No abstract

No abstract

No abstract

Duplex stainless steels are desirable for use in power generation systems due to their attractive combination of strength, corrosion resistance, and cost. However, thermal embrittlement at intermediate homologous temperatures of ~475°C and below, limits upper service temperatures for many applications. New lean grade duplex alloys have improved thermal stability over standard grades and potentially increase the upper service temperature or the lifetime at a given temperature for this class of material. The present work compares the thermal stability of lean grade, alloy 2003 to standard grade, alloy 2205, through a series of isothermal agings between 260°C and 482°C for times between 1 and 10,000 hours. Aged samples were characterized by changes in microhardness and impact toughness. Additionally, atom probe tomography was performed to illustrate the evolution of the α-α′ phase separation in both alloys at select conditions. Atom probe tomography confirmed that phase separation occurs via spinodal decomposition for both alloys and identified the presence of Ni-Cu-Si-Mn-P clusters in alloy 2205 that may contribute to embrittlement of this alloy. The impact toughness model predictions for upper service temperature show that alloy 2003 may be viable for use in 288°C applications for 80 year service lifetimes based on a Charpy V-notch criteria of 47 J at room temperature. In comparison, Alloy 2205 should be limited to 260°C applications for the same room temperature toughness of 47 J. © 2015. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ 2 Keywords: duplex stainless steels, 475°C embrittlement, spinodal decomposition, atom probe tomography 1.0 Introduction Duplex stainless steels (DSS) comprise a unique class of materials that possess desirable properties of both the face-centered cubic (austenitic) and body-centered cubic (ferritic) phases within their microstructures. The ferrite and austenite phases are present in roughly equal volume fractions,typically ranging from 30-70% ferrite. Relative to their austenitic counterparts, DSS tend to have higher strength, higher toughness, improved corrosion resistance (especially to localized corrosion), and exceptional resistance to halide stress corrosion cracking [1,2]. Additionally, their relatively low nickel content lowers the cost of these alloys and helps ensure price stability.DSS are widely used in chemical processing, desalination, pulp and paper, storage, and transportation industries due to their high strength and good corrosion resistance [1]. Components commonly manufactured from DSS include storage tanks, pipes, pressure vessels, heat exchangers, seawater systems, rotors, and structural members. However, DSS have had little application in power generation industries, in part due to concerns with thermal embrittlement. The thermal embrittlement that limits broader applications of DSS generally occurs at temperatures between 204°C and 538°C with a peak embrittlement rate near 475°C. ...

Three-dimensional chemical imaging of Fe-Cr alloys showing Fe-rich (α)/Cr-rich (α') phase separation is reported using atom probe tomography techniques. The extent of phase separation, i.e., amplitude and wavelength, has been quantitatively assessed using the Langer-Bar-on-Miller, proximity histogram, and autocorrelation function methods for two separate Fe-Cr alloys, designated 2101 and 2205. Although the 2101 alloy possesses a larger wavelength and amplitude after annealing at 427 °C for 100-10 000 h, it exhibits a lower hardness than the 2205 alloy. In addition to this phase separation, ultra-fine Ni-Mn-Si-Cu-rich G-phase precipitates form at the α/α' interfaces in both alloys. For the 2101 alloy, Cu clusters act to form a nucleus, around which a Ni-Mn-Si shell develops during the precipitation process. For the 2205 alloy, the Ni and Cu atoms enrich simultaneously and no core-shell chemical distribution was found. This segregation phenomenon may arise from the exact Ni/Cu ratio inside the ferrite. After annealing for 10 000 h, the number density of the G-phase within the 2205 alloy was found to be roughly one order of magnitude higher than in the 2101 alloy. The G-phase precipitates have an additional deleterious effect on the thermal embrittlement, as evaluated by the Ashby-Orowan equation, which explains the discrepancy between the hardness and the rate of phase separation with respect to annealing time (Gladman T 1999 Mater. Sci. Tech. Ser. 15 30-36).

No abstract

Mechanical property degradation due to the disorder to order phase transformation is of potential concern for alloys based on the Ni-Cr binary system, particularly in nuclear power applications where component lifetimes can exceed 80 years. In the present research, the disorderorder phase transformation has been studied in the Ni-33 at% Cr model alloy by a combined experimental and computational approach. The multiscale modeling framework utilizes grand canonical and kinetic Monte Carlo simulation techniques based upon density functional theory calculations to treat both the thermodynamic and kinetic aspects of the phase transformation. The simulation results are used to generate a simple model for the ordering kinetics based upon the Kolmogorov-Johnson-Mehl-Avrami equation. Experimental measurements of the change in lattice parameter as a function of aging time and temperature are obtained in order to assess the model accuracy. The resulting model shows reasonable agreement with experimental data at 470 and 418 °C, however additional experimental data at longer aging times are needed to confirm the accuracy of the model at lower temperatures. The model predicts that the initiation of the ordering transformation will occur in Ni-33Cr at temperatures and timescales relevant to nuclear power systems, though longer times are required for the transformation to proceed to completion.

No abstract