Welding Metallurgy of

Fossil fuel will continue to be the major source of energy for the foreseeable future To meet the demand for clean and affordable energy, an increase in the operating efficiency of fossil fired power plants is necessary There are several initiatives worldwide to achieve efficiencies >45% higher heating value (HHV) through an increase in steam temperature (700 to 760°C) and pressure (276 to 345 MPa) Realising this goal requires materials with excellent creep rupture properties and corrosion resistance at elevated temperatures In order to accomplish this, three classes of materials have been identified: creep strength enhanced ferritic steels, austenitic stainless steels and nickel base superalloys Although new alloys have been designed and developed to meet this need, welding can have a significant and often detrimental effect on the required mechanical and corrosion resistant properties Two previous papers addressed the welding and weldability of ferritic and austenitic stainless steels W

Weldability and weld performance of candidate nickel base superalloys for advanced ultrasupercritical fossil power plants part I: fundamentals

Transition of creep damage region in dissimilar welds between Inconel 740H Ni-based superalloy and P92 ferritic/martensitic steel

Hot deformation characteristics and processing map analysis of a new designed nickel-based alloy for 700 °C A-USC power plant

The hot deformation behavior of a new-designed nickel-based alloy with different initial states were investigated by isothermal compression at temperature range of 1000–1200 °C with strain rate of 0.01–20 s −1 and nominal strain of 15–70% on a Gleeble-1500 thermo-mechanical simulator. The correlation of dynamic recrystallization (DRX) mechanism and initial alloy states as well as deformation parameters were analyzed in detail by means of transmission electron microscope (TEM) and electron backscattered diffraction (EBSD). For homogenized alloy with extremely large grains, the DRX mechanism transforms partly from discontinuous dynamic recrystallization (DDRX) characterized with serrated and bulging grain boundary into continuous dynamic recrystallization (CDRX) operated by progressive sub-grain merging and rotation with the increasing strain rate and decreasing deformation temperature, which results from the poor deformation compatibility in the alloy with extremely large grains. However, the dominant DRX mechanism in as-forged alloy is DDRX almost at all the deformation conditions, and CDRX is only an assistant mechanism, which caused by the initial inhomogeneous dislocation distribution and fine grains. In consequence, the initial alloy states have a significant influence on the DRX, which cannot be ignored in the hot deformation research.

Dynamic recrystallization of a new nickel-based alloy for 700 °C A-USC power plant applications with different initial states: as-homogenized and as-forged

Electron microscopy techniques have been used to investigate the cause of premature creep failure in the fusion zone of INCONEL® Alloy 740H® (INCONEL and 740H are registered trademarks of Special Metals Corporation) welds. The reduced creep rupture lives of all-weld-metal and cross-weld creep specimens (relative to base metal specimens) have been attributed to the presence of large grain boundary regions that were denuded in fine γ′ but contained coarse, elongated particles. Investigation of creep rupture specimens has revealed four factors that influence the formation of these coarsened zones, and the large particles found within them have been identified as γ′. Comparisons of the microstructural characteristics of these zones to the characteristics that are typical of denuded zones formed by a variety of mechanisms identified in the literature have been made. It is concluded that the mechanism of γ′-denuded zone formation in alloy 740H is discontinuous coarsening of the γ′ phase. The discontinuous reaction is catalyzed by the grain boundary migration and sliding which occur during creep and likely promoted by the inhomogeneous weld metal microstructure that results from solute segregation during solidification. The increased susceptibility to the formation of the observed γ′-denuded zones in the weld metal as compared to the base metal is discussed in the context of differences in the contributions to the driving force for the discontinuous coarsening reaction.

Microstructural Evolution of INCONEL® Alloy 740H® Fusion Welds During Creep

Thermodynamic and kinetic modeling were used to determine appropriate heat treatment schedules for homogenization and second phase dissolution in INCONEL® alloy 740H® (INCONEL and 740H are registered trademarks of Special Metals Corporation) fusion welds. Following these simulations, a two-step heat treatment process was applied to specimens from a single pass gas tungsten arc weld (GTAW). Scanning electron microscopy (SEM) has been used to assess the changes in the distribution of alloying elements as well as changes in the fraction of second phase particles within the fusion zone. Experimental results demonstrate that adequate homogenization of alloy 740H weld metal can be achieved by a 1373 K/4 h (1100 °C/4 h) treatment. Complete dissolution of second phase particles could not be completely achieved, even at exposure to temperatures near the alloy’s solidus temperature. These results are in good agreement with thermodynamic and kinetic predictions.

Homogenization and Dissolution Kinetics of Fusion Welds in INCONEL® Alloy 740H®

Characterization of the microstructural evolution of fusion welds in alloys slated for use in advanced ultrasupercritical (A-USC) boilers during creep has been performed. Creep-rupture specimens involving INCONEL® 740, NIMONIC® 263 (INCONEL and NIMONIC are registered trademarks of Special Metals Corporation), and Haynes® 282® (Haynes and 282 are registered trademarks of Haynes International) have been analyzed via light optical microscopy, scanning electron microscopy, X-ray diffraction, and thermodynamic and kinetic modeling. Focus has been given to the microstructures that develop along the grain boundaries in these alloys during creep at temperatures relevant to the A-USC process cycle, and particular attention has been paid to any evidence of the formation of local γ′-denuded or γ′-free zones. This work has been performed in an effort to understand the microstructural changes that lead to a weld strength reduction factor (WSRF) in these alloys as compared to solution annealed and aged alloy 740 base metal. γ′ precipitate-free zones have been identified in alloy 740 base metal, solution annealed alloy 740 weld metal, and alloy 263 weld metal after creep. Their development during long-term thermal exposure is correlated with the stabilization of phases that are rich in γ′-forming elements (e.g., η and G) and is suppressed by precipitation of phases that do not contain the γ′ formers (e.g., M23C6 and μ). The location of failure and creep performance in terms of rupture life and WSRF for each welded joint is presented and discussed.

Microstructural Evolution and Creep-Rupture Behavior of A-USC Alloy Fusion Welds

Microstructural Evolution and Creep Rupture Behavior of INCONEL® Alloy 740H® Fusion Welds

Characterization of γ′ coarsened zones (CZs) in alloy 740H fusion welds via a variety of electron microscopy techniques was conducted. The effects of solute partitioning during nonequilibrium solidification on the amount of strengthening precipitates along the grain boundaries were evaluated via electron-probe microanalysis and scanning electron microscopy. Electron backscatter diffraction was used to present evidence for the preferential growth of CZs toward regions of lower γ′ content, even if growth in that direction increases grain boundary area. Scanning electron microscopy and image analysis were used to quantify the propensity for CZs to develop along certain segments of the grain boundaries, as governed by the local variations in γ′ content. Scanning transmission electron microscopy with X-ray energy-dispersive spectrometry (XEDS) was used to assess the compositions of the matrix and precipitate phases within the CZs and to quantify the segregation of alloying components to the reaction front. Thermodynamic and kinetic modeling were used to compare calculated and experimental compositions. The work presented here provides new insight into the progression of the discontinuous coarsening (DC) reaction in a complex engineering alloy.

Daniel H. Bechetti

Papers

Microstructural Evolution of INCONEL® Alloy 740H® Fusion Welds During Creep

Homogenization and Dissolution Kinetics of Fusion Welds in INCONEL® Alloy 740H®

Microstructural Evolution and Creep-Rupture Behavior of A-USC Alloy Fusion Welds

Microstructural Evolution and Creep Rupture Behavior of INCONEL® Alloy 740H® Fusion Welds

Characterization of Discontinuous Coarsening Reaction Products in INCONEL® Alloy 740H® Fusion Welds