oxides of less than 0.1 ␮m thickness, the Mn-rich layer outside the Cr2 O3 -rich oxides and the Si enrichment at the interface between Cr2 O3 -rich oxides and alloy matrix, is substantially the same as that just after the pre-oxidation treatment shown in Figs. 3 and 4. Again, the particles that seem to contain high Si and having a size of about 0.2 ␮m in Fig. 12 are the precipitates of Fe2 W Laves phase but not particles containing Si, similar as those in Fig. 3. The growth of sur-

face oxide scale scarcely takes place during oxidation in steam at 650 ◦ C for 9000 h, as shown by the increase in thickness from 550 to 600 nm after the pre-oxidation treatment shown in Fig. 3 and 3 to 600 to 650 nm after oxidation in steam for 9000 h shown in Fig. 12. The present results indicate that the thin scale of Cr2 O3 -rich oxides formed by the pre-oxidation treatment is stable for up to long times in steam at 650 ◦ C. No evidence was found for the exfoliation nor cracking of the thin scale of Cr2 O3 -rich oxides. In order to examine the possibility of exfoliation of the thin scale of Cr2 O3 -rich oxides formed by the pre-oxidation treatment, the cyclic oxidation test was carried out for MARB2 in steam. At first the specimens were subjected to the pre-oxidation treatment in argon gas containing 0.3 ppm oxygen at 700 ◦ C for 50 h and then thermal cycling test was carried out for up to 40 cycles in steam between 650 ◦ C and 150 ◦ C, using trapezoidal thermal cycling with a temperature hold at 650 ◦ C for 20 h. But the weight gain curves of MARB2 during cyclic oxidation test were the same as those during continuous or isothermal oxidation test in steam at 650 ◦ C, indicating no breakaway in the weight gain curves and hence no exfoliation of Cr2 O3 -rich oxides formed by the pre-oxidation treatment during cyclic oxidation test [27]. This suggests that the thin scale of Cr2 O3 rich oxides is highly resistant to exfoliation, because of very thin scale. Thermal cycling tests under stress will be required in future to assess the exfoliation properties of the thin scale of Cr2 O3 -rich oxides.

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steel. Further investigation will be necessary to make clear the effect of Si oxide layer on the outward diffusivity of metal cations, the lattice structure of Si oxide layer and the thermodynamic calculation of phase stability of Cr2 O3 and Si oxide. 4. Conclusion

Fig. 12. STEM-EDS analysis of cross-section of MARB2 after pre-oxidation in argon gas containing 0.3 ppm oxygen at 700 ◦ C for 50 h and then oxidation in steam at 650 ◦ C for 9000 h.

3.5. Effect of pre-oxidation treatment on subsequent oxidation resistance in steam The oxidation resistance of 9Cr steel in steam depends on the oxide scale structure formed during the pre-oxidation treatment. The present results show that the formation of very thin scale of Cr2 O3 -rich oxides during the pre-oxidation treatment significantly improves the oxidation resistance of 9Cr steel in steam. During the pre-oxidation treatment in argon gas containing 0.1 ppm to 1 vol% oxygen, both Cr2 O3 and (Fe,Cr)3 O4 form on the surface of 9Cr steel. The formation of Cr2 O3 is enhanced by reducing oxygen concentration in argon gas as shown by Figs. 5 and 6, and by the formation of Si oxides at the interface between Cr2 O3 -rich oxide scale and alloy matrix as shown by Figs. 4 and 12. It has been reported that at lower partial pressure of oxygen in gas, the formation of Cr2 O3 by selective oxidation can be achieved at lower Cr concentrations for Ni-Cr and Fe-Ni-Cr alloys containing less than 10 mass% Cr [12,37,38]. This suggests that lower partial pressure of oxygen in argon gas enhances the formation of Cr2 O3 in the present 9 mass% Cr steel. The amount of oxygen atoms diffusing inward in the steel decreases with decreasing partial pressure of oxygen in gas and the amount of Cr atoms diffusing outward in the steel decreases with decreasing the concentration of Cr in the steel. This indicates that the amount of Cr atoms diffusing outward needed for the formation of Cr2 O3 is achieved at lower Cr concentrations in the steel at lower partial pressure of oxygen in gas. It has been also reported that the formation of SiO2 at the interface between Cr2 O3 -rich oxide scale and alloy matrix improves oxidation resistance of steels and Ni alloys [17,35]. Although the crystal structure of Si oxides formed at the interface between Cr2 O3 -rich oxide scale and alloy matrix was not identified in the present work, the Si oxide layer is considered to stabilize the Cr2 O3 rich oxide scale, which improves the oxidation resistance of 9Cr

(1) The formation of thin scale of Cr2 O3 -rich oxides, having a thickness of less than 0.1 ␮m, is achieved on the surface of 9Cr steel by the pre-oxidation treatment in argon gas containing 0.1 ppm to 1 vol% oxygen, which significantly improves the subsequent oxidation resistance in steam at 650 ◦ C. The weight gain of the 9Cr steel after oxidation in steam at 650 ◦ C becomes reduced with increasing pre-oxidation time and temperature and with increasing Si concentration in the steel. Lower oxygen concentration in argon gas used for the pre-oxidation treatment enhances the formation of Cr2 O3 scale. (2) The major component of the thin scale of Cr2 O3 -rich oxides formed by the pre-oxidation treatment in argon gas containing 0.3 ppm oxygen at 700 ◦ C for 50 h is identified as Cr2 O3 containing small amount of Fe. The Mn-rich layer, identified as (Fe, Cr, Mn)3 O4 , is located close to the scale/steam interface. Si oxides are enriched at the interface between the Cr2 O3 scale and alloy matrix. (3) The weight gain of the 9Cr-3W-0.2V-0.05Nb steel with 0.3 mass% Si is smaller in steam at 650 ◦ C than that of T91 at 600 ◦ C, if the present steel is subjected to the pre-oxidation treatment in argon gas containing 0.3 ppm oxygen for 50 h or more at 650 ◦ C or that for 100 h at 500 to 650 ◦ C. This suggests that the present steel can be applied to boiler components in steam at 650 ◦ C from a point of view of oxidation resistance in steam. (4) The thin scale of Cr2 O3 -rich oxides formed on MARN, MARB1 and MARB2 by the pre-oxidation treatment in argon gas containing 0.3 ppm oxygen is stable during subsequent oxidation test in steam at 650 ◦ C for a long time exceeding 20,000 h. No evidence is found for the breakaway in the weight gain curves for the specimens with pre-oxidation treatment. The thin scale of Cr2 O3 -rich oxides is also highly resistant to exfoliation during cyclic oxidation test in steam between 150 and 650 ◦ C. Acknowledgements The authors are grateful to Mr. O. Yoshida, formerly Graduate student at Yokohama National University and presently at Nissan Motor Corporation, for his assistance in oxidation test. The present research was funded by National Institute for Materials Science. References [1] R. Blum, R.W. Vanstone, Proc. of 6th International Charles Parsons Turbine Conference, Dublin, Ireland, September 16–18, 2003, pp. 498–510. [2] H. Tschaffon, The European way to 700 ◦ C coal fired power plant, in: Proc. of 8th Liege Conference on Materials for Advanced Power Engineering 2006, Liege, Belgium, September 18–20, 2006, pp. 61–67. [3] K.H. Metzger, K. Maile, A. Klenk, A. Helmrich, Q. Chen, W, in: Proc. of 7th International Conference on Advances in Materials Technology for Fossil Power Plants, Waikoloa, Hawaii, USA, October 22–25, 2013, pp. 6–95. [4] A. Di Gianfrancesco, A. Tizzanini, C. Stolzenberger, Proc. of 7th International Conference on Advances in Materials Technology for Fossil Power Plants, Waikoloa, Hawaii, USA, October 22–25, 2013, pp. 19–23. [5] R. Viswanathan, J.F. Henry, J. Tanzosh, G. Stanko, J. Shingledecker, B. Vitalis, Proc. of 4th International Conference on Advances in Materials Technology for Fossil Power Plants, Hilton Head Island, South Carolina, USA, October 25–28, 2005, pp. 3–19. [6] J. Shingledecker, R. Purgert, P. Rawls, Proc. of 7th International Conference on Advances in Materials Technology for Fossil Power Plants, Waikoloa, Hawaii, USA, October 22–25, 2013, pp. 41–52. [7] M. Fukuda, E. Saito, H. Semba, J. Iwasaki, S. Izumi, S. Takano, T. Takahashi, Y. Sumiyoshi, Proc. of 7th International Conference on Advances in Materials Technology for Fossil Power Plants, Waikoloa, Hawaii, USA,O ctober 22–25, 2013, pp. 24–40.

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