Evolution of Nonlinear Acoustics during Creep in ASME Grade 91 Steel Welded Joint

ASME Grade 91 (9Cr-1Mo-MnVNb) has been used for boiler components in ultra-supercritical (USC) thermal power plants at approximately 873 K. The creep life of the welded joints in this steel decreased as a result of Type IV creep damage that forms in the heat-affected zone (HAZ) under long-term use at high temperatures [1]. In this study, we investigated the relationship between microstructural change and the evolutions of two nonlinear acoustic characterizations with electromagnetic acoustic resonance (EMAR) [2] throughout the creep life in the welded joints and the correlation between two nonlinear acoustic characterizations. One was resonant frequency shift [3] and other three-wave mixing [4, 5]. EMAR was a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT) [2]. We used a bulk-wave EMAT, which transmits and receives shear wave propagating in thickness direction of a plate specimen. Creep tests of thick welded joints specimens were interrupted at several time steps at 873 K and 90 MPa. Two nonlinear acoustic parameters and ultrasonic attenuation decreased from the start to 20% of creep life. They gradually increased from 50% of creep life to rupture. We interpreted these phenomena in terms of dislocation recovery, recrystallization, and restructuring related to the initiation and growth of creep void, with support from the SEM and TEM observation. This noncontact resonance-EMAT measurement can monitor the evolution of nonlinear acoustics throughout the creep life and has a potential to assess the Type IV creep damage advance and to predict the creep life of high Cr ferritic heat resisting steels

Evaluation of creep strength reduction factors for welded joints of Grade 122 steel

ABSTRACT HCM12A (ASME Grade 122) is used for boiler components in thermal power plants because of its high creep strength. However, type IV creep damage formed in heat affected zone brings about considerable decrease in creep strength of the weldment and a failure of large diameter piping in a thermal power plant due to this damage took place recently. In order to update the design method and develop life estimation method for this kind of piping system with axial weld, creep rupture data of base metal and welded joints has been collected and analyzed in the SHC (Strength of HighChromium Steel) committee in Japan since 2004. In the present paper, the creep rupture data of over 400 points for welded joint specimens of HCM12A offered from six Japanese organizations were analyzed. These data clearly indicated that the long-term creep strength of welded joints becomes weaker than that of base metal at above 600C due to Type IV fracture in fine grain heat-affected zone. After discussing the effects of product form, welding procedure and specimen sampling etc. on the creep strength, the master creep life equation for the welded joints was developed. So-called region decomposition technique was adopted to fit the data both in high and low stress regimes with a reasonable accuracy. The creep strength reduction factor obtained from 100,000 hours creep strength of welded joints and base metal was given as a function of temperature