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Creep rates and strengthening mechanisms in tungsten-strengthened 9Cr steels
Materials Science and Engineering A319– 321 (2001) 770– 773 www.elsevier.com/locate/msea
Creep rates and strengthening mechanisms in tungsten-strengthened 9Cr steels Fujio Abe * National Research Institute for Metals (NRIM), Frontier Research Center for Structural Materials, 1 -2 -1 Sengen, Tsukuba 305 -0047, Japan
Abstract The creep deformation behavior has been investigated for simple 9Cr– W and solute modified 9Cr– WVTa steels containing high W, where Fe2W Laves phase precipitates during creep. Creep tests were carried out at 823, 873 and 923 K for up to 15 000 h. The precipitation of Fe2W effectively decreases a minimum creep rate, but the large coarsening of Fe2W promotes the acceleration of creep rate after reaching a minimum creep rate. As a result, the effect of Fe2W on the extension of creep rupture time is rather small. The fine MC carbides are significantly effective for the stabilization of lath martensitic microstructure. © 2001 Elsevier Science B.V. All rights reserved. Keywords: 9Cr steel; Tungsten; Tempered martensite; Creep rate; Creep rupture strength
1. Introduction Recently, high-strength 9 – 12Cr heat resistant steels having tempered martensitic microstructure have been developed for the boiler and turbine of an ultra-supercritical power plant, at a temperature higher than 873 K [1–3]. These steels are alloyed with many elements: C, Cr, W, Mo, V, Nb, N, B etc., for improving solid solution and precipitation strengthening, but the main alloying constituents are C – Cr – W. At present, understanding of creep deformation behavior and strengthening mechanisms are not sufficient for these 9–12Cr steels containing high W, where Fe2W Laves phase precipitates during creep. The purpose of the present research is to analyze the creep rate curves and to discuss the strengthening mechanisms by W for simple 9Cr –W and solute modified 9Cr – WVTa steels.
given in Table 1. The steel rods were austenitized, quenched and then tempered [4,5]. The 9Cr –4W steel contained 10 vol.% d-ferrite in the matrix of tempered martensite because of a high concentration of W. However, the other steels were observed to be 100% tempered martensite. The major component of carbides in as-tempered condition was M23C6 of : 0.07 mm in each steel. The M23C6 were distributed mainly along prior austenite grain and lath boundaries. Fine MC carbides of several tens nm or less were also distributed in the matrix of the 9Cr –1WVTa and 9Cr –3WVTa steels in as-tempered condition. Creep tests were carried out at 823, 873 and 923 K for up to : 15 000 h, using specimens of 6 mm in gage diameter and 30 mm in gage length.
3. Results and discussion 2. Experimental procedure
3.1. Creep rupture strength
Four simple 9Cr – (0, 1, 2, 4)W – 0.1C steels and two solute modified 9Cr – 1WVTa and 9Cr – 3WVTa steels were used. The chemical compositions of the steels are
Fig. 1 shows the creep rupture data for the steels at 923 K. The creep rupture strength increases with increasing W concentration, or by the addition of MC carbide forming elements V and Ta. The small addition of boron may also contribute to the increase in the creep rupture strength of the 9Cr –1WVTa and 9Cr –
* Tel.: +81-298-592215; fax: +81-298-592201. E-mail address: [email protected] (F. Abe).
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 2 0 0 2 - 5
F. Abe / Materials Science and Engineering A319–321 (2001) 770–773
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Table 1 Chemical compositions of the 9Cr–W steels examined (wt.%)
9Cr 9Cr–1W 99Cr–2W 9Cr–4W 9Cr–1WVTa 9Cr–3WVTa
C
Cr
W
V
Ta
B
Mn
Si
O
N
0.104 0.101 0.100 0.101 0.125 0.167
8.96 9.01 8.92 9.09 9.24 9.16
– 0.99 1.92 3.93 1.06 3.08
– – – – 0.18 0.16
– – – – 0.10 0.10
– – – – 0.005 0.007
0.49 0.48 0.48 0.50 – –
0.30 0.29 0.28 0.29 – –
0.009 0.011 0.012 0.006 0.003 0.003
0.001 0.002 0.002 0.002 0.003 0.003
3WVTa steels. The maximum creep rupture strength is obtained for the 9Cr – 3WVTa steel. The present results indicate that a combination of maximized W of 3% in martensitic phase, fine MC carbides and small amount of boron significantly improves the creep rupture strength. The creep rupture strength of the 9Cr–4W steel is the same as that of the 9Cr– 1WVTa steel at short times B1000 h, but it decreases rapidly at longer times \1000 h.
The TEM observations show that the coarsening rate of Fe2W is much larger than that of M23C6 and MC carbides [6,7]. Fig. 3 shows the stress dependence of minimum creep rate for the 9Cr–4W, 9Cr –1WVTa and 9Cr –3WVTa steels. The stress dependence is approximately described by a power law as m; min = A| n,
(1)
3.2. Creep rate and creep deformation beha6ior The creep rate curves consist of a primary or transient creep region, where the creep rate decreases with time and of a tertiary or acceleration creep region, where the creep rate increases with time after reaching a minimum creep rate, as shown in Fig. 2. We have revealed that the transient creep is a consequence of the movement and annihilation of excess dislocations and that the acceleration creep is a consequence of gradual loss of creep strength due to the microstructural evolution, such as the agglomeration of precipitates and the coarsening of martensite lath [4]. In the high-W steels, such as the 9Cr– 2W and 9Cr– 4W steels, the decrease in creep rate with time becomes more significant at longer times above :20 h, as shown by the downward deviation from the extrapolated lines from the shorttime conditions, resulting from the precipitation of Fe2W Laves phase during creep. This causes the longer duration of transient creep and the retardation of the onset of acceleration creep, resulting in lower minimum creep rate. The precipitation of Fe2W Laves phase is also observed in the 9Cr– 3WVTa steel, but not in the low-W steels, such as the 9Cr, 9Cr– 1W and 9Cr– 1WVTa steels. The creep rate of the 9Cr– 1WVTa steel is much lower than that of the simple 9Cr– 1W steel from the initial stage of creep. This results from the precipitation strengthening by fine MC carbides that form during tempering before creep. The transient creep region of the 9Cr– 1WVTa steel continues for a longer time than in the simple 9Cr– 1W steels, resulting in the lower minimum creep rate. This suggests that fine MC carbides are effective for the stabilization of tempered martensite and for the suppression of the transition from the transient to the acceleration creep region.
Fig. 1. Creep rupture data for the 9Cr steels at 923 K.
Fig. 2. Creep rate versus time curves for the 9Cr, 9Cr – 1W, 9Cr–2W, 9Cr– 4W and 9Cr – 1WVTa steel at 923 K and 78 MPa.
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F. Abe / Materials Science and Engineering A319–321 (2001) 770–773
Fig. 3. Stress dependence of minimum creep rate for the 9Cr – 4W, 9Cr– 1WVTa and 9Cr–3WVTa steels at 823, 873 and 923 K.
Fig. 4. Effect of fine precipitation and subsequent coarsening of Fe2W on creep rate.
where A is a constant and n is the stress exponent, with the exception of the 9Cr– 4W and 9Cr – 3WVTa steels at 923 K, where significant coarsening of Fe2W Laves phase occurs. In the 9Cr– 4W steel, the minimum creep rate is described by Eq. (1) with n =15 at 873 K, which is larger than at 823 K (n = 13). At 923 K, however, the n again exhibits a small value and decreases with decreasing stress. The change in n with temperature results from the effect of Fe2W precipitation on creep rate, as shown in Fig. 4. At low temperature 823 K, the precipitation rate of Fe2W is low and hence the decrease in creep rate by the precipitation of Fe2W is small in the transient creep region. With increasing temperature, the precipitation rate increases and hence the decrease in creep rate by the precipitation of fine Fe2W is larger at 873 K than at 823 K, causing
apparently larger n at 873 K than at 823 K. A further increase in temperature to 923 K results in the large coarsening of Fe2W and the decrease in creep rate by the precipitation of Fe2W becomes less pronounced with decreasing stress and increasing test duration. This results in a decrease in n with decreasing stress. Similar behavior is also observed in the 9Cr–2W and 9Cr– 3WVTa steels. At 923 K, the minimum creep rate is approximately the same between the 9Cr–4W and 9Cr –1WVTa steels at high stresses \100 MPa, but the decrease in minimum creep rate with decreasing stress becomes less pronounced in the 9Cr–4W steel than in the 9Cr –1WVTa steel B100 MPa. Fig. 5 shows the increase in creep rate with strain, d ln m; /dm, in the acceleration creep region after reaching a minimum creep rate as a function of time to rupture for the 9Cr –4W and 9Cr–1WVTa steels. Assuming exponential function of strain, the creep rate in the acceleration region is described by [5,8] m; = m; 0 exp(nm)exp(mm)exp(dm)
(2)
d ln m; /dm =n+ m+ d
(3)
where m; 0 is the initial creep rate, n the stress exponent in Eq. (1), m the microstructure degradation and d, the other parameter associated with damage, such as creep voids. Because the microstructure observations gave no evidence of any formation of creep voids showing the development of creep damage, the parameter d in Eqs. (2) and (3) is neglected and the acceleration of creep rate results from the parameters n and m. The n and m correspond to the acceleration of creep rate by an increase in stress due to a decrease in cross section with strain at constant load test and by strength loss due to microstructural evolution, respectively. The value of n has been reported to be 4–6 for simple alloys and is evaluated to be :5 for the present 9Cr steel without W. Because the values of d ln m; /dm for the 9Cr– 1WVTa
Fig. 5. Acceleration of creep rate by strain, d ln m; /dm, after reaching a minimum creep rate.
F. Abe / Materials Science and Engineering A319–321 (2001) 770–773
and 9Cr –4W steels are much larger than 5, a greater part of d ln m; /dm comes from the parameter m. The contribution of the coarsening of Fe2W to the parameter m is considered to be much larger than that of M23C6 and MC carbides, because the coarsening rate of Fe2W is much larger than that of these carbides. In the 9Cr–4W steel, the d ln m; /dm is evaluated to be : 20 or less at short times below 100 h and increases with increasing test duration and then decreases after reaching a maximum. The increase in d ln m; /dm with test duration shifts to shorter times with increasing temperature, reflecting the temperature dependence of coarsening rate of Fe2W. The shape of the d ln m; /dm versus time curves exhibiting a maximum at 923 K results from the time dependence of the amount of fine Fe2W at the time showing a minimum creep rate. As shown in Fig. 4, at high temperature, where the coarsening rate is large, the amount of fine Fe2W has a maximum at relatively short time.
3.3. Effect of Fe2W precipitation on the impro6ement of creep rupture time The time to rupture was found to depend on the acceleration of creep rate d ln m; /dm in the acceleration creep region, as well as the minimum creep rate m; min and is described by tr= 1.5/(m; min·d ln m; /dm)
(4)
for all the steels examined. As described previously, the precipitation of Fe2W effectively decreases the m; min but the large coarsening of Fe2W increases the d ln m; /dm. Therefore, the extension of creep rupture time by the precipitation of Fe2W is rather small. At 923 K, the large coarsening of Fe2W occurs at relatively short times, which causes the less pronounced decreases in minimum creep rate at low stresses and long times above 1000 h at 923 K (Fig. 3) and hence, causes the rapid decrease in creep rupture strength at long times (Fig. 1). The increase in time to rupture with increasing W concentration in the simple 9Cr – W steels is closely correlated with the reduced coarsening rate of M23C6 carbides, which effectively exert pinning force for coarsening lath [6,7]. On the other hand, the fine MC carbides are stable during creep and are effective for the stabilization of lath martensitic microstructure. A rapid decrease in creep rupture strength is not seen for the 9Cr –1WVTa steel even at 923 K. The decrease in creep rupture strength with time in Fig. 1 is less pronounced for the 9Cr–3WVTa steel at 923 K than for the 9Cr–
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4W steel, indicating less pronounced microstructure evolution in the 9Cr–3WVTa steel than in the 9Cr– 4W steel. The stabilization of martensite lath by the fine MC carbides may compensate a loss of creep strength due to coarsening of Fe2W in the 9Cr –3WVTa steel.
4. Conclusion In the high-W steels, such as the 9Cr–2W, 9Cr –4W and 9Cr–3WVTa steels, the fine precipitation of Fe2W Laves phase occurs during creep and the decrease in creep rate with time becomes more significant at long times above : 20 h in the transient creep region at 923 K. This causes a decrease in minimum creep rate. The coarsening rate of Fe2W is much larger than that of M23C6 and MC carbides. The large coarsening of Fe2W promotes the acceleration of creep rate after reaching a minimum creep rate. As a result, the effect of Fe2W on the extension of creep rupture time is rather small. The increase in time to rupture of the simple 9Cr–W steels with increasing W concentration is closely correlated with the reduced coarsening rate of M23C6 carbides. The fine MC carbides are effective for the stabilization of lath martensitic microstructure during creep. The combination of maximized W of : 3% and fine MC carbides significantly improves the creep rupture strength of tempered martensitic 9Cr steel.
References [1] F. Masuyama, Proceedings of the Sixth Liege Conference on Materials for Advanced Power Engineering, Part III, Liege, Belgium, 1998, p. 1807. [2] D.H. Allen, J.E. Oakey, B. Scarlin, Proceedings of the Sixth Liege Conference on Materials for Advanced Power Engineering, Part III, Liege, Belgium, 1998, p. 1825. [3] F. Abe, M. Igarashi, N. Fujitsuna, K. Kimura, S. Muneki, Proceedings of the Sixth Liege Conference on Materials for Advanced Power Engineering, Part I, Liege, Belgium, 1998, p. 259. [4] F. Abe, S. Nakazawa, H. Araki, T. Noda, Met. Trans. 23A (1992) 469. [5] F. Abe, Mater. Sci. Eng. A234 – 236 (1997) 1045. [6] F. Abe, Proceedings of the Fourth International Conference on Recrystallization and Related Phenomena, Tsukuba, Japan, 1999, p. 289. [7] F. Abe, Proceedings of the Eighth International Conference on Creep and Fracture of Engineering Materials and Structures, Tsukuba, Japan, 1999, p. 395. [8] M. Prager, Press. Vessel Piping 288 (1994) 401.
Creep rates and strengthening mechanisms in tungsten-strengthened 9Cr steels Fujio Abe * National Research Institute for Metals (NRIM), Frontier Research Center for Structural Materials, 1 -2 -1 Sengen, Tsukuba 305 -0047, Japan
Abstract The creep deformation behavior has been investigated for simple 9Cr– W and solute modified 9Cr– WVTa steels containing high W, where Fe2W Laves phase precipitates during creep. Creep tests were carried out at 823, 873 and 923 K for up to 15 000 h. The precipitation of Fe2W effectively decreases a minimum creep rate, but the large coarsening of Fe2W promotes the acceleration of creep rate after reaching a minimum creep rate. As a result, the effect of Fe2W on the extension of creep rupture time is rather small. The fine MC carbides are significantly effective for the stabilization of lath martensitic microstructure. © 2001 Elsevier Science B.V. All rights reserved. Keywords: 9Cr steel; Tungsten; Tempered martensite; Creep rate; Creep rupture strength
1. Introduction Recently, high-strength 9 – 12Cr heat resistant steels having tempered martensitic microstructure have been developed for the boiler and turbine of an ultra-supercritical power plant, at a temperature higher than 873 K [1–3]. These steels are alloyed with many elements: C, Cr, W, Mo, V, Nb, N, B etc., for improving solid solution and precipitation strengthening, but the main alloying constituents are C – Cr – W. At present, understanding of creep deformation behavior and strengthening mechanisms are not sufficient for these 9–12Cr steels containing high W, where Fe2W Laves phase precipitates during creep. The purpose of the present research is to analyze the creep rate curves and to discuss the strengthening mechanisms by W for simple 9Cr –W and solute modified 9Cr – WVTa steels.
given in Table 1. The steel rods were austenitized, quenched and then tempered [4,5]. The 9Cr –4W steel contained 10 vol.% d-ferrite in the matrix of tempered martensite because of a high concentration of W. However, the other steels were observed to be 100% tempered martensite. The major component of carbides in as-tempered condition was M23C6 of : 0.07 mm in each steel. The M23C6 were distributed mainly along prior austenite grain and lath boundaries. Fine MC carbides of several tens nm or less were also distributed in the matrix of the 9Cr –1WVTa and 9Cr –3WVTa steels in as-tempered condition. Creep tests were carried out at 823, 873 and 923 K for up to : 15 000 h, using specimens of 6 mm in gage diameter and 30 mm in gage length.
3. Results and discussion 2. Experimental procedure
3.1. Creep rupture strength
Four simple 9Cr – (0, 1, 2, 4)W – 0.1C steels and two solute modified 9Cr – 1WVTa and 9Cr – 3WVTa steels were used. The chemical compositions of the steels are
Fig. 1 shows the creep rupture data for the steels at 923 K. The creep rupture strength increases with increasing W concentration, or by the addition of MC carbide forming elements V and Ta. The small addition of boron may also contribute to the increase in the creep rupture strength of the 9Cr –1WVTa and 9Cr –
* Tel.: +81-298-592215; fax: +81-298-592201. E-mail address: [email protected] (F. Abe).
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 2 0 0 2 - 5
F. Abe / Materials Science and Engineering A319–321 (2001) 770–773
771
Table 1 Chemical compositions of the 9Cr–W steels examined (wt.%)
9Cr 9Cr–1W 99Cr–2W 9Cr–4W 9Cr–1WVTa 9Cr–3WVTa
C
Cr
W
V
Ta
B
Mn
Si
O
N
0.104 0.101 0.100 0.101 0.125 0.167
8.96 9.01 8.92 9.09 9.24 9.16
– 0.99 1.92 3.93 1.06 3.08
– – – – 0.18 0.16
– – – – 0.10 0.10
– – – – 0.005 0.007
0.49 0.48 0.48 0.50 – –
0.30 0.29 0.28 0.29 – –
0.009 0.011 0.012 0.006 0.003 0.003
0.001 0.002 0.002 0.002 0.003 0.003
3WVTa steels. The maximum creep rupture strength is obtained for the 9Cr – 3WVTa steel. The present results indicate that a combination of maximized W of 3% in martensitic phase, fine MC carbides and small amount of boron significantly improves the creep rupture strength. The creep rupture strength of the 9Cr–4W steel is the same as that of the 9Cr– 1WVTa steel at short times B1000 h, but it decreases rapidly at longer times \1000 h.
The TEM observations show that the coarsening rate of Fe2W is much larger than that of M23C6 and MC carbides [6,7]. Fig. 3 shows the stress dependence of minimum creep rate for the 9Cr–4W, 9Cr –1WVTa and 9Cr –3WVTa steels. The stress dependence is approximately described by a power law as m; min = A| n,
(1)
3.2. Creep rate and creep deformation beha6ior The creep rate curves consist of a primary or transient creep region, where the creep rate decreases with time and of a tertiary or acceleration creep region, where the creep rate increases with time after reaching a minimum creep rate, as shown in Fig. 2. We have revealed that the transient creep is a consequence of the movement and annihilation of excess dislocations and that the acceleration creep is a consequence of gradual loss of creep strength due to the microstructural evolution, such as the agglomeration of precipitates and the coarsening of martensite lath [4]. In the high-W steels, such as the 9Cr– 2W and 9Cr– 4W steels, the decrease in creep rate with time becomes more significant at longer times above :20 h, as shown by the downward deviation from the extrapolated lines from the shorttime conditions, resulting from the precipitation of Fe2W Laves phase during creep. This causes the longer duration of transient creep and the retardation of the onset of acceleration creep, resulting in lower minimum creep rate. The precipitation of Fe2W Laves phase is also observed in the 9Cr– 3WVTa steel, but not in the low-W steels, such as the 9Cr, 9Cr– 1W and 9Cr– 1WVTa steels. The creep rate of the 9Cr– 1WVTa steel is much lower than that of the simple 9Cr– 1W steel from the initial stage of creep. This results from the precipitation strengthening by fine MC carbides that form during tempering before creep. The transient creep region of the 9Cr– 1WVTa steel continues for a longer time than in the simple 9Cr– 1W steels, resulting in the lower minimum creep rate. This suggests that fine MC carbides are effective for the stabilization of tempered martensite and for the suppression of the transition from the transient to the acceleration creep region.
Fig. 1. Creep rupture data for the 9Cr steels at 923 K.
Fig. 2. Creep rate versus time curves for the 9Cr, 9Cr – 1W, 9Cr–2W, 9Cr– 4W and 9Cr – 1WVTa steel at 923 K and 78 MPa.
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F. Abe / Materials Science and Engineering A319–321 (2001) 770–773
Fig. 3. Stress dependence of minimum creep rate for the 9Cr – 4W, 9Cr– 1WVTa and 9Cr–3WVTa steels at 823, 873 and 923 K.
Fig. 4. Effect of fine precipitation and subsequent coarsening of Fe2W on creep rate.
where A is a constant and n is the stress exponent, with the exception of the 9Cr– 4W and 9Cr – 3WVTa steels at 923 K, where significant coarsening of Fe2W Laves phase occurs. In the 9Cr– 4W steel, the minimum creep rate is described by Eq. (1) with n =15 at 873 K, which is larger than at 823 K (n = 13). At 923 K, however, the n again exhibits a small value and decreases with decreasing stress. The change in n with temperature results from the effect of Fe2W precipitation on creep rate, as shown in Fig. 4. At low temperature 823 K, the precipitation rate of Fe2W is low and hence the decrease in creep rate by the precipitation of Fe2W is small in the transient creep region. With increasing temperature, the precipitation rate increases and hence the decrease in creep rate by the precipitation of fine Fe2W is larger at 873 K than at 823 K, causing
apparently larger n at 873 K than at 823 K. A further increase in temperature to 923 K results in the large coarsening of Fe2W and the decrease in creep rate by the precipitation of Fe2W becomes less pronounced with decreasing stress and increasing test duration. This results in a decrease in n with decreasing stress. Similar behavior is also observed in the 9Cr–2W and 9Cr– 3WVTa steels. At 923 K, the minimum creep rate is approximately the same between the 9Cr–4W and 9Cr –1WVTa steels at high stresses \100 MPa, but the decrease in minimum creep rate with decreasing stress becomes less pronounced in the 9Cr–4W steel than in the 9Cr –1WVTa steel B100 MPa. Fig. 5 shows the increase in creep rate with strain, d ln m; /dm, in the acceleration creep region after reaching a minimum creep rate as a function of time to rupture for the 9Cr –4W and 9Cr–1WVTa steels. Assuming exponential function of strain, the creep rate in the acceleration region is described by [5,8] m; = m; 0 exp(nm)exp(mm)exp(dm)
(2)
d ln m; /dm =n+ m+ d
(3)
where m; 0 is the initial creep rate, n the stress exponent in Eq. (1), m the microstructure degradation and d, the other parameter associated with damage, such as creep voids. Because the microstructure observations gave no evidence of any formation of creep voids showing the development of creep damage, the parameter d in Eqs. (2) and (3) is neglected and the acceleration of creep rate results from the parameters n and m. The n and m correspond to the acceleration of creep rate by an increase in stress due to a decrease in cross section with strain at constant load test and by strength loss due to microstructural evolution, respectively. The value of n has been reported to be 4–6 for simple alloys and is evaluated to be :5 for the present 9Cr steel without W. Because the values of d ln m; /dm for the 9Cr– 1WVTa
Fig. 5. Acceleration of creep rate by strain, d ln m; /dm, after reaching a minimum creep rate.
F. Abe / Materials Science and Engineering A319–321 (2001) 770–773
and 9Cr –4W steels are much larger than 5, a greater part of d ln m; /dm comes from the parameter m. The contribution of the coarsening of Fe2W to the parameter m is considered to be much larger than that of M23C6 and MC carbides, because the coarsening rate of Fe2W is much larger than that of these carbides. In the 9Cr–4W steel, the d ln m; /dm is evaluated to be : 20 or less at short times below 100 h and increases with increasing test duration and then decreases after reaching a maximum. The increase in d ln m; /dm with test duration shifts to shorter times with increasing temperature, reflecting the temperature dependence of coarsening rate of Fe2W. The shape of the d ln m; /dm versus time curves exhibiting a maximum at 923 K results from the time dependence of the amount of fine Fe2W at the time showing a minimum creep rate. As shown in Fig. 4, at high temperature, where the coarsening rate is large, the amount of fine Fe2W has a maximum at relatively short time.
3.3. Effect of Fe2W precipitation on the impro6ement of creep rupture time The time to rupture was found to depend on the acceleration of creep rate d ln m; /dm in the acceleration creep region, as well as the minimum creep rate m; min and is described by tr= 1.5/(m; min·d ln m; /dm)
(4)
for all the steels examined. As described previously, the precipitation of Fe2W effectively decreases the m; min but the large coarsening of Fe2W increases the d ln m; /dm. Therefore, the extension of creep rupture time by the precipitation of Fe2W is rather small. At 923 K, the large coarsening of Fe2W occurs at relatively short times, which causes the less pronounced decreases in minimum creep rate at low stresses and long times above 1000 h at 923 K (Fig. 3) and hence, causes the rapid decrease in creep rupture strength at long times (Fig. 1). The increase in time to rupture with increasing W concentration in the simple 9Cr – W steels is closely correlated with the reduced coarsening rate of M23C6 carbides, which effectively exert pinning force for coarsening lath [6,7]. On the other hand, the fine MC carbides are stable during creep and are effective for the stabilization of lath martensitic microstructure. A rapid decrease in creep rupture strength is not seen for the 9Cr –1WVTa steel even at 923 K. The decrease in creep rupture strength with time in Fig. 1 is less pronounced for the 9Cr–3WVTa steel at 923 K than for the 9Cr–
773
4W steel, indicating less pronounced microstructure evolution in the 9Cr–3WVTa steel than in the 9Cr– 4W steel. The stabilization of martensite lath by the fine MC carbides may compensate a loss of creep strength due to coarsening of Fe2W in the 9Cr –3WVTa steel.
4. Conclusion In the high-W steels, such as the 9Cr–2W, 9Cr –4W and 9Cr–3WVTa steels, the fine precipitation of Fe2W Laves phase occurs during creep and the decrease in creep rate with time becomes more significant at long times above : 20 h in the transient creep region at 923 K. This causes a decrease in minimum creep rate. The coarsening rate of Fe2W is much larger than that of M23C6 and MC carbides. The large coarsening of Fe2W promotes the acceleration of creep rate after reaching a minimum creep rate. As a result, the effect of Fe2W on the extension of creep rupture time is rather small. The increase in time to rupture of the simple 9Cr–W steels with increasing W concentration is closely correlated with the reduced coarsening rate of M23C6 carbides. The fine MC carbides are effective for the stabilization of lath martensitic microstructure during creep. The combination of maximized W of : 3% and fine MC carbides significantly improves the creep rupture strength of tempered martensitic 9Cr steel.
References [1] F. Masuyama, Proceedings of the Sixth Liege Conference on Materials for Advanced Power Engineering, Part III, Liege, Belgium, 1998, p. 1807. [2] D.H. Allen, J.E. Oakey, B. Scarlin, Proceedings of the Sixth Liege Conference on Materials for Advanced Power Engineering, Part III, Liege, Belgium, 1998, p. 1825. [3] F. Abe, M. Igarashi, N. Fujitsuna, K. Kimura, S. Muneki, Proceedings of the Sixth Liege Conference on Materials for Advanced Power Engineering, Part I, Liege, Belgium, 1998, p. 259. [4] F. Abe, S. Nakazawa, H. Araki, T. Noda, Met. Trans. 23A (1992) 469. [5] F. Abe, Mater. Sci. Eng. A234 – 236 (1997) 1045. [6] F. Abe, Proceedings of the Fourth International Conference on Recrystallization and Related Phenomena, Tsukuba, Japan, 1999, p. 289. [7] F. Abe, Proceedings of the Eighth International Conference on Creep and Fracture of Engineering Materials and Structures, Tsukuba, Japan, 1999, p. 395. [8] M. Prager, Press. Vessel Piping 288 (1994) 401.