Effect of Thermal Sprayed Al on the Steam Oxidation Resistance of 9Cr-1Mo Steel

Transcription

1 Thermal Spray 2003: Advancing the Science & Applying the Technology, (Ed.) C. Moreau and B. Marple, Published by ASM International, Materials Park, Ohio, USA, 2003 Effect of Thermal Sprayed Al on the Steam Oxidation Resistance of 9Cr-1Mo Steel T. Sundararajan, S. Kuroda, T. Itagaki and F. Abe National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, , Japan Abstract Thermal spray of Al was carried out on the modified 9Cr-1Mo steel to evaluate the steam oxidation resistance of the sprayed Al coating. Atmospheric plasma spray process (APS) was used to coat aluminum on sandblasted 9Cr-1Mo steel substrate. The coating thickness was around 40 µm. The coated specimens were steam oxidized in four different temperatures, ranging from 600 to 750 o C. The results show that the scale growth occurred in the interface between coating and substrate subsequently it penetrated into the coating structure. Al diffused into the alloy substrate with high solubility. The diffusion increased with increase in the steam temperature and test duration. Diffused aluminum formed the high hardness intermetallic compound in the substrate near the coating/substrate interface. With increase in the test duration, the intermetallic compound moved towards the bulk and at prolonged aging, it became dissolved. This was identified from the decrease in the micro hardness values at coating/substrate interface at prolonged duration. The scale growth at the substrate surface of Al sprayed steel was much controlled compared to the uncoated specimens. Introduction There is continuous trend in the efficiency improvement of modern steam power plants, since this reduces both specific emission of environmentally damaging gases and the plant operating costs and thereby cost of electricity generation. In general, the efficiency is increased by raising the temperature (and also pressure) of the steam generated in the boiler. To withstand these advanced operating conditions materials are required with high temperature capability [1]. This challenge was seen to be appropriate for the pre-competitive development program and it has been taken up as STX-21 in our institute. The existing 9Cr-1Mo steel is severely affected by both creep and oxidation at 650 o C. To overcome the creep and oxidation problem, new alloys developments are under progress. Addition of W, Si, B, N and V in the Cr-Mo type steels showed a marked improvement in their creep resistance. However, these newly developed alloys showed only a marginal improvement in their steam oxidation resistance [2]. There are several attempts to improve the oxidation resistance of the ferritic steel used for steam power plants [3]-[5]. One such attempt was made in our earlier studies by thermal spray coating of Ni-Cr alloys [6], [7]. The coatings showed an excellent protection against the steam oxidation till the tested duration of 1000 hours. The excellent protection of the coating against steam oxidation was attributed to the formation of Chromium oxide film at the coating surface. In general, the ability of current generation high temperature alloys to protect against oxidation depends upon the potential to form and maintain the thin oxide film of Al or Cr. So in the present study thermal spray of Al have been attempted on the same modified 9Cr-1Mo steel to evaluate their steam oxidation resistance. Also, in the economy point of view Al is less costly than the Ni-Cr powders and in the process point of view it is easy to spray. Materials and Method Modified 9Cr-1Mo type ferritic steel specified as ASME T91 was used as a substrate specimen with following dimensions. 1cm 2 area with 4mm thick specimens coated with Al powders (Manufacturer: Showa Denko). The compositions of alloy 503

2 Table 1: Chemical composition of substrate and coating powder (mass%). Material C P S Si Mn Cr Ni Mo Cu V Al N Nb Fe 9Cr-1Mo bal Al bal substrate and the powder are represented in Table 1. Atmospheric Plasma Spray (APS) process was adopted in the present study to coat this material on the sandblasted steel substrate. Totally 6 passes were adopted including the 2 preheating passes for pre-heating the substrate. Thickness of the coating obtained was around 40 µm. The parameters used for APS are given in Table 2. The coated specimens were introduced to XRD and SEM investigations before placing them in to the steam oxidation chambers. Steam oxidation tests were carried out in different temperatures viz., 600, 650, 700 and 750 o C. The details of the test procedure is described elsewhere [6]. The surfaces of oxidized specimens were analyzed by XRD to identify the new phases formed during steam oxidation as compared with as-coated condition. SEM/EDS analysis was carried out on the cross sections using backscattered electron image. Micro hardness studies were carried out on the cross sections to reveal their change in hardness values along the coating and near interface and correlate them to the compound formation during the steam oxidation. Figure 1: XRD spectra for Al powder and their coating by APS process on modified 9Cr-1Mo steel. Table 2: Spray parameters used for APS process. Plasma gas Ar flow rate 45 l/min Arc current 700 A Arc voltage 30 V Gun speed 30 cm/s Stand off 100 mm Powder size µm Figure 2: BSE image of the cross section of Al coated 9Cr-1Mo steel. Results and Discussion Figure 1 shows the XRD spectra for the Al powder used for spraying process and the coating produced by APS process. The pattern responsible for powder showed four major peaks corresponding to the cubic Al. After the coating were carried out by APS process the spectrum showed a similar pattern to that of powder. It indicated that the coating essentially consists of metallic Al. Figure 2 shows the backscattered electron image for the cross section of this coated specimens. The image shows that the thickness of the coating is around µm. The coating appears to be dense and less porous. In our earlier studies, [6],[7] we tried the Ni-Cr coatings by APS process. The coating showed variation in the thickness along with more number of pores. Also, it produced the oxides of Ni and Cr in the coating. In the present investigation the Al coatings showed neither non-uniform coating thickness nor oxides. This suggests that APS could be a good process to produce the Al coatings. 600 o C 700 o C 50 µm 650 o C 750 o C Figure 3: BSE image of 9Cr-1Mo steel steam oxidized at various temperatures for 100 hours. 504

3 Steam Oxidation of the Base Alloy: Figure 3 shows the backscattered electron (BSE) image of the cross section of the base modified 9Cr-1Mo steel oxidized at different temperatures for 100 hours duration. At 600 C, the scale grown on the steel surface was around 25 µm. The scale thickness is gradually increased with increase in steam temperature. At 750 C, the scale growth reached the thickness of 120 µm. The scale developed at all temperatures showed a double layer. EDX mapping was carried out on these scales shown that the outer layer mainly consists of iron oxide and the inner layer possesses the combination of Cr and Fe rich oxides. The results have a good agreement with the earlier studies reported that ferritic steel contains 9% Cr develops Cr rich Fe oxide spinels with outer magnetite layer [8]. At 1000 hours tested conditions, all the four temperature specimens found to lose their scales by de-lamination. The scales were very brittle and peeled off from the surface while transferring them from the furnace. This indicated that the material is very much prone to steam oxidation even at 600 C and the scales formed on the surface is very brittle resulting in the delamination from the surface and exposes the new surface for the steam to continue the scale formation. XRD Studies: Figure 4 shows the XRD spectra for specimens steam oxidized at 600 o C for 10, 100 and 1000 hours. At 10 hours duration, the spectrum showed the major Al peaks along with a few minor peaks. The peaks are indexed to Al 2 O 3, Fe 2 O 3 and peaks responsible for Fe-Al intermetallics. Appearance of Al 2 O 3 is desirable to protect the substrate against the steam oxidation whereas, formation of Fe 2 O 3 reveal their failure in the complete protection against the steam oxidation. The intermetallics peaks appeared in the spectrum are matching with more than one Fe-Al type intermetallic compound. Formation of the Fe-Al intermetallic phase may arise from the reaction occurred between Fe present in the substrate and Al present in the coating. Increases in the test duration from 10 hours to 100 hours, the intensities of the oxides and intermetallic peaks were enhanced. At 1000 hours duration the Fe 2 O 3 and intermetallic peaks significantly increased. The above phenomenon indicated that at prolonged durations of steam oxidation at 600 o C, intermetallic phases continued to grow along with Fe 2 O 3 scale growth. In all the tested durations the Al peaks remained as the major peaks. Figure 5 shows the XRD spectra for steam oxidized Al coatings at 750 o C for different durations. The 10 hours spectrum showed the major peak for the Fe-Al type intermetallics along with peaks responsible for Fe 2 O 3, Al and Al 2 O 3. Al showed less intense peaks compared to the specimens oxidized at 600 o C indicating the drastic change in the coating structure. The intermetallic phase appeared as the major peaks indicating that the reactive diffusion between coating and the substrate took place. Also, presence of Fe 2 O 3 suggested the inefficiency of the coating to protect against the steam oxidation at this temperature. At 100 and 1000 hours spectra showed the total absence of Al peak. Gradual decrease of the intermetallic phase was noticed along with increase in the Fe 2 O 3 peaks. The Fe 2 O 3 peaks showed the highest intensities at 1000 hours tested conditions. Figure 5: XRD spectra for Al sprayed 9Cr-1Mo steel steam oxidized at 750 o C for different durations. Figure 4: XRD spectra for Al sprayed 9Cr-1Mo steel steam oxidized at 600 o C for different durations. SEM Studies: Figure 6 shows the backscattered electron image for the specimens steam oxidized at 600 and 750 o C for different durations. At 600 o C, the cross section of 10 hours steam oxidized specimen showed coating similar to the as-coated condition except few root like growth in the coating from the 505

4 substrate. The EDS investigations revealed this roots like structures are responsible for iron oxides. This inferred that the Al coating cannot restrict the scale growth across the coating structure. Also the Fe diffusion occurred into the coating structure. Presence of intermetallic compounds noticed in the XRD studies may arise from the diffused Fe, which would have reacted with Al to form the intermetallics on the coating surface. Specimen oxidized for 100 hours at the same temperature showed more root like scale growth across the coating structure. Further increase in the test duration to 1000 hours leads to the non-homogeneity in the coating with more root like scale growth across the coating structure. Also, the diffusion of Al towards the steel substrate was noticed at this condition. In the case of specimens steam oxidized at 750 o C, the 10 hours specimens showed the diffusion of Al towards the substrate along with the scale growth from the substrate across the coating. At 100 hours, the Al sprayed coating was diffused to the bulk substrate and the coating totally disappeared. The diffused Al reached around 40 micron into the substrate. At 1000 hours test duration the diffusion of Al reached further deeply into the bulk. This can be seen from the needle like 50 µm Figure 6: BSE Cross section image for steam oxidized Al sprayed coatings on 9Cr-1Mo steel substrate. precipitates present in the substrate much deeper (100 µm) from the coating/substrate interface. The EDS mapping of the precipitates confirmed the presence of Fe and Al, which are Figure 7: BSE image of Al sprayed 9Cr-1Mo steel steam oxidized at various temperatures for 1000 hours. 506

5 possibly the Fe-Al intermetallics. It should be noted that at 600 o C/10 hours, the coating did not diffuse to the bulk substrate whereas at 750 o C/10 hours the diffusion occurred. At 100 hours, the 600 o C specimen showed the coating intact with no diffusion to the substrate whereas at 750 o C specimen it showed the total absence of coating i.e all the Al diffused to the bulk substrate. This indicated that steam temperature played an important role in the diffusion of Al. Figure 7 shows the BSE image of specimens steam oxidized at different temperature at constant test duration (1000 hours). At 600 o C Coating Substrate Figure 8: Effect of steam temperature on the micro hardness values for Al sprayed 9Cr-1Mo steel the diffusion of Al is occurred around 10 µm to the substrate. In the case of 650 o C the diffusion of Al reached more than 20 µm in the substrate region. Whereas, at 700 and 750 o C specimens showed the total diffusion of the coating to the substrate. Both conditions showed the needle like precipitates present in the substrate which arise from the diffusion of Al and then form Fe-Al intermetallics. This had the good correlation with the XRD results. The XRD spectra for specimens steam oxidized at 750 o C did not show the peak for the Al. The total diffusion of the coating lead to the absence of Al in the steam oxidized specimens at this temperatures. Micro hardness Studies: Micro hardness studies were carried out on the cross sections of the steam oxidized Al sprayed 9Cr-1Mo specimens. The purpose of this study is to find out the diffusion of Fe and Al and hence the formation of Fe-Al type intermetallics. Intermetallic compounds generally posses higher harness value than the respective elements [9]. In Fig. 8, the dotted line in figure approximately separates the coating and substrate region. However, it is very difficult to identify the interface for all the cases since it showed a very high diffusion rate at higher test temperatures and hence degradation in the coating thickness. The specimens tested at 600 o C showed an enhanced hardness values in the coating region compared to the as-coated condition. Whereas in the substrate region the values for this temperature tested conditions showed a similar values to the as-coated specimen. This indicated that the diffusion of Fe occurred to the coating whereas the substrate was unaltered. EDS mapping for specimen oxidized at 600 o C/10 hours confirmed the above phenomenon. At 650 and 700 o C, the enhanced hardness values have been in both coating and substrate region. This indicated that at these temperatures, both inward Al diffusion and outward Fe diffusion occurred which resulted in the high hardness Fe-Al type intermetallics. In the case of 750 o C, the higher hardness values were noticed at the coating and in the deep substrate. This may be due to the higher diffusion rate of Al at this temperature. In the 100 hours test condition, the 600 o C specimen also showed the higher hardness values in the substrate and coating region. This suggested that at this test duration the diffusion of Al into the substrate started to occur. The 650 and 700 o C specimens also showed the similar trend to that of 600 o C. On the other hand, the specimen oxidized at 750 o C showed higher hardness values only near surface of the coating region and in the substrate region the hardness values are approaching to the ascoated specimen. This is attributed to the dissociation/dissolution of the intermetallic phase and further diffusion of Al to the bulk substrate. In 1000 hours case, the specimen steam oxidized at 600 o C showed the higher hardness in both coating and substrate region. The rest of all other tested temperatures showed higher harness only at the coating surface but it fell rapidly the similar hardness values of ascoated specimen in the substrate region. Though the entire coating was diffused at higher steam temperatures, the amount of scale growth (iron oxide) was much controlled compared to the uncoated substrate (Figure 3). At 750 o C, the uncoated specimen showed scale growth of 120 µm in 100 hours whereas, the coated specimen at the same condition showed an uneven surface with small amount of scale (Figure 6). This indicated that the Al coating can protect the scale formation to a certain extent. However, the Al diffusion to the substrate is very fast and it did not stop till 1000 hours of test. Also Fe-Al intermetallics found in the early stages of test would be dissociated/dissolved in the longer test durations. This is because the Al solubility in the steel is very high. The optimum thickness required to protect the scale formation in the steel needs to be determined. Also intermetallics formed during oxidation should be retained in 507

6 the longer test duration. Some methods to achieve this should be investigated for the complete protection of the coating layer against the scale growth. Conclusion 1. Thermal spray of Al coating by APS process could provide a thick and dense coating. The coating process did not produce any detectable oxides in the thermal sprayed coatings. 2. Fe present in the substrate diffused to the coating to form high hardness Fe-Al intermetallics. Similarly, Al diffused from the coating to the substrate. The Al diffusion rate increased with increase in the steam temperature. 3. Steam oxidation resistance of the Al coating remained moderately good till the 1000 hours of tested durations. Acknowledgement The authors very much thankful to Mr.T.Awane for the help in SEM/EDS investigations References 1. R.Viswanathan and W.Bekker, Materials for Ultrasupercritical Coal Power Plants-Boiler Materials: Part 1, J. Mater. Engg. Perfom., 10(2001), F.Abe, H.Okada, S.Wanikawa, M.Tabuchi, T.Itagaki, K.Kimura and K.Yamaguchi, Guiding Principle for Development of Advanced Ferritic Steels for 650 o C USC Boilers, First International Conf on Advances in Structural Steels, May 22-24, 2002, Tsukuba, Japan 3. A.Aguero, J.Garciade Blas, R.Muelas, A.Sanchez and S.Tsipas, Steam Oxidation Coating Resistance of Steam Turbine Components: A Feasibility Study, Material Science Forum , 2001, p J.Porcayo-calderon, J.G.Gonzalez-Rodriguez and L.Martinez, Protection of Carbon Steel Against Hot Corrosion Using Thermal Spray Si and Cr Base Coatings, J. Mater. Engg and Perform., 7, (1998), J.Porcayo-calderon, E.Brito-Figueroa and J.G.Gonzalez- Rodriguez, Oxidation Behaviour of Fe-Si Thermal Spray Coatings, Mater, Lett., 38(1999), T.Sundararajan, S.Kuroda, T.Itagaki and F.Abe, Steam Oxidation Resistance of Ni-Cr Thermal Spray Coatings on 9Cr-1Mo Steel. Part 1: 80Ni-20Cr, ISIJ Inter., 43, (2003), T.Sundararajan, S.Kuroda, T.Itagaki and F.Abe, Steam Oxidation Resistance of Ni-Cr Thermal Spray Coatings on 9Cr-1Mo Steel. Part 2: 50Ni-50Cr, ISIJ Inter., 43, (2003), A.H.Sully and E.A.Brandes: Chromium, 2 nd edition, Chapter-7; 1967, London, Butterworths. 9. Hosoda H, Yoshimi K, Inoue K, Hanada S, Effect of wet environment on hardness and yield stress of B2 Fe-Al alloys, Mater. Sci. Engg A258 (1998),