Proceedings of the First International Conference on Self Healing Materials April 2007, Noordwijk aan Zee, The Netherlands N.Shinya et al.

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1 SELF HEALING OF CREEP DAMAGE THROUGH AUTONOMOUS BORON SEGREGATION AND BORON NITRIDE PRECIPITATION DURING HIGH TEMPERATURE USE OF AUSTENITIC STAINLESS STEELS N. Shinya 1*, J. Kyono 1, K. Laha 2 and C. Masuda 3 1 National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki , Japan 2 Indira Gandhi Centre for Atomic Research, Kalpakkam , Tamil Nadu, India 3 Waseda University, , Nishiwaseda, Shinjuku-ku, Tokyo , Japan *Phone: , Fax: , SHINYA.Norio@nims.go.jp Long time high temperature use of heat resisting steels leads to the premature and low ductility creep fracture by cavitation. The premature and low ductility creep fracture is caused by the nucleation, growth and coalescence of creep cavities on grain boundaries. The long time creep fracture properties of heat resisting steels depend mainly on the growth of creep cavities. As creep cavities are thought to grow by diffusive transport of matter from the cavity surface to the grain boundaries, the physical properties of the cavity surface are closely connected to the cavity growth. In this study, the chemical composition of austenitic stainless steels have been modified with addition of minute amount of elements such as boron and cerium with the aim to alter the physical properties of the cavity surface by segregation of elemental boron and precipitation of boron nitride onto the creep cavity surface. Surface chemistry of creep cavities in creep ruptured specimens was analyzed by Auger electron spectroscopy. On creep cavity surface in the usual austenitic stainless steels without boron and cerium, extensive sulfur segregation was observed. It is well known that the sulfur segregation enhances the creep cavity growth remarkably. In the modified steels, the segregation of boron and precipitation of boron nitride were observed on the creep cavity surfaces. The boron segregation and boron nitride precipitation were thought to alter the physical properties of the creep cavity surface and to suppress the surface diffusion of creep cavity, since boron and boron nitride are very stable at high temperatures, whereas the sulfur segregation accelerates the surface diffusion remarkably. Cerium acted as a getter for soluble sulfur in the steels by the precipitation of cerium-oxysulfide to facilitate the segregation of boron and precipitation of boron nitride. The segregation of boron and the precipitation of boron nitride reduced creep cavity growth rate substantially and improved long time creep rupture strength coupled with long time ductility. The boron segregation and boron nitride precipitation onto the creep cavity surface are autonomously developed during the high temperature use of the modified austenitic stainless steels, suppressing creep cavity growth. It was considered that the segregation and the precipitation provided the steels with the function of autonomous self-healing for the creep damage. Keywords: self-healing, creep damage, stainless steel, creep cavity, boron segregation, boron nitride precipitation, rupture properties 1 Springer 2007

2 1 Self-healing materials research in Japan In Japan the research works on self-healing materials are becoming active and have attracted considerable attention. The self-healing paint and the self-healing catalyst have been actually used for automobiles lately. Among the many research works of the self-healing materials, self-healing structural materials are especially expected to be developed and turned to practical use. Typical works on the autonomous self-healing structural materials 1-3) are shown in Table 1. All of the self-healing structural materials in Table 1 can heal the material damage of atomic to micro sizes without special treatments in the usual environments of the material use. Although it may take comparatively long time for the self-healing, the material performance can be recovered due to the healing of the damage. In this report, our research work on the self-healing heat resisting steels is introduced. Table 1 Self-healing materials research in Japan Material Material damage Healing environment Healing reaction Polymer 1) (PPE, PC) Scission of chain Room temperature Chains are recombined by eliminating two protons from the ends with change from Cu(II) to Cu(I) in the polymer Ceramic 2) (Si 3 N 4, Mullite) Surface micro cracks High temperature Oxidation of SiC particles at crack surface SiC+2O 2 SiO 2 +CO 2 SiO 2 heals surface cracks Metals 3) (321, 347 steels) Creep cavities High temperature B segregation and BN precipitation at creep cavity surface suppress cavity growth 2 Self-healing heat resisting steels research - creep cavities leading to creep fracture of heat resisting steels - Most of the heat resisting steels exhibit a premature and low ductility creep fracture at specific range of temperatures and stresses, most of which correspond to a long time rupture and actual service conditions. 4) The premature and low ductility creep fracture arises from the formation, growth and interlinking of cavities on grain boundaries. This phenomenon, called the creep cavitation, is of great technical importance because it often limits either the life or service conditions of structural components. Figure 1 shows the progress of creep cavitation during creep in a commercial type 304 stainless steel. The creep rupture life and the ductility of the steel working at the cavitation range depend on the creep cavity nucleation and growth rates. 2 Springer 2007

3 Figure 1: Progress of creep cavitation with increasing life fraction consumed for commercial austenitic stainless steel (SUS304H) at 750 and 37MPa Among the two processes of creep cavitation, cavity growth is the most important because the nucleation of creep cavities is believed to occur almost completely during the loading of creep specimens. Figure 2 shows the creep cavity growth mechanism, which is assumed to act in the usual cavitation range, especially at lower applied stresses and high temperature service. The cavity growth rate is expected to be controlled by the slower process of either grain boundary diffusion or the creep cavity surface diffusion. 5) Figure 2: Illustration of creep cavity growth mechanism wherein atomic transport occurs along the creep cavity surface and then down the grain boundary. 3 Concept of self-healing of creep cavities It is known that the creep cavity surface diffusion is influenced by the segregation of trace elements on its surface. Holt and Wallace 6) classified the most common trace elements according to whether they have detrimental or beneficial effects on the creep rupture strength. 3 Springer 2007

4 Among them, O and S can cause severe embrittlement during creep even at low ppm levels, whereas B and Zr can cause a beneficial effect. Some trace elements diffuse to grain boundary and creep cavity surface, and segregate there during high temperature services. It is known that the soluble S segregates on the creep cavity surface very easily, and the cavity growth rate increases with the increase in surface diffusion rate. 5) Because of the low melting point of S (112.8 C), the creep cavity surface contaminated with S becomes very active at high temperatures, and then the surface diffusion rate increases by several orders of magnitude. 7) When the soluble S is removed almost completely, other elements such as B and N can segregate onto the creep cavity surface. Segregation of B may suppresses the surface diffusion at creep cavity surface and retards the cavity growth rate because of high melting point of B (2080 C), and co-segregation of B and N may forms BN at creep cavity surface. As BN is very stable at high temperatures (melting point: 3000 C), the BN precipitation on the creep cavity surface is expected to suppress the surface diffusion completely and retard the cavity growth remarkably. The B segregation and the BN precipitation on the creep cavity surface occur continuously during high temperature use of the heat resisting steels and heal the creep cavity autonomously. Figure 3 shows the S segregation in usual steels, and B segregation and BN precipitation in self-healing heat resisting steels. Figure 3: Illustration of S an B segregation and BN precipitation on creep cavity surface 4 Springer 2007

5 4 Trial development of self-healing austenitic stainless steels 4.1 Self-healing of creep cavities by B segregation in modified 347 austenitic stainless steel Two 347 austenitic stainless steels of chemical compositions shown in Table 2 were melted in vacuum arc furnace. The chemical composition of the standard 347 austenitic stainless steel has been modified with the addition of 0.07 wt% of B and wt% of Ce. Cerium has a strong affinity to S and O, and removes them by formation of Ce 2 O 2 S. The steels were given a solution heat treatment at 1200 C for 20min, followed by water quenching before creep test. Table 2: Chemical composition of the melted 347 steels (wt%) Alloy C Si Mn P S Cr Ni Nb N B Ce B-free steel B-added steel Creep rupture tests were carried out at 750. The variation of creep rupture life and ductility of both steels with applied stress are shown in Figs. 4 and 5. Applied stress, MPa B-free steel B-added steel Rupture life, hours Figure 4: Variation of creep rupture life of the steels with applied stress, tested at Springer 2007

6 Addition of minute amount of B and Ce had remarkable effects on the creep rupture strength and ductility of the steel. Creep strength and ductility of the steel increased with addition of B and Ce, effects of which were more pronounced on longer creep exposure Elongation, % B-free steel 5 B-added steel Rupture Life, hours Figure 5: Variation of creep rupture elongation % with rupture life of the steels, tested at 750 Interrupted creep tests at 78MPa, 750 C were carried out on the steels in argon atmosphere to measure the growth rate of the cavities on the specimen surface by SEM. Figure 6 compares the growth rate of the creep cavities. Measurements were carried out on several cavities until they coalescence with each other. Boron addition in the steel decreased the cavity growth rate almost by an order of magnitude. Cavity growth rate, μm / hour 10-1 B-free steel B-added steel Time, hours Figure 6: Variation of cavity growth rate with creep exposure at 78MPa, 750 for both the steels The chemistry of the creep cavity surface in both the steels was examined by Auger electron spectroscopy (AES). The crept specimens of the steels were fractured by impact loading at liquid nitrogen temperature in the AES chamber to expose the creep cavity surface. Figures 7 and 8 shows the fracture surfaces with creep cavities on grain boundaries and the Auger spectra obtained from the cavity surfaces of B-free and B-added steels. 6 Springer 2007

7 Figure 7: SEM micrograph showing creep cavity surface, exposed by breaking at liquid nitrogen temperature under impact loading, of 347 stainless steel ruptured for 998h at 750 C and 69MPa, and Auger spectrum obtained from the creep cavity surface Figure 8: SEM micrograph showing creep cavity surface, exposed by breaking at liquid nitrogen temperature under impact loading, of Mod.347 stainless steel ruptured for 5461h at 750 C and 69MPa, and Auger spectrum obtained from the creep cavity surface. 7 Springer 2007

8 Presence of S segregation was observed on the cavity surface of B, Ce-free steel (Fig.7) whereas Auger peak of elemental B, instead of S, was observed in B, Ce-added steel. The soluble S in the B, Ce-added steel may be removed almost completely by formation of Ce 2 O 2 S. In B-added steel, in the absence of S contamination, most of the nucleated cavity surfaces were covered with a filmy segregation of elemental B. Boron segregation is expected to decrease the diffusivity along the cavity surface because of its relatively high melting point of around 2080 C. The suppression of creep cavitation in the B-added steel was observed, and this is thought to be caused by the segregation of B on the cavity surface. The suppression may have increased the creep strength associated with increase in creep ductility. As the segregation of B on creep cavity surface occurs autonomously during high temperature use of the steel, the segregation provides the steel with self-healing function for creep damage. 4.2 Self-healing of creep cavity by BN precipitation in modified 321 austenitic stainless steel A 321 austenitic stainless steel was modified with addition of 0.07 wt% of B, wt% of N and wt% of Ce. The results of creep rupture tests carried out at 750 C showed that the addition of minute amount of B and Ce increased creep rupture strength coupled with ductility, which was more pronounced on longer creep exposure. The creep cavities in the ruptured specimens of the modified 321 steel are few and fine, compared to those of the usual 304 and 321 austenitic stainless steels. The crept specimens were fractured by impact loading at liquid nitrogen temperature in the AES chamber to expose the creep cavity surface. Figure 9 shows the fracture surface of the modified 321 steel with creep cavities and the Auger spectrum obtained from the cavity surface. The presence of B and N on the cavity surface was observed, whereas the presence of S was not observed. The energy positions and the shapes of B and N peaks indicate that the segregated B and N form a stable compound of BN. X-ray diffraction pattern obtained from the precipitates of the modified 321 steel after creep rupture test showed the presences of Ti 4 C 2 S 2 and Ce 2 O 2 S. Figure 9: SEM micrograph showing creep cavity surface, exposed by breaking at liquid nitrogen temperature under impact loading, of Mod.321 steel crept for 10200h (t/tr=0.86) at 750 C 63MPa, and Auger spectrum obtained from the creep cavity surface 8 Springer 2007

9 The co-addition of Ce and Ti is highly effective to remove the soluble S in the steel and to prevent from its depositing on the cavity surface. In the absence of S segregation, the precipitation of BN film on the cavity surface decreases the cavity growth rate remarkably and provides the steel with the property of the self-healing effect of creep cavitation. The selfhealing of creep cavitation by BN precipitation increases the creep rupture strength and ductility of the steel, particularly for long rupture time region. REFERENCES (1) K. Takeda, M. Tanahashi and H. Unno, Science and Technology of Advanced Materials, 4(2003), p.435 (2) K. Ando, K. Houjyou, M.C. Chu, S. Kakeshita, K. Takahashi, S. Sakamoto and S. Sato, Journal of the European Ceramic Society, 22(2002), p.1339 (3) N. Shinya, J. Kyono and K. Laha, Journal of Intelligent Material Systems and Structures, 17(2006), p (4) N. Shinya, J. Kyono and H. Kushima, ISIJ International, 46(2006), p (5) W.D. Nix, K.S. Yu and J.S. Wang, Metall. Trans. A, 14A(1983), p.563 (6) R.T. Holt and W. Wallace, Int. Met. Rev., 21(1976), p.1 (7) G.E. Rhead, Surf. Sci. 47(1975), p Springer 2007