Creep Property of Boron Added 9Cr Heat Resistant Steels after Welding

Abstract:

Although welding results in premature failure by type IV fracture under high temperature creep conditions, the alloy design of light elements such as boron addition and nitrogen reduction enhances the creep lifetime of 9Cr heat resistant steel. In particular, the simulated heat affected zone (SHAZ) sample of new 9Cr steel (called TA steel) shows about 10 times longer creep lifetime than that of the standard Gr. 91 steel. The welded TA steel is thus expected to exhibit good creep properties because its SHAZ sample has coarser grains and suppresses type IV fracture. The preservation of base metal’s microstructure after welding results from the precipitate morphology, such as high grain boundary coverage by precipitates and low amount of MX being nucleation sites of ferrite grains during the a-g phase transformation. In addition, the increase of stability of M₂₃C₆ affects high pinning pressure toward grain boundary migration upon rapid heating during welding. First-principles calculations confirm the increased stability when boron is absorbed by M₂₃C₆. Moreover, the calculations reveals that boron decreases the coherency between matrix and M₂₃C₆, suppressing grain coarsening during creep tests in TA steel. It is concluded that the increased microstructural stability during welding and long high temperature exposure generates the elongated creep lifetime in welded TA steel including about 0.01 wt% boron and less than 0.01 wt% nitrogen.

• Periodical: Materials Science Forum (Volume 941)
• Pages: 340-345
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.941.340
• Online since: December 2018
• Authors: Tetsuya Matsunaga*, Maaouia Souissi, Ryoji Sahara, Hiromichi Hongo, Masaaki Tabuchi, Wei Zhang, Michael J. Mills
• Keywords: Boron, Creep, First-Principles Calculation, Heat Resistant Steel, Weld

References

[1] M. Lenzen, Life cycle energy and greenhouse gas emissions of nuclear energy: A review, Energy Conv. Manage. 49 (2008) 2177-2199.

DOI: 10.1016/j.enconman.2008.01.033

[2] H. Kimura, T. Sato, C. Bergins, S. Imano, E. Saito, Development of technologies for improving efficiency of large coal-fired thermal power plants, Hitachi Review 60 (2011) 365-371.

[3] F. Masuyama, Creep degradation in welds of Mod. 9Cr-1Mo steel, Int. J. Press. Vessels Pip. 83 (2006) 819-825.

DOI: 10.1016/j.ijpvp.2006.08.010

[4] M. Matsui, M. Tabuchi, T. Watanabe, K. Kubo, J. Kinugawa, F. Abe, Degradation of creep strength in welded joint of 9%Cr steel, ISIJ Int. 41 (2001) S126-S130.

DOI: 10.2355/isijinternational.41.suppl_s126

[5] T. Matsunaga, H. Hongo, M. Tabuchi, R. Sahara, Suppression of grain refinement in heat-　affected zone of 9Cr-3W-3Co-VNb steels, Mater. Sci. Eng. A, 655 (2016) 168-174.

DOI: 10.1016/j.msea.2015.12.095

[6] T. Matsunaga, H. Hongo, M. Tabuchi, M. Souissi, R. Sahara, H.C. Whitt, T.K. Payton, W. Zhang, M.J. Mills, Submitted.

[7] I. Fedorova, A. Kostka, E. Tkachev, A. Belyakov, R. Kaibyshev, Tempering behavior of a low nitrogen boron-added 9%Cr steel, Mater. Sci. Eng. A. 662 (2016) 443-455.

DOI: 10.1016/j.msea.2016.03.092

[8] P.E. Blochl, Projector augmented-wave method, Phys. Rev. B. 50 (1994) 17953-17979.

DOI: 10.1103/physrevb.50.17953

[9] G. Kresse, D. Joubert, From ultrasoft pseudopotentials to the projector augmented-wave method, Phys. Rev. B. 59 (1999) 1758-1775.

DOI: 10.1103/physrevb.59.1758

[10] G. Kresse, J. Hafner, Ab initio molecular dynamics for liquid metals, Phys. Rev. B. 47 (1993) 558-561.

DOI: 10.1103/physrevb.47.558

[11] G. Kresse, J. Furthmuller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B. 54 (1996) 11169-11186.

DOI: 10.1103/physrevb.54.11169

[12] R. Sahara, T. Matsunaga, H. Hongo, M. Tabuchi, Theoretical investigation of stabilizing mechanism by boron in body-centered cubic iron through (Fe, Cr)23(C, B)6 precipitates, Metall. Mater. Trans. A. 47 (2016) 2487-2497.

DOI: 10.1007/s11661-016-3397-7

[13] C.M. Fang, M.H.F. Sluiter, M.A. van Huis, C.K. Ande, H.W. Zandbergen, Origin of Predominance of Cementite among Iron Carbides in Steel at Elevated Temperature, Phys. Rev. Lett. 105 (2010) 055503.

DOI: 10.1103/physrevlett.105.055503

[14] C.M. Fang, M.A. van Huis, M.H.F. Sluiter, H.W. Zandbergen, Stability, structure and electronic properties of γ-Fe23C6 from first-principles theory, Acta Mater. 58 (2010) 2968-2977.

DOI: 10.1016/j.actamat.2010.01.025

[15] N.I. Medvedeva, D.C. Van Aken, J.E. Medvedeva, Stability of binary and ternary M23C6 carbides from first principles, Comput. Mater. Sci. 96 (2015) 159-164.

DOI: 10.1016/j.commatsci.2014.09.016

[16] C.S. Smith, Grains, phases, and interfaces–an interpretation of microstructure, Trans. Met. Soc. AIME. 175 (1948) 15-51.

[17] F. Ishikawa, T. Takahashi, T. Ochi, Intragranular ferrite nucleation in medium-carbon vanadium steels, Metall. Mater. Trans. A. 25 (1994) 929-936.

DOI: 10.1007/bf02652268