Microstructural Evolution in a 9%Cr-3%Co-3%W-VNb Steel during Creep

Abstract:

Microstructural evolution in a 9Cr-3Co-3W-0.2V-0.06Nb-0.05N-0.005B steel crept at T=650°C under an applied stress of 140 MPa up to strains of 1, 3, 5.75 and 12%, which represent primary, secondary and tertiary creep stages and rupture, respectively, was studied. The steel was initially normalized from 1050°C, and finally tempered at 750°C for 3h. After tempering the boundaries of tempered martensite lath structure (TMLS) were decorated by M₂₃C₆ carbides, M₆C carbides and Laves phase particles. The 3% W additives provide the narrow size distribution of the boundary particles excepting M₆C carbides. The depletion of thermodynamically none-equilibrium content of W from the solid solution during creep leads to following events. (i) Continuous precipitation of small Laves phase particles occurs during all creep stages and results in the formation of bimodal size distribution. As a result, the average size of Laves phase particles remains unchanged during creep. (ii) Coarsening of M₂₃C₆ carbides starts to occur only at the transition to tertiary creep. (iii) Transformation of laths to subgrains followed by their growth is observed during all stages of creep. The density of particle located at lath/subgrain boundaries decreases from 5.6 to 2.6 μm^-1 during creep up to rupture. However, no full transformation of TMLS into subgrain structure has been revealed.

• Periodical: Materials Science Forum (Volume 879)
• Pages: 548-553
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.879.548
• Online since: November 2016
• Authors: Alexandra Fedoseeva*, Nadezhda Dudova, Rustam Kaibyshev
• Keywords: High-Chromium Martensitic Steels, Laves Phase, M₂₃C₆ Carbide, Particle Density, Tempered Martensite Lath Structure

References

[1] F. Abe, T.U. Kern, R. Viswanathan, Creep resistant steels, Woodhead Publishing in Materials, Cambridge, England, (2008).

[2] T. -U. Kern, M. Staubli, B. Scarlin, The European Efforts in Material Development for 650°C USC Power Plants—COST522, ISIJ International 42 (2002) 1515-1519.

DOI: 10.2355/isijinternational.42.1515

[3] A. Kipelova, M. Odnobokova, A. Belyakov, R. Kaibyshev, Effect of Co on Creep Behavior of a P911 Steel, Metall. Mater. Trans. A 44A (2013) 577-583.

DOI: 10.1007/s11661-012-1390-3

[4] N. Dudova, A. Plotnikova, D. Molodov, A. Belyakov, R. Kaibyshev, Structural changes of tempered martensitic 9%Cr–2%W–3%Co steel during creep at 650°C, Mat. Sci. Eng. A 534 (2012) 632-639.

DOI: 10.1016/j.msea.2011.12.020

[5] F. Abe, Analysis of creep rates of tempered martensitic 9%Cr steel based on microstructure evolution, Mater. Sci. Eng. A 510–511 (2009) 64–69.

DOI: 10.1016/j.msea.2008.04.118

[6] K. Hashimoto, M. Yamanaka, Y. Otugara, T. Zaizen, M. Onoyama, T. Fujita, Proc. In: Topical Conf. Ferritic Alloys for Use in Nuclear Energy Technologies, ed. by J.W. Davis and D.J. Michel, The Metallurgical Society of AIME, Snowbird, Utah (1983).

[7] F. Abe, H. Araki, T. Noda, The effect of tungsten on dislocation recovery and precipitation behavior of low-activation martensitic 9Cr steels, Met. Trans. A 22A (1991) 2225- 2236.

DOI: 10.1007/bf02664988

[8] Y. Tsuchida, K. Okamoto, Improvement of creep rupture strength of high Cr ferritic steel by addition of W, ISIJ International 35 No. 3 (1995) 317-323.

DOI: 10.2355/isijinternational.35.317

[9] V. Knezevic, J. Balun, G. Sauthoff, G. Inden, A. Schneider, Martensitic/ferritic super heat-resistant 650°C steels - Design and testing of model alloys, Mat. Sci. and Eng. A 477 (2008) 334–343.

DOI: 10.1016/j.msea.2007.05.047

[10] R.L. Klueh, A.T. Nelson, Ferritic/martensitic steels for next-generation reactors, Journal of Nuclear Materials 371 (2007) 37–52.

DOI: 10.1016/j.jnucmat.2007.05.005

[11] A. Fedoseeva, N. Dudova, R. Kaibyshev, Creep strength breakdown and microstructure evolution in a 3%Co modified P92 steel, Mat. Sci. Eng. A 654 (2016) 1-12.

DOI: 10.1016/j.msea.2015.12.027