Session: CS-01-02/03-01 Structural Integrity of Pressure Components & Environmental Fatigue Issues (Joint M&F, D&A)

Paper Number: 60160

Start Time: Tuesday, July 13, 2021, 09:00 AM

60160 - Steam Generator Grade P91 Steel Components Creep-Assessment By Test After Extended Service 

      Previous study carried out creep analysis for steam generator high-temperature-section two components, outflow tubing and manifold of the superheater harp: they may have been critical because of the long continued service (109000 hours or twelve years) and loading conditions, including maximum operation temperature (565°C) and applied stress (65 MPa). Metallographic methods by replica had showed no evidence of the creep cavitation in all the positions considered for both tubing and manifold. In particular, they had not found any cavitation or phases affecting creep strength of the material in the base, HAZ and weld metal microstructure. Analysis by TEM had detected precipitation particles as Laves phases, inclusions as M₂₃C₆ carbides and secondary phases as MX: their size and frequency have been reference for the present study. Any growth of antagonist phases such as Z-phase, supposed detrimental for both creep-strengthening and ductility, had not yet showed up. Hardness values measured were similar to those declared by the steel maker. Thus, material appeared still without signs of relevant microstructure degradation. Long creep lives obtained through the analysis were congruent with test results confirming the possibility of life extension. Study considered operation temperature below 600°C as the relevant factor for the worked-case components’ safety. Creep results’ reliability primarily depends on the constitutive law used and whether another might be more adequate for the P91’ microstructure evolution. In the previous study, the lower operation temperature with material's good conditions justified assumption of secondary creep-regime with a Norton power-law in the analysis. That way, life results obtained through the API 579-1 Level 3 assessment had appeared consistent with those predicted by the Italian creep code and ECCC recommendations: the latter base both on master-curve method application. Preliminary stress analysis had been by pressure formula (elastic, inelastic in the creep-redistribution steady-state regime). The critical zones considered had been the ligament between openings on the cylinder wall and outflow tubing/manifold intersection (tee weld). Now, present study carries out investigation for the two components based on the next plant outage outcome, after further 25000-hours service. Analysis would use different creep law, depending also on the microstructure changes detected; it should include the weld, provided creep strain and strain rate data. Both metallographic methods and hardness measurements' results would compare with previous ones providing microstructure evolution in the period. 

Presenting Author: Ottaviano Grisolia INAIL, Central Research Directorate, Department of Technology

Authors:

Ottaviano Grisolia INAIL, Central Research Directorate, Department of Technology
Lorenzo Scano Studio Scano Associato, Safety & Integrity
Francesco Piccini Studio Scano Associato, Safety & Integrity
Antonietta Lo Conte Politecnico di Milano, Dipartimento di Meccanica
Massimiliano De Agostinis Università di Bologna, DIN
Stefano Fini Università di Bologna, DIN