Session: CS-01-01 Structural Integrity of Pressure Components

Paper Number: 102473

102473 - Steam-Generator Grade P91 Steel Component Weld Creep-Assessment 

Previous study carried out creep assessment 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 (109,000 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 cavitations in all the positions considered for both tubing and manifold. In particular, they had not found any cavitations or phases affecting creep strength of the material in the base, heat affected zone and weld metal microstructure. Observation through the transmission-electronic microscope 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 following study. Any growth of antagonist phases such as Z-phase, supposed detrimental for both strengthening and ductility, had not yet showed up. Hardness values measured were similar to those declared by the steel maker. Therefore, material appeared still without signs of relevant microstructure degradation. In the analysis, lower operation temperature with material’s good conditions had justified assumption of secondary creep-regime with a Norton power-law. 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 the outflow-tube / manifold intersection. Life results through API 579-1 Level 3, FEM stress analysis and those predicted by Italian creep code and ECCC recommendations had been consistent: the latter base both on master-curve method application, stress analysis by formula. Long creep lives obtained had been congruent with test results confirming the possibility of life extension. Second study had included in-situ repetition of the additional controls previously carried out at 109,000 hours. Infield test-results comparison allowed characterizing microstructure evolution after 20,000 hours of extended service. Precipitation particles’ size and frequency observed through energy dispersive X-ray spectrometry were still within range of acceptable variation; antagonist phases for creep strengthening and ductility were missing. Operation temperature showed to be the leading factor than pressure, which for the worked case is high enough. Below 600°C, safety beyond 100,000 hours should be until appearance of the cavitations, which may represent the break point for the secondary regime. Study should have included creep simulation for the tertiary regime. Material's good conditions found out allowed excluding the circumstance with reasonable confidence. Now, present study extends the analysis to the weld: To carry out the creep simulation requires extraction of constituent law's coefficients for the P91 weld metal. Analytical methods base on creep strain and strain rate data. As an alternative to their planned experimental derivation from an ex-service welded-material sample, study benefits from literature availability. Specifically, data come from rupture creep-tests carried out by Idaho University in 2009. They were on cross-weld P91 specimens at 600°C and 650°C; the weld sample consisted of as-fabricated P91 two plates welded by a V-butt joint. Coherently, instead of the outflow-tube / manifold tee-weld, study has considered the butt joint weld on manifold and the outflow tube. It adopted this choice also for economy of the simulation, weld creep damage calculated through both Larson-Miller theory and Monkman-Grant relation. LMP-stress data from API-579-1 base-metal minimum-LMP and from Idaho-University weld-metal show good consistency. For the manifold’s weld, API 579-1 and FEM stress analysis have predicted expended life percentage of 24%. For the outflow tube’s weld, master curve method and manual stress analysis have predicted similar expended life percentage. Thus, creep behavior of weld metal appears not worsening strength of the joints and components as a whole. It confirms the cavitations’ absence be prerequisite for P91’s integrity beyond 100,000 hours (operation temperatures below 600°C). For the worked case, this means safe operation until the next plant outage (159,000 hours).

Presenting Author: Ottaviano Grisolia INAIL, Central Research Dir., Technology Dept.

Presenting Author Biography: Dr. Ottaviano Grisolia is a senior researcher of the technology department at the INAIL seat in Rome, Italy. The department is responsible for ensuring that pressure vessels design meets Italian safety codes. Dr. Grisolia is at INAIL since 2008 and earlier at the former certification agency ISPESL: he is currently involved with R & D activities supporting standards’ development and life extension assessments for high-temperature components operating in Italy. In particular, he is carrying out structural analysis programs and creep analysis based on experimental data published in a scientific journal. Wrote a PC program for the application of the Italian creep code, master curve method and a manual procedure of static, flexibility analysis, internal piping. INAIL published his final report on residual life technologies. Since 2008 annually, Dr. Grisolia participates as lead author/presenter in the ASME PVP conference, codes & standards’ section, high temperature and components’ integrity. Before joining ISPESL in 1994, Dr. Grisolia has been a technician employed at different Italian manufacturing companies in the aerospace field: the Commercial Aircraft Group of Alenia was among the others. As stress and weight engineer, he was involved with carrying out structural analysis programs on remote piloted and commercial and space vehicles. As lead structural designer, he participated in the Get Away Special program of European Space Agency for the space shuttle mission flown in 1993. He won postgraduate scholarship developing a static analysis PC program for commercial aircraft seats. In 1983, Dr. Grisolia received the master degree in aeronautical engineering and the professional qualification from State University of Naples, Italy.

Authors:

Ottaviano Grisolia INAIL, Central Research Dir., Technology Dept.
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