Study of Creep Strength for P91 Steels after Short Term Overheating above Ac₃ Temperature

Abstract:

Advanced ferritic steels containing 9 wt% Cr are widely used in the construction of supercritical and ultra supercritical boiler components. The microstructure of the as supplied 91 materials consists of a tempered martensite matrix, a fine dispersion of intergranular chromium rich M₂₃C₆ precipitates and intragranular carbonitrides MX particles rich in V and Nb. This steel requires post weld heat treatment (PWHT) to produce a tempered microstructure after welding to develop excellent creep strength for high temperature service. Based on past experience, situations may arise whereby the components are subjected to an accidental overshoot in temperature during PWHT. The short excursion to high temperature beyond Ac₃ would have resulted in the formation of deleterious phases, for example, soft α-ferrite which has poor creep strength and hard martensite which has a low toughness. In this study, the degraded specimens with soft α ferrite as a result of cooling transformation from 900°C are proven to have a limited creep rupture life where the creep rupture strength dropped remarkably after 1000 hours. As the peak temperature increased to 950°C and 1000°C, hard and brittle martensite was formed on cooling. The creep specimens were found to exhibit better creep strength; most probably the creep behavior was improved by the tempering effect at 600°C during creep tests. Nevertheless, despite the tempering which might have improved the toughness slightly, the high temperature creep rupture stress still had dropped approximately 40%, as compared to the virgin alloys in the range of rupture time from 1,000 hours to 10,000 hours.

• Periodical: Applied Mechanics and Materials (Volume 393)
• Pages: 94-101
• DOI: https://doi.org/10.4028/www.scientific.net/AMM.393.94
• Online since: September 2013
• Authors: Ng Guat Peng, Badrol Ahmad, Mohd Razali Muhamad, M. Ahadlin
• Keywords: Ac₁, Ac₃, Creep Strength, Martensitic, Microstructure, PWHT

References

[1] B.K. Choudhary, E. Issac Samuel. Creep Behavior of Modified 9Cr-1Mo Ferritic Steel. Journal of Nuclear Materials. 412 (2011). Pp 82-89.

DOI: 10.1016/j.jnucmat.2011.02.024

[2] S. Spigarelli, E. Quadrini. Analysis of the Creep Bahaviour of Modified P91 (9Cr-1Mo-NbV) weld. Materials and Design 23 (2002). pp.547-552.

DOI: 10.1016/s0261-3069(02)00026-2

[3] Nippon Steel Corporation, Data Package for NF616 Ferritic Steel, January 1993, second edition, March (1994).

[4] A. Czyrska-Filemonowicz, A. Zielinska_Lipiec, P.J. Ennis. Modified 9% Cr Steels for advanced power generation: microstructure and properties. Journal of achievements in materials and manufacturing engineering, Vol 19, Issue 2, Dec (2006).

[5] R.P. Chen, H. Ghassemi Armaki, K. Maruyama, Y. Minami, M. Igarashi. Microstructure Degradation during High Temperature Exposure Up to 105 H and Its Effects on Creep of Gr. 91 Steel. Proceedings from the sixth international conference on Advances In Materials Technology for Fossil Power Plants. August 31-September 3, 2010, Sante Fe, New Mexico, USA. Copyright @2011 Electric Power Research Institute, distributed by ASM International. pp.654-665.

DOI: 10.1016/j.msea.2011.02.060

[6] Alexandrov B., Wang L., Siefert J., Tatman J., Lippold J. Phase Transformations In Creep Strength Enhanced Ferritic Steel Welds.

[7] J. Siefert, J. Sanders, J. Tanzosh, B. Alexandrov, J. Lippold, An Update of Phase Transformation During PWHT of Grade 91. Materials Science and Technology (MS & T) 2009, October 25-29, 2009, Pittsburgh, Pennsylvania. Joining of Advanced and Specialty Materials 2009 ( JASM XI).

[8] J Hald, L Korcakova. Precipitate Stability in Creep Resistant Ferritic Steels – Experimental Investigation and Modeling. ISIJ International, Vol 43 (2003), No 3, pp.420-427.

DOI: 10.2355/isijinternational.43.420

[9] R.C. MacLachlan, J.J. Sanchez-Hanton and R.C. Thomson. The effect of simulated post weld heat treatment temperature overshoot on micro structural evolution in P91 and P92 power plant steels. Proceedings from the sixth international conference on Advances In Materials Technology for Fossil Power Plants. August 31-September 3, 2010, Sante Fe, New Mexico, USA. Copyright @2011 Electric Power Research Institute, distributed by ASM International. pp.787-799.

[10] I. a. Shibli, S. r. Holdsworth, G. Merckling. Creep and fracture in high temperature components: Design and life assessment issues. Proceeding from the ECCC Creep Conference, September 12-14, 2005, London, U. K.

[11] K. Haarmann, J.C. Vaillant, B. Vandenbergha. The T91/P91 Book, VALLOUREC & MANNESMANN TUBES, Mannesmann Research Center & Vallourec Research Center, 2002 (2nd edition).

[12] Kazuhiro Kimura, Kota Sawada, Hidenki Kushima, Creep Rupture Ductility of Creep Strength Enhanced Ferritic Steels. Journal of Pressure Vessel Technology, Volume 134, Issue 3, Materials and Fabrication. 031403 (May 18, 2012) (7 pages) doi: 10. 1115/1. 4005876.

DOI: 10.1115/1.4005876