Metallographic Atlas for 2.25Cr-1Mo Steels and Degradation due to Long-term Service at the Elevated Temperatures

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1 ECCC Creep Conference, September 2005, London. Metallographic Atlas for 2.25Cr-1Mo Steels and Degradation due to Long-term Service at the Elevated Temperatures Hideaki Kushima, Takashi Watanabe, Masaharu Murata, Kazushige Kamihira, Hideo Tanaka and Kazuhiro Kimura, National Institute for Materials Science (NIMS) Abstract In a series of Metallographic Atlas of Long-term Crept Materials as a part of NIMS Creep Data Sheet, new volume of No.M-4 on 2.25Cr-1Mo steels have been published in March Microstructural evolution during long-term creep up to about,000h has been described on three type steels with ferrite/pearlite, bainite and tempered martensite microstructures in the as heat treated condition. Higher short-term creep strength of the steel with tempered martensite microstructure than those of the other steels decreased with increase in creep exposure, and no difference was observed in the long-term as a result of microstructural change. Keywords: creep; 2.25Cr-1Mo steel; metallographic atlas; long-term creep strength; initial microstructure; microstructural change. 1. Introduction The 2.25Cr-1Mo steel has been widely used for high temperature structural components serviced at temperatures of 400 to 600 o C. Long-term service at the elevated temperature causes deterioration of mechanical property of materials as a result of changes in microstructure. The loss of strength should be evaluated accurately in order to assure security and reliability for long-term serviced high temperature plants. Mechanical property is strongly influenced by microstructure and consequently microstructural changes during aging and creep exposure has been reported on 2.25Cr-1Mo steels in association with strength [1-5]. Mechanical property and microstructure of the long-term serviced materials taken from pressure vessels has been also investigated [6,7]. Database on microstructural evolution during long-term exposure at the elevated temperatures should be useful information for risk management of high temperature plant. National Institute for Materials Science (NIMS) has been conducting Creep Data Sheet Project since 1966, and a large amount of creep data and crept test pieces have been accumulated. Recently, NIMS has started to publish Metallographic Atlas of Long-term Crept Materials which contains a number of micrographs on crept materials. The first volume of it published in 1999 was on a 18Cr-8Ni steel (JIS SUS304H TB) [8]. Those of a 18Cr-12Ni-Mo steel (JIS SUS 316H TB) [9] and a 18Cr-10Ni-Ti steel (JIS SUS321H TB) [10] were published in 2003 and 2004, respectively. In a series of NIMS Metallographic Atlas, new volume of No.M-4 on three type 2.25Cr-1Mo steels [11-13] with different initial microstructure has been published in March The Metallographic Atlas of 2.25Cr-1Mo steels contains a number of optical micrographs showing the changes in microstructure during long-term creep exposure beyond,000h at the temperatures of 450 to 650 o C, in addition to creep rupture data and hardness. In this paper, the outline of Metallographic Atlas of 2.25Cr-1Mo steels is described and influence of initial microstructure and microstructural change during creep exposure on creep rupture strength and rupture ductility have been mentioned. 223

2 H.Kushima, T.Watanabe, M.Murata, K.Kamihira, H.Tanaka and K.Kimura Table 1. Chemical compositions (mass%) and heat treatment conditions of three 2.25Cr-1Mo steels C Si Mn P S Ni Cr Mo Cu Al N Ann. (JIS STBA 24) NT (JIS SCMV 4NT) QT (ASTM A542) Ann. (JIS STBA 24) 930ºC /20min 720ºC /130min Air cooling 930ºC /1h Air cooling NT 740ºC /2h Air cooling (JIS SCMV 4NT) 700ºC /4h Furnace cooling 930ºC /6h Water quenching QT 635ºC /6h Air cooling (ASTM A542) 600ºC /2h Air cooling a b c a) Ann. (JIS STBA 24) b) NT (JIS SCV 4NT) c) QT (ASTM A542) Fig. 1. Optical micrographs of the as heat treated steels 20µm 2. Metallographic Atlas of 2.25Cr-1Mo steels 2.1 Materials The 2.25Cr-1Mo steels with different initial microstructure of ferrite/pearlite, bainite and tempered martensite were used for the Metallogaphic Atlas No. M-4. Creep rupture data of those steels has been reported in the Creep Data Sheets No.3B [11], No.11B [12] and No.36B[13], respectively. A typical heat with standard creep strength has been selected from multi heats of each material. The chemical compositions and heat treatment conditions of the selected 2.25Cr-1Mo steels are shown in Table 1. Chemical composition of the three steels was essentially the same, however, heat treatment condition was different each other. Optical micrographs of the annealed 2.25Cr-1Mo steel tube (JIS STBA24), normalized and tempered (NT) steel plate (JIS SCMV4 NT), and quenched and tempered (QT) steel plate (ASTM A542) are shown in Figure 1. Microstructure of the steels in the as heat treated condition was ferrite/pearlite (Fig.1a), bainite (Fig.1b) and tempered martensite (Fig.1c) for annealed steel tube, normalized and tempered steel plate, quenched and tempered steel plate, respectively. 2.2 Contents of Metallographic Atlas The Metallographic Atlas of 2.25Cr-1Mo steels contains relating creep rupture data, profile of creep ruptured specimens, optical and transmission electron micrographs and hardness. Creep ruptured pecimens tested over the range of temperatures from 450 to 650 o C were examined for each steels. 224

3 Metallographic Atlas of 2.25Cr-1Mo Steels and Degradation due to Creep 500 Ann. (JIS STBA 24) Stress / MPa o C 475 o C 500 o C 525 o C 550 o C 600 o C 450 o C 500 o C 550 o C 600 o C 650 o C n = o C Time to rupture / h Fig.2. Stress versus time to rupture curves of the 2.25Cr-1Mo steel tube (JIS STBA 24) tested over a range of temperatures from 450 to 650 o C Optical micrographs of near fractured surface, gauge portion and head portion of the creep ruptured specimen are displayed with a magnification of, 400 and 1,000. In addition to exhibit of those for every crept specimen, micrographs are summarized as a view format for different temperature and creep exposure time. Transmission electron micrographs of the creep ruptured specimens crept at 550 o C are indicated. Vickers hardness of the steels in the as heat treated condition, gauge and head portion of the creep ruptured specimens is also shown. Stress versus time to rupture curves of the 2.25Cr-1Mo steel tube (JIS STBA 24) tested over a range of temperatures from 450 to 650 o C are shown in Figure 2. Creep rupture data of the crept specimens subjected to microstructural examination for the Metallographic Atlas was indicated by solid symbol in Fig.2. The crept specimens tested over a range of rupture life from to exceed,000h have been used. Creep testing condition of the NT and QT steels is essentially the same as that of annealed one. Number of the specimens examined was 28 pieces for annealed steel, 27 pieces for NT and QT steels, respectively. Profiles of creep ruptured specimen crept at 500, 550 and 650 o C of QT steel are shown in Figure 3. High ductility with necking is observed on the specimen creep ruptured at 500 o C and short-term (Fig.3 (a)). However, a profile of the specimen creep ruptured at 500 o C and long-term (Fig.3 (b)) indicates a low ductility. Similar feature of low ductility was observed on the specimens creep ruptured at 550 o C (Figs.3 (c) and (d)). Creep rupture ductility tends to decrease with increase in creep exposure. On the other hand, high ductility is observed on creep ruptured specimen crept at 650 o C (Fig3 (e)). It has been observed that creep rupture ductility decreases with increase in creep exposure, however, it recover after a sufficient extent of creep exposure. In this way, rupture ductility is understood from the profile of creep ruptured specimen, as well as an extent of oxidation. 225

4 H.Kushima, T.Watanabe, M.Murata, K.Kamihira, H.Tanaka and K.Kimura a) 500ºC, 300MPa, t R =552.2 h b) 500ºC, 157MPa, t R =124,452.1 h c) 550ºC, 216MPa, t R =956.5 h d) 550ºC, 78MPa, t R =66,055.2 h e) 650ºC, 53MPa, t R =978.5 h 10mm Figure 3. Profiles of creep ruptured specimen crept at 500, 550 and 650ºC of QT steel (ASTM A542). Optical micrographs of the specimen gauge portion of annealed 2.25Cr-1Mo steel (JIS STBA24) crept for about 300 to,000h at temperatures of 450 to 650 o C are shown in Figure 4. A lamellar morphology of pearlite is observed on the specimens creep ruptured at 450 o C through short-term to long-term, however, it has collapsed after long-term creep exposure beyond 10,000h at 500 o C. With increase in temperature, collapse of lamellar morphology of pearlite takes place at the shorter creep exposure time. Influences of time and temperature on evolution of microstructure during creep exposure are systematically demonstrated. Similar view format is displayed on not only specimen gauge portion, but also specimen head portion and near fracture surface for each 2.25Cr-1Mo steels. Moreover, optical micrographs of the same condition of the three steels are also summarized in a similar way for comparison. 3. Creep rupture properties and microstructural change 3.1 Creep rupture strength Stress versus time to rupture curves at 450, 550 and 650 o C of the three 2.25Cr-1Mo steels are shown in Figure 5. At the lower temperature of 450 o C, creep rupture strength of the QT steel is higher than the other steels and it shows about ten times longer creep rupture life than the others. Short-term creep rupture strength at 550 o C of the QT steel was also higher than the other steels, however, difference in creep rupture strength decreased with decrease in applied stress and disappeared at the stresses below MPa. No difference in creep rupture strength was observed at 650 o C for the three steels. 226

5 Metallographic Atlas of 2.25Cr-1Mo Steels and Degradation due to Creep Fig.4. Optical micrographs of the specimen gauge portion of annealed 2.25Cr-1Mo steel (JIS STBA 24) crept for about 300 to,000h at temperatures of 450 to 650 o C. 500 Stress / MPa o C 550 o C 40 Ann. (JIS STBA 24) NT (JIS SCMV 4NT) QT (ASTM A542) 650 o C Time to rupture / h Fig.5. Stress versus time to rupture curves at 450, 550 and 650 o C of the three 2.25Cr-1Mo steels. 227

6 H.Kushima, T.Watanabe, M.Murata, K.Kamihira, H.Tanaka and K.Kimura Cr-1Mo steels Stress / MPa Ann. (JIS STBA 24) NT (JIS SCMV 4NT) QT (ASTM A542) Larson-Miller Parameter (T (K) (20 + log t R(h) )) Fig.6. Creep data of the 2.25Cr-1Mo steels at temperatures of 450 to 650 o C plotted against a Larson-Miller parameter. Creep data of the 2.25Cr-1Mo steels at temperatures of 450 to 650 o C are plotted against a Larson- Miller parameter (LMP, C=20) and shown in Figure 6. In the small parameter region corresponding to lower temperature and short-term, creep rupture strength of the QT steel with tempered martensite microstructure is higher than the other steels with ferrite/pearlite and bainite microstructures. However, the curve of QT steel is steeper than the other steels in the range of LMP of 19,000 to 20,000, and a magnitude of difference in creep rupture strength of the steels decreases. In the large LMP range exceed 20,000, difference in creep rupture strength disappeared and those of the three 2.25Cr-1Mo steels are almost the same. Vickers hardness of gauge portion of the creep ruptured 2.25Cr-1Mo steels is plotted against a LMP and shown in Figure 7. In the as heat treated condition, hardness of the QT steel is HV239 and that is much greater than those of the NT steel (HV179) and annealed steel (HV152). Specimens of NT and annealed steels creep ruptured in the short-term indicate larger hardness than that in the as heat treated condition, however, it decreases monotonously with increase in LMP. On the other hand, QT steel holds higher hardness than the other steels in the short-term up to about 19,000 of LMP. A large hardness value of the QT steel, however, rapidly drop in the range of LMP of 19,000 to 20,000, as well as decrease in creep rupture strength of the QT steel as shown in Fig.6. In the long-term where a LMP exceeds 20,000, hardness of the three 2.25Cr-1Mo steels are almost the same. It has been supposed that change in creep rupture strength with increase in LMP is caused by microstructural change during long-term creep exposure. 3.2 Creep rupture ductility Changes in reduction of area with increase in creep exposure time at 500, 550, 600 and 650 o C of the three 2.25Cr-1Mo steels are shown in Figure 8. The NT and annealed steels possess high ductility independent of temperature and creep exposure time. On the other hand, rupture ductility of the QT steel is strongly 228

7 Metallographic Atlas of 2.25Cr-1Mo Steels and Degradation due to Creep influenced by creep exposure condition. In the short-term at 500 o C, the QT steel holds high ductility as well as the other steels, however, it rapidly drop in excess of 1,000h. A similar drop in rupture ductility is observed also at 550 o C, however, it tends to recover in the long-term over about 10,000h. Recovery of rupture ductility with increase in creep exposure time is observed at 600 o C, and high ductility is observed at 650 o C independent of creep exposure time. Vickers hardness (HV10) Ÿ As received Ann. (JIS STBA 24) NT (JIS SCMV 4NT) QT (ASTM A542) Gauge portion Larson-Miller Parameter (T (K) (20 + log t R(h) )) Fig.7. Vickers hardness of gauge portion of the creep ruptured 2.25Cr-1Mo steels plotted against a Larson-Miller parameter. Reduction of area / % o C 600 o C 550 o C 650 o C Ann. (JIS STBA 24) NT (JIS SCMV 4NT) QT (ASTM A542) Time to rupture / h Fig.8. Changes in reduction of area with increase in creep exposure time at 500, 550, 600 and 650 o C of the three 2.25Cr-1Mo steels. 229

8 H.Kushima, T.Watanabe, M.Murata, K.Kamihira, H.Tanaka and K.Kimura 300 h level h level h level 000 h level 500ºC 550ºC 600ºC 650ºC Á Á 550 µ µm m h (78MPa) h (37MPa) h (160MPa) h (78MPa) h (41MPa) h (250MPa) h (177MPa) h (108MPa) h (78MPa) h (333MPa) h (294MPa) h (216MPa) h (157MPa) Fig.9. Optical micrographs of the near fractured surface of the QT steel (ASTM A542). Stress Optical micrographs of the near fractured surface of the QT steel are shown in Figure 9. A transgranular fracture is observed on the specimens creep ruptured after 438.1h at 500 o C, however, fracture mode shifts from transgranular to intergranular with increase creep exposure time. An intergranular fracture is observed on the specimens creep ruptured at 550 and 600 o C in conjunction with intergranular cracking. On the other hand, a transgranular fracture is observed on the specimens creep ruptured at 650 o C, and change in rupture ductility is clearly recognized from these micrographs. The reduction of area of the three 2.25Cr-1Mo steels shown in Fig.8 is plotted against a LMP (C=20) and shown in Figure 10. The NT and annealed steels hold good rupture ductility which is higher than 70% of reduction of area throughout the tested condition. Although a rupture ductility of the QT steel is high in the short-term region where a LMP is smaller than 18,000, it rapidly decreases from about 70% to less than 10% with increase in creep exposure time and recovers in the long-term after showing significantly low ductility in the range of a LMP of 18,000 to 20,000. A poor ductility of the QT steel in the intermediate-term is correspondent to significant drop in hardness and creep rupture strength. 3.3 Microstructural change Bright field TEM images of the QT steel (a) in the as heat treated condition and specimens creep ruptured at 550 o C after (b) 99.8h at 265MPa, (c) 8,203.0h at 137MPa and (d) 46,816.8h at 88MPa are shown in Figure 11. Tempered martensitic microstructure with fine lath and high dislocation density was observed in the as tempered condition (Fig.11(a)). Homogeneously recovered microstructure consists of increased lath width and decreased dislocation density has been observed in the specimen creep ruptured after 99.8h (Fig.11(b)). On the other hand, inhomogeneous progress in recovery was found in the specimen creep ruptured after 8,203.0h (Fig.11(c)). In the specimen creep ruptured after 46,816.8h (Fig.11(d)), martensitic lath structure 230

9 Metallographic Atlas of 2.25Cr-1Mo Steels and Degradation due to Creep has completely disappeared and dislocation density has significantly decreased. Reduction of area / % Ÿ Ann.(JIS STBA 24) NT(JIS SCMV 4NT) QT(ASTM A542) Larson-Miller Parameter (T (K) (20 + log t R(h) )) Fig.10. Reduction of area of the three 2.25Cr-1Mo steels shown in Fig.8 plotted against a Larson-Miller parameter. a b c d a) as heat treated b) t R =99.8h, El=22%, RA=68% c) t R =8,203.0h, El=3%, RA=4% d) t R =46,816.8h, El=14%, RA=37% Fig.11. Bright field TEM images of the QT steel (ASTM A542) (a) in the as heat treated condition and specimen creep ruptured at 550 o C after (b) 99.8h at 265MPa, (c) 8,203.0h at 137MPa and (d) 46,816.8h at 88MPa. 231

10 H.Kushima, T.Watanabe, M.Murata, K.Kamihira, H.Tanaka and K.Kimura Fig.12. Bright field TEM images of the 2.25Cr-1Mo steels in the as heat treated and creep ruptured at 550 o C. Bright field TEM images of the 2.25Cr-1Mo steels in the as heat treated and creep ruptured at 550 o C are shown in Figure 12. A dislocation density of the annealed steel is significantly low even in the as heat treated condition and changes in microstructure during creep exposure is small. Although high dislocation density is observed for the NT steel in the as heat treated condition, it has rapidly decreased during short-term creep exposure. On the other hand, decrease in dislocation density in the QT steel during creep exposure is slower than that in the NT steel. Inhomogeneous progress in recovery of tempered martensite microstructure is observed with expanding of low dislocation density area from the vicinity of prior austenite grain boundary towards grain interior. After long-term creep exposure of several tens of thousands hours at 550 o C, tempered martensite microstructure has been completely recovered and no significant difference in microstructures is observed for the three 2.25Cr-1Mo steels regardless of different initial microstructure. Rapid decreases in hardness and creep rupture strength of the QT steel correspond to expansion of recovered area. Fine martensite microstructure and high dislocation density of the QT steel is maintained for longer duration than the NT steel, and higher hardness and creep rupture strength of the QT steel in the short-term originates in such stable microstructure. However, hardness and creep strength rapidly decrease with extending of soft recovered region and differences in hardness, creep strength and microstructure disappear in the long-term of a LMP exceeds about 20,000. Common creep rupture strength in the long-term regardless of initial microstructure is thought to be governed by the inherent creep strength of the 2.25Cr-1Mo steel [14-16]. Corresponding to extending of soft recovered region, significant drop in rupture ductility was observed in the QT steel, and it recovered in the long-term where a tempered martensite microstructure has been completely recovered. It has been concluded that, consequently, significant decrease in rupture ductility is caused because creep deformation is concentrated in the soft recovered region. Rupture ductility increased in the long-term, since soft recovered region covered whole grain and creep deformation took place throughout the material. 232

11 Metallographic Atlas of 2.25Cr-1Mo Steels and Degradation due to Creep 4. Conclusions The outline of the Metallographic Atlas of 2.25Cr-1Mo steels was described and influence of initial microstructure and microstructural change during creep exposure on creep rupture strength and rupture ductility were mentioned. 1. The NIMS Metallographic Atlas of 2.25Cr-1Mo steels (No.M-4) published in March 2005, contains information about microstructural evolution during long-term creep exposure of the annealed, normalized/tempered and quenched/tempered steels with ferrite/pearlite, bainite and tempered martensite microstructures, respectively. A number of optical micrographs of the steels in the as heat treated condition and creep ruptured at the temperature of 450 to 650 o C are displayed in conjunction with relating creep rupture data, profile of crept specimens, bright field TEM images and hardness data. 2. Creep rupture strength and hardness of the quenched/tempered steel was higher than the other annealed and normalized/tempered steels in the short-term, however, differences of the three steels disappeared after rapid drop in strength and hardness of the quenched/tempered steel. No difference in creep strength and hardness was observed in the long-term. 3. Large drops in creep strength and hardness of the quenched/tempered steel originated in an extending of soft recovered area from the vicinity of prior austenite grain boundary towards grain interior. Although high rupture ductility of the annealed and normalized/tempered steels are maintained independent of creep testing condition, that of the quenched/tempered steel shows rapid drop corresponding to extension of soft region and recovery in the long-term. 4. It has been concluded that significant decrease in rupture ductility of the quenched/tempered steel is caused by concentration of creep deformation in the soft recovered region, and recovery of ductility is attained in the long-term since soft recovered region covered whole grain and creep deformation can take place throughout the material. References 1. Murphy M.C. and Branch G.D. Metallurgical changes in 2.25CrMosteels during creep-rupture test. Journal of The Iron and Steel Institute. 1971; 209: Lonsdale D. and Flewitt P.E.J. Damage accumulation and microstructural changes occurring during the creep of a 21/4%Cr1%Mo steel. Materials Science and Engineering. 1979; 39: Abdel-Latif A.M., Corbett J.M. and Taplin D.M.R. Analysis of carbides formed during accelerated aging of 2.25Cr-1Mo steel. Metal Science. 1982; 16: Thomson R.C. and Bhadeshia H.K.D.H. Changes in chemical composition of carbides in 2.25Cr-1Mo power plant steel Part 1 Bainitic microstructure. Materials Science and Technology. 1994; 10: Thomson R.C. and Bhadeshia H.K.D.H. Changes in chemical composition of carbides in 2.25Cr-1Mo power plant steel Part 2 Mixed microstructure. Materials Science and Technology. 1994; 10: Wada T. and Biss V.A. Restoration of elevated temperature tensile strength in 2.25Cr-1Mo steel. Metallurgical Transactions A. 1983; 14A: Nishizawa Y., Hara Y., Hori A., Tsukahara H., Miyano K., Wada T. and Cox T.B. Changes in microstructure and mechanical properties of Cr-Mo reactor vessel steels during long term service. Proc. Pressure Vessels and Piping 1985; 98(1):

12 H.Kushima, T.Watanabe, M.Murata, K.Kamihira, H.Tanaka and K.Kimura 8. NRIM Creep Data Sheet. National Research Institute for Metals, No.M-1, NIMS Creep Data Sheet. National Institute for Materials Science, No.M-2, NIMS Creep Data Sheet. National Institute for Materials Science, No.M-3, NRIM Creep Data Shee. National Research Institute for Metals, No.3B, NRIM Creep Data Sheet. National Research Institute for Metals, No.11B, NIMS Creep Data Sheet. National Institute for Materials Science, No.36B, Kimura K., Kushima H., Yagi K. and Tanaka C. Fundamental properties of long-term creep strength for ferritic heat resistant steels. Tetsu-to-Hagané. 1991; 77(5): Kimura K., Kushima H., Yagi K. and Tanaka C. Effects of minor alloying elements on inherent creep strength properties of ferritic steels. Tetsu-to-Hagané. 1995; 81(7): Kimura K., Kushima H., Abe F. and Yagi K. Inherent creep strength and long term creep strength properties of ferritic steels. Materials Science and Engineering. 1997; A :