Analysis of the creep behaviour of modified P91 (9Cr–1Mo–NbV) welds.pdf

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Analysis of the creep behaviour of modified P91 (9Cr–1Mo–NbV) welds.pdf

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Analysis of the creep behaviour of modified P91 (9Cr–1Mo–NbV) welds

Materials and Design 23 (2002) 547–552 0261-3069/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S0261-3069 ?02 .00026-2 Analysis of the creep behaviour of modified P91 (9Cr–1Mo–NbV) welds S. Spigarelli*, E. Quadrini INFMyDepartment of Mechanics, University of Ancona, I-60131, Via Brecce Bianche I 60131, Ancona, Italy Received 26 February 2002; accepted 29 April 2002 Abstract The creep response at 600 8C of a P91 (9Cr–1Mo–NbV) steel welded joint was investigated by testing small samples from different weld locations. The microstructure of the weld was analysed by light and transmission electron microscopy. The heat- affected zone material exhibiting a fine-grained structure showed the highest minimum creep-rate values, i.e. the lowest creep strength. The coarse-grained weld metal generally had the greatest creep strength, whereas the parent metal exhibited an intermediate behaviour. The minimum creep-rate dependence on applied stress for weld metal, fine-grained heat-affected zone, and base material was described by means of the conventional power-law equation with a stress exponent close to 15. The experimental results, both in the form of strain vs. time creep curve and of minimum creep rate, were successfully used to simulate the creep response of cross-weld samples. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Creep; Weld; Steel 1. Introduction The modified 9Cr–1Mo steel (PyT91) has found several applications in power-generation plants. The creep strength of this steel, and of its more recent W- bearing developments, derives mainly from the disper- sion of M C and fine Nb and V carbonitrides (MX) 23 6 that precipitate during tempering treatment andyor creep w1–19x. Several studies have investigated the microstruc- ture of these steels and its evolution during creep exposure. These materials exhibit a fully martensitic structure, with a dispersion of M C precipitated during 23 6 tempering on the boundaries of martensi