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Ferritic-martensitic steels constitute a key class of materials for power generation due to their creep-resistant properties. The current study examines the creep and creep-fatigue properties of two ferritic-martensitic steels, 22V (21⁄4Cr-1MoV) and P91 (9Cr-1MoV). These steels are commonly used in boiler and piping applications at elevated temperatures. While base metal properties are important and are investigated in the present work, component failure most commonly occurs in the welded region of components in-service. The majority of the present work consists of studies on welded ferritic-martensitic components, examining the effect of weldment processing on microstructure and elevated temperature performance. First, creep-fatigue properties of P91 base metal are examined. In service, P91 components are subjected to elevated temperatures and cyclic stresses, leading to the accumulation of creep-fatigue damage. This study examines the creep-fatigue behavior of P91, including mechanical response under various loading conditions at 600°C and 650°C. Microstructural studies utilize techniques including: scanning electron microscopy, (SEM) scanning transmission electron microscopy (STEM), electron backscattered diffraction (EBSD) and transmission Kikuchi diffraction (TKD). Quantitative microstructural studies track substructure coarsening and dislocation density as a function of creep-fatigue deformation. Significant anelastic backflow is observed at minimum load during every creep-fatigue test conducted. The effects of loading parameters on creep-fatigue rupture life and anelastic backflow are also studied. The differences between monotonic creep and creep-fatigue, which lead to accelerated failure under creep-fatigue deformation are examined. Creep-fatigue properties of P91 weldments are also assessed by studying a conventional flux-cored arc welding process (FCAW) as well as a non-conventional cold metal transfer (CMT) welding process. Specimens from each weldment were deformed using a purpose-built, load-controlled creep-fatigue testing apparatus under multiple loading conditions at 650°C. Ruptured weldments are examined using characterization techniques including SEM and STEM diffraction-contrast imaging (STEM-DCI). Specimens welded using the CMT process significantly outperform the FCAW weldments. Further characterization reveals that changes in precipitate size and distribution as well as differences in subgrain size and dislocation density between the two welding processes result in the differences in creep-fatigue strength. Concerning 22V, a systematic comparison is performed to determine the effect of welding polarity and post-weld heat treatment (PWHT) conditions on weld metal microstructure. DC+, AC 50% balance and AC 75% balance waveforms are used to weld 22V submerged arc weldments (SAW). All weldments are PWHT at either 1275°F (690°C) or 1310°F (710°C). The present work examines grain size, subgrain and dislocation content, and second phase distribution as a function of welding polarity and PWHT conditions using SEM, STEM, EBSD, optical microscopy (OM) and energy dispersive spectroscopy (EDS). The most critical microstructural difference is a change in precipitate distribution as a function of weld processing parameters. AC 50% weldments produce a refined distribution of MX carbonitrides compared to the DC+ weldments. Also, MX carbonitrides in the DC+ weldments nucleate preferentially on high-angle grain boundaries which reduce the creep strengthening effects of the MX phase. The change in precipitate distribution is attributed to differences in heat distribution during welding as a function of welding polarity. In addition, the effect of welding polarity and PWHT conditions on mechanical properties of 22V SAW are also studied. AC 50% weldments with a refined intragranular MX carbonitride distribution exhibit a stable substructure and the best creep performance. In addition, the AC 50% weldments exhibit the highest fracture toughness when PWHT at 1310°F. Ductile fracture modes include microvoid coalescence, with microvoids nucleating at Cr-Mo rich carbides. Based on the current work, the most favorable SAW parameters for 22V include welding with AC 50% polarity and PWHT at 1310°F, which produces favorable fracture toughness while retaining excellent creep strength.
