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Effect of aluminizing on surface microstructure of an HH309 stainless steel
Surface & Coatings Technology 200 (2006) 5048 – 5051 www.elsevier.com/locate/surfcoat
Effect of aluminizing on surface microstructure of an HH309 stainless steel S. Sharafi *, M.R. Farhang Materials Science and Engineering Department, Shahid Bahonar University of Kerman, P.O. Box No. 76135-133, Kerman, Iran Received 12 October 2004; accepted in revised form 17 May 2005 Available online 27 June 2005
Abstract Oxidation resistance of chromium and heat resistant steels is due to formation of Cr2O3 on the surface. This surface layer will be destabilized above 1000 -C and will not protect the metal. Recent investigations show that aluminizing process increases oxidation resistance of these steels by formation of Al2O3 which is more stable than Cr2O3 at high temperatures. In this investigation aluminizing process is done on austenitic heat resistant stainless steel HH309. The microstructure of the samples was then examined by SEM and EDS and phases were identified by XRD. The results show that the coating consists of two layers. The first consists of Fe – Al intermetallic phases and the second one is an interdiffusion layer consists of ferrite associated with NiAl and Ni3Al precipitates. Oxidation resistance of these samples was studied in air at 700 and 1100 -C. The results show that aluminized samples have higher oxidation resistance. D 2005 Elsevier B.V. All rights reserved. Keywords: Cementation; Aluminum oxide; Chromium oxide; Intermettalic phases; Oxidation resistance
1. Introduction Heat resistant stainless steels such as HH309 are widely used in petroleum, chemical, nuclear and other applications due to their excellent high temperature strength. Oxidation resistance of these steels is due to the formation of Cr2O3 on the surface. However, this surface layer will be destabilized at high temperature (above 1000 -C) and will not protect the metal anymore. It has been reported that the addition of Al to stainless steels or nickel base alloys would cause an increase in high temperature oxidation resistance [1]. The beneficial effect of Al on the high temperature oxidation resistance is recognized by providing a protective alumina surface layer during high temperature exposure [2 –5]. The pack cementation technique is most widely used for the deposition of Al to improve the performance of the steels in high temperature corrosive environments [6,7]. The complex aluminide intermetallic coatings formed during the
process exhibit superior resistance to oxidation, carburization and sulfidation [8]. The amount of Al introduced and the associated changes in the microstructure depends upon raw materials, temperature, time and other processing factors [6].
2. Experimental procedures In this investigation, an aluminizing process is conducted on an HH309 stainless steel. Chemical composition of this steel is shown in Table 1. For solution treatment, this steel was exposed in argon gas for 3 h at 1060 -C. Then specimens were cut from a square bar with 10 10 2 mm dimensions. These coupons were ground through 600-grit SiC paper, cleaned and dried. The powder mixture for Table 1 Chemical composition of heat resistance stainless steel HH309 in wt.%
* Corresponding author. Tel.: +98 3412114041; fax: +98 3412111865. E-mail address: Sh [email protected] (S. Sharafi). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.05.024
Fe
Ni
Cr
S
P
Mn
Si
C
Bal.
10.46
26.1
0.007
0.032
1.02
1.91
0.4
S. Sharafi, M.R. Farhang / Surface & Coatings Technology 200 (2006) 5048 – 5051
5049
Table 2 EDS results of the spectrums of Fig. 2 in wt.% Element
Spectrum 1
Spectrum 3
Spectrum 4
Spectrum 5
Fe O Cr Ni Al
65.31 29.16 3.6 1.93 –
36.74 – 3.59 1.55 58.12
65.24 – 2.86 2.36 29.51
– 47.05 – – 52.95
were derived by measuring the specimen weight before and after oxidation and the surface area of specimens.
3. Results and discussion
Fig. 1. Cross-section micrograph of aluminized sample.
aluminizing consists of 85 wt.% alumina, 10 wt.% Al and 5 wt.% ammonium chloride. The samples and pack materials were placed in steel crucible, then the crucible was placed into furnace with inert atmosphere and held at 1050 -C for 6 h. After cementation treatment, the pack was furnace-cooled to room temperature. Then the samples were removed from the pack and cleaned in distilled water. Microstructure and chemical composition of the cross-section of the coated specimens were analyzed using scanning electron microscopy (SEM) (CamScan MV320) with energy dispersive spectroscopy (EDS). The different phases of the surface layers were determined with an X-ray diffraction (XRD) technique using Cu-Ka radiation at 40 kV and 30 mA (Advance Bruker D8). For oxidation tests, two groups of samples were used: bare steels and aluminized steels. These samples were exposed in air at 700 and 1100 -C for oxidation resistance evaluation. The weight gains in mg/cm2 of the specimens
Fig. 2. Cross-section micrograph of first layer.
By aluminizing process on the HH309 steel, the coating formed on the surface consists of two layers. The cross-section micrograph of the aluminized sample is depicted in Fig. 1. Two distinct layers are observed. Fig. 2
Fig. 3. XRD analysis of aluminized sample (first layer).
Fig. 4. SEM micrograph of second layer of the coating.
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S. Sharafi, M.R. Farhang / Surface & Coatings Technology 200 (2006) 5048 – 5051
Table 3 EDS results of the spectrums of Fig. 4 in wt.% Spectrum 1
Spectrum 2
Spectrum 3
Fe Al Cr Ni
58.01 11.35 21.93 8.71
6.19 11.82 2.46 79.53
5.43 26.41 4.07 64.09
shows the SEM micrograph for the first layer of the coating. Table 2 shows the EDS results of the spectrums shown on this micrograph. Fig. 3 shows the X-ray diffraction pattern of the surface of alumnized sample. The characteristic peaks of FeAl, Fe24Al78, AlFeO3, Fe2O3 and Al2O3 are seen. From XRD and EDS (Fig. 3 and Table 2) results, it is found that the outer layer consists of Fe – Al intermetallic phases such as FeAl and Fe24Al78. Although the substrate was austenitic stainless steel, its characteristic peaks are absent, indicating that a rather thick aluminized layer was formed. The peaks of Al2O3 and Fe2O3 are also seen. The presence of Al2O3 was believed to be the residue filler material remained after cleaning. The existence of Fe2O3 was due to oxidation of Fe during the aluminizing process. Fig. 4 shows the SEM micrograph for second layer of the coating. This figure shows that two different precipitates are distributed in the matrix. Table 3 shows the EDS results obtained from different area of this micrograph. EDS results (Table 3) show that the bright precipitates (spectrum 2 in the figure) contain high concentration of Ni and Al, and the bright grey precipitates (spectrum 3 in the figure) contain Ni and Al. The dark grey matrix (spectrum 1 in the figure) consists of Fe, Al, Cr and Ni. Fig. 5 shows the X-ray diffraction pattern for the second layer of the coating. The peaks of ferrite, NiAl and Ni3 Al are shown. By observation of XRD results (Fig. 5), it is found that the matrix is ferrite with high concentration of Al, and the precipitates are NiAl and Ni3Al.
Fig. 6. Concentration profiles of Al, Cr, Fe and Ni in aluminized steel.
The effect of alloying elements on microstructure of stainless steels has often been expressed in terms of Ni equivalent if they tend to stabilize austenite and as Cr equivalent if they stabilize ferrite. To determine these equivalents, different empirical formulae have been reported in the literature. Eq. (1) and (2) show the relationship
(a) 1.24 1.22
Weight increase (mg/cm2)
Element
1.2 1.18 1.16 1.14 1.12 1.1 1.08 1.06 0
50
100
Time (hr.)
(b)
40
35
Weight increase (mg/cm2)
30
25
20
15
10
5
0 0
5
10
15
Time (hr.)
Fig. 5. XRD analysis for second layer of the coating.
Fig. 7. Weight change curves at 1100 -C for (a) aluminized and (b) bare steels.
S. Sharafi, M.R. Farhang / Surface & Coatings Technology 200 (2006) 5048 – 5051
5051
Fig. 7a,b shows the weight changes of aluminized and bare samples oxidize in air at 1100 -C. According to this figure the weight changes of aluminized specimens are lower than the others one. Therefore the oxidation resistance of the steel has been increased by aluminizing process. Fig. 8a,b shows the XRD analysis of surfaces of the uncoated and aluminized steels oxidized at 1100 -C for 100 h. Surface of the bare steel is covered by iron oxide (Fe3O4), but the surface of the aluminized specimen was protected by aluminum oxide (Al2O3). Therefore good oxidation resistance of aluminized sample (Fig. 7a) is due to the formation of Al2O3 on its surface. The low oxidation resistance of the other sample (Fig. 7b) depends on Cr for the formation of Cr 2O3 on its surface, but Cr2O3 destabilized at high temperatures (above 1000 -C) and converted to volatile oxide (CrO3) [1,2]. Therefore Fe has been oxidized rapidly due to the lack of protective layer on the surface (Fig. 8a).
4. Conclusions
Fig. 8. XRD analysis of surface oxidized at 1100 -C for 100 h: (a) bare and (b) aluminized steels.
between Creq and Nieq with other alloying elements developed by Schoefer [9]. Creq ¼ Cr þ 1:21Mo þ 0:48Si þ 2:4Al þ N
ð1Þ
Nieq ¼ Ni þ 0:11Mn þ 24:5C þ 0:44Cu þ N
ð2Þ
By diffusion of Al into substrate, Creq increases and increasing Creq stabilizes ferrite. This phase transformation from austenite to ferrite shows good agreement with Schoefer diagram [9]. The concentration profiles of Al, Cr, Fe and Ni are illustrated in Fig. 6. This figure shows that the amount of Al in the edge of second layer is about 17 wt.%. The minimum amount of Al to destabilize austenite is about 15 wt.% for HH309 steel estimated by Schoefer diagram. In this region, austenite has been transformed to ferrite. The sharp variations in the concentration profiles of different elements in this figure are due to the formations of different intermetallic phases. The number of phases found in this investigation is less than the phases which exist in Fe – Al equilibrium phase diagram. This is due to the problems of nucleation and growth of most phases [10].
1. The coating obtained by aluminizing the HH309 stainless steel consists of two layers. The outer layer consists of iron aluminde intermetallics (FeAl, Fe24Al78). The interdiffusion layer consists of ferrite that associated with NiAl and Ni3Al precipitates. 2. The aluminum diffusion inside the sample has caused the matrix transformed from austenite to ferrite in the interdiffusion layer of the coating. 3. Aluminizing process increases oxidation resistance at high temperatures by the formation of Al2O3 on the surface.
References [1] I. Saeki, T. Saito, R. Furuchi, H. Konno, T. Nakamura, K. Mabuchi, M. Itoh, Corros. Sci. 40 (1998) 1295. [2] G. Lai, High Temperature Corrosion of Engineering Alloys, ASM International Handbook, 1997, p. 25. [3] S.E. Sadique, M.A.H. Mollah, M.M. Ali, M.M. Haque, S. Basri, M.H.M. Ahmad, S.M. Sapuan, J. Corros. Sci. Eng. 1 (2003) (paper 18). [4] C. Badini, F. Laurella, Surf. Coat. Technol. 135 (2001) 291. [5] K. Messaoudi, A.M. Huntz, B. Lesage, Mater. Sci. Eng., A 247 (1998) 248. [6] W.T. Tsai, K.E. Huang, Thin Solid Films 366 (2000) 164. [7] D.M. Mattox, in: S.R. Lampman, F. Reidenbach (Eds.), Surface Engineering, ASM International Handbook, vol. 5, 1994, p. 611. [8] N.H. Heo, M.T. Kim, J.H. Shin, C.Y. Kim, Surf. Coat. Technol. 124 (2000) 39. [9] E.A. Schoefer, in: D. Peckner, I.M. Bernstein (Eds.), Handbook of Stainless Steels, Mcgraw-Hill, 1997, p.12/1. [10] S.R. Shatynskiet, J.P. Hirth, R.A. Rapp, Acta Metall. 24 (1976) 1071.
Effect of aluminizing on surface microstructure of an HH309 stainless steel S. Sharafi *, M.R. Farhang Materials Science and Engineering Department, Shahid Bahonar University of Kerman, P.O. Box No. 76135-133, Kerman, Iran Received 12 October 2004; accepted in revised form 17 May 2005 Available online 27 June 2005
Abstract Oxidation resistance of chromium and heat resistant steels is due to formation of Cr2O3 on the surface. This surface layer will be destabilized above 1000 -C and will not protect the metal. Recent investigations show that aluminizing process increases oxidation resistance of these steels by formation of Al2O3 which is more stable than Cr2O3 at high temperatures. In this investigation aluminizing process is done on austenitic heat resistant stainless steel HH309. The microstructure of the samples was then examined by SEM and EDS and phases were identified by XRD. The results show that the coating consists of two layers. The first consists of Fe – Al intermetallic phases and the second one is an interdiffusion layer consists of ferrite associated with NiAl and Ni3Al precipitates. Oxidation resistance of these samples was studied in air at 700 and 1100 -C. The results show that aluminized samples have higher oxidation resistance. D 2005 Elsevier B.V. All rights reserved. Keywords: Cementation; Aluminum oxide; Chromium oxide; Intermettalic phases; Oxidation resistance
1. Introduction Heat resistant stainless steels such as HH309 are widely used in petroleum, chemical, nuclear and other applications due to their excellent high temperature strength. Oxidation resistance of these steels is due to the formation of Cr2O3 on the surface. However, this surface layer will be destabilized at high temperature (above 1000 -C) and will not protect the metal anymore. It has been reported that the addition of Al to stainless steels or nickel base alloys would cause an increase in high temperature oxidation resistance [1]. The beneficial effect of Al on the high temperature oxidation resistance is recognized by providing a protective alumina surface layer during high temperature exposure [2 –5]. The pack cementation technique is most widely used for the deposition of Al to improve the performance of the steels in high temperature corrosive environments [6,7]. The complex aluminide intermetallic coatings formed during the
process exhibit superior resistance to oxidation, carburization and sulfidation [8]. The amount of Al introduced and the associated changes in the microstructure depends upon raw materials, temperature, time and other processing factors [6].
2. Experimental procedures In this investigation, an aluminizing process is conducted on an HH309 stainless steel. Chemical composition of this steel is shown in Table 1. For solution treatment, this steel was exposed in argon gas for 3 h at 1060 -C. Then specimens were cut from a square bar with 10 10 2 mm dimensions. These coupons were ground through 600-grit SiC paper, cleaned and dried. The powder mixture for Table 1 Chemical composition of heat resistance stainless steel HH309 in wt.%
* Corresponding author. Tel.: +98 3412114041; fax: +98 3412111865. E-mail address: Sh [email protected] (S. Sharafi). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.05.024
Fe
Ni
Cr
S
P
Mn
Si
C
Bal.
10.46
26.1
0.007
0.032
1.02
1.91
0.4
S. Sharafi, M.R. Farhang / Surface & Coatings Technology 200 (2006) 5048 – 5051
5049
Table 2 EDS results of the spectrums of Fig. 2 in wt.% Element
Spectrum 1
Spectrum 3
Spectrum 4
Spectrum 5
Fe O Cr Ni Al
65.31 29.16 3.6 1.93 –
36.74 – 3.59 1.55 58.12
65.24 – 2.86 2.36 29.51
– 47.05 – – 52.95
were derived by measuring the specimen weight before and after oxidation and the surface area of specimens.
3. Results and discussion
Fig. 1. Cross-section micrograph of aluminized sample.
aluminizing consists of 85 wt.% alumina, 10 wt.% Al and 5 wt.% ammonium chloride. The samples and pack materials were placed in steel crucible, then the crucible was placed into furnace with inert atmosphere and held at 1050 -C for 6 h. After cementation treatment, the pack was furnace-cooled to room temperature. Then the samples were removed from the pack and cleaned in distilled water. Microstructure and chemical composition of the cross-section of the coated specimens were analyzed using scanning electron microscopy (SEM) (CamScan MV320) with energy dispersive spectroscopy (EDS). The different phases of the surface layers were determined with an X-ray diffraction (XRD) technique using Cu-Ka radiation at 40 kV and 30 mA (Advance Bruker D8). For oxidation tests, two groups of samples were used: bare steels and aluminized steels. These samples were exposed in air at 700 and 1100 -C for oxidation resistance evaluation. The weight gains in mg/cm2 of the specimens
Fig. 2. Cross-section micrograph of first layer.
By aluminizing process on the HH309 steel, the coating formed on the surface consists of two layers. The cross-section micrograph of the aluminized sample is depicted in Fig. 1. Two distinct layers are observed. Fig. 2
Fig. 3. XRD analysis of aluminized sample (first layer).
Fig. 4. SEM micrograph of second layer of the coating.
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S. Sharafi, M.R. Farhang / Surface & Coatings Technology 200 (2006) 5048 – 5051
Table 3 EDS results of the spectrums of Fig. 4 in wt.% Spectrum 1
Spectrum 2
Spectrum 3
Fe Al Cr Ni
58.01 11.35 21.93 8.71
6.19 11.82 2.46 79.53
5.43 26.41 4.07 64.09
shows the SEM micrograph for the first layer of the coating. Table 2 shows the EDS results of the spectrums shown on this micrograph. Fig. 3 shows the X-ray diffraction pattern of the surface of alumnized sample. The characteristic peaks of FeAl, Fe24Al78, AlFeO3, Fe2O3 and Al2O3 are seen. From XRD and EDS (Fig. 3 and Table 2) results, it is found that the outer layer consists of Fe – Al intermetallic phases such as FeAl and Fe24Al78. Although the substrate was austenitic stainless steel, its characteristic peaks are absent, indicating that a rather thick aluminized layer was formed. The peaks of Al2O3 and Fe2O3 are also seen. The presence of Al2O3 was believed to be the residue filler material remained after cleaning. The existence of Fe2O3 was due to oxidation of Fe during the aluminizing process. Fig. 4 shows the SEM micrograph for second layer of the coating. This figure shows that two different precipitates are distributed in the matrix. Table 3 shows the EDS results obtained from different area of this micrograph. EDS results (Table 3) show that the bright precipitates (spectrum 2 in the figure) contain high concentration of Ni and Al, and the bright grey precipitates (spectrum 3 in the figure) contain Ni and Al. The dark grey matrix (spectrum 1 in the figure) consists of Fe, Al, Cr and Ni. Fig. 5 shows the X-ray diffraction pattern for the second layer of the coating. The peaks of ferrite, NiAl and Ni3 Al are shown. By observation of XRD results (Fig. 5), it is found that the matrix is ferrite with high concentration of Al, and the precipitates are NiAl and Ni3Al.
Fig. 6. Concentration profiles of Al, Cr, Fe and Ni in aluminized steel.
The effect of alloying elements on microstructure of stainless steels has often been expressed in terms of Ni equivalent if they tend to stabilize austenite and as Cr equivalent if they stabilize ferrite. To determine these equivalents, different empirical formulae have been reported in the literature. Eq. (1) and (2) show the relationship
(a) 1.24 1.22
Weight increase (mg/cm2)
Element
1.2 1.18 1.16 1.14 1.12 1.1 1.08 1.06 0
50
100
Time (hr.)
(b)
40
35
Weight increase (mg/cm2)
30
25
20
15
10
5
0 0
5
10
15
Time (hr.)
Fig. 5. XRD analysis for second layer of the coating.
Fig. 7. Weight change curves at 1100 -C for (a) aluminized and (b) bare steels.
S. Sharafi, M.R. Farhang / Surface & Coatings Technology 200 (2006) 5048 – 5051
5051
Fig. 7a,b shows the weight changes of aluminized and bare samples oxidize in air at 1100 -C. According to this figure the weight changes of aluminized specimens are lower than the others one. Therefore the oxidation resistance of the steel has been increased by aluminizing process. Fig. 8a,b shows the XRD analysis of surfaces of the uncoated and aluminized steels oxidized at 1100 -C for 100 h. Surface of the bare steel is covered by iron oxide (Fe3O4), but the surface of the aluminized specimen was protected by aluminum oxide (Al2O3). Therefore good oxidation resistance of aluminized sample (Fig. 7a) is due to the formation of Al2O3 on its surface. The low oxidation resistance of the other sample (Fig. 7b) depends on Cr for the formation of Cr 2O3 on its surface, but Cr2O3 destabilized at high temperatures (above 1000 -C) and converted to volatile oxide (CrO3) [1,2]. Therefore Fe has been oxidized rapidly due to the lack of protective layer on the surface (Fig. 8a).
4. Conclusions
Fig. 8. XRD analysis of surface oxidized at 1100 -C for 100 h: (a) bare and (b) aluminized steels.
between Creq and Nieq with other alloying elements developed by Schoefer [9]. Creq ¼ Cr þ 1:21Mo þ 0:48Si þ 2:4Al þ N
ð1Þ
Nieq ¼ Ni þ 0:11Mn þ 24:5C þ 0:44Cu þ N
ð2Þ
By diffusion of Al into substrate, Creq increases and increasing Creq stabilizes ferrite. This phase transformation from austenite to ferrite shows good agreement with Schoefer diagram [9]. The concentration profiles of Al, Cr, Fe and Ni are illustrated in Fig. 6. This figure shows that the amount of Al in the edge of second layer is about 17 wt.%. The minimum amount of Al to destabilize austenite is about 15 wt.% for HH309 steel estimated by Schoefer diagram. In this region, austenite has been transformed to ferrite. The sharp variations in the concentration profiles of different elements in this figure are due to the formations of different intermetallic phases. The number of phases found in this investigation is less than the phases which exist in Fe – Al equilibrium phase diagram. This is due to the problems of nucleation and growth of most phases [10].
1. The coating obtained by aluminizing the HH309 stainless steel consists of two layers. The outer layer consists of iron aluminde intermetallics (FeAl, Fe24Al78). The interdiffusion layer consists of ferrite that associated with NiAl and Ni3Al precipitates. 2. The aluminum diffusion inside the sample has caused the matrix transformed from austenite to ferrite in the interdiffusion layer of the coating. 3. Aluminizing process increases oxidation resistance at high temperatures by the formation of Al2O3 on the surface.
References [1] I. Saeki, T. Saito, R. Furuchi, H. Konno, T. Nakamura, K. Mabuchi, M. Itoh, Corros. Sci. 40 (1998) 1295. [2] G. Lai, High Temperature Corrosion of Engineering Alloys, ASM International Handbook, 1997, p. 25. [3] S.E. Sadique, M.A.H. Mollah, M.M. Ali, M.M. Haque, S. Basri, M.H.M. Ahmad, S.M. Sapuan, J. Corros. Sci. Eng. 1 (2003) (paper 18). [4] C. Badini, F. Laurella, Surf. Coat. Technol. 135 (2001) 291. [5] K. Messaoudi, A.M. Huntz, B. Lesage, Mater. Sci. Eng., A 247 (1998) 248. [6] W.T. Tsai, K.E. Huang, Thin Solid Films 366 (2000) 164. [7] D.M. Mattox, in: S.R. Lampman, F. Reidenbach (Eds.), Surface Engineering, ASM International Handbook, vol. 5, 1994, p. 611. [8] N.H. Heo, M.T. Kim, J.H. Shin, C.Y. Kim, Surf. Coat. Technol. 124 (2000) 39. [9] E.A. Schoefer, in: D. Peckner, I.M. Bernstein (Eds.), Handbook of Stainless Steels, Mcgraw-Hill, 1997, p.12/1. [10] S.R. Shatynskiet, J.P. Hirth, R.A. Rapp, Acta Metall. 24 (1976) 1071.