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Title: Microstructure and High Temperature Oxidation Behavior of Cr-W Alloys

Abstract

Cr alloys containing 0-30%W by weight were investigated for use in elevated temperature applications. The alloys were melted in a water-cooled, copper-hearth arc furnace. Microstructure of the alloys was characterized using x-ray diffraction, scanning electron microscopy, and light microscopy. A pseudocyclic oxidation test was employed to study scale formation at 1000ºC in dry air. The scale was predominantly chromia and spalled upon cooling. Alloying with aluminum up to 8 weight percent reduced the spalling drastically. Furthermore, aluminizing the surface of the Cr-W alloys completely stopped the spalling.

Authors:
Publication Date:
Research Org.:
National Energy Technology Laboratory (NETL), Pittsburgh, PA, and Morgantown, WV
Sponsoring Org.:
USDOE - Office of Fossil Energy (FE)
OSTI Identifier:
912976
Report Number(s):
DOE/NETL-IR-2007-080
TRN: US200802%%502
DOE Contract Number:
None cited
Resource Type:
Conference
Resource Relation:
Conference: Refractory Metals 2007, a symposium at the 136th TMS Annual Meeting, Orlando, FL, Feb. 25-Mar. 1, 2007
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ALLOYS; ALUMINIUM; ARC FURNACES; MICROSCOPY; MICROSTRUCTURE; OXIDATION; REFRACTORY METALS; SCANNING ELECTRON MICROSCOPY; X-RAY DIFFRACTION

Citation Formats

Dogan, O.N. Microstructure and High Temperature Oxidation Behavior of Cr-W Alloys. United States: N. p., 2007. Web.
Dogan, O.N. Microstructure and High Temperature Oxidation Behavior of Cr-W Alloys. United States.
Dogan, O.N. Thu . "Microstructure and High Temperature Oxidation Behavior of Cr-W Alloys". United States. doi:. https://www.osti.gov/servlets/purl/912976.
@article{osti_912976,
title = {Microstructure and High Temperature Oxidation Behavior of Cr-W Alloys},
author = {Dogan, O.N.},
abstractNote = {Cr alloys containing 0-30%W by weight were investigated for use in elevated temperature applications. The alloys were melted in a water-cooled, copper-hearth arc furnace. Microstructure of the alloys was characterized using x-ray diffraction, scanning electron microscopy, and light microscopy. A pseudocyclic oxidation test was employed to study scale formation at 1000ºC in dry air. The scale was predominantly chromia and spalled upon cooling. Alloying with aluminum up to 8 weight percent reduced the spalling drastically. Furthermore, aluminizing the surface of the Cr-W alloys completely stopped the spalling.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Thu Feb 01 00:00:00 EST 2007},
month = {Thu Feb 01 00:00:00 EST 2007}
}

Conference:
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  • The oxidation behavior of Cr(X) solid solution (Cr{sub ss}) and Cr{sub 2}X Laves phases (X = Nb, Ta) was studied individually and in combination at 950--1,100 C in air. The Cr{sub ss} phase was significantly more oxidation resistant than the Cr{sub 2}X Laves phase. At 950 C, two-phase alloys of Cr-Cr{sub 2}Nb and Cr-Cr{sub 2}Ta exhibited in-situ internal oxidation, in which remnants of the Cr{sub 2}X Laves phase were incorporated into a growing chromia scale. At 1,100 C, the Cr-Cr{sub 2}Nb alloys continued to exhibit in-situ internal oxidation, which resulted in extensive O/N penetration into the alloy ahead of themore » alloy-scale interface and catastrophic failure during cyclic oxidation. IN contrast, the Cr-Cr{sub 2}Ta alloys exhibited a transition to selective Cr oxidation and the formation of a continuous chromia scale. The oxidation mechanism is interpreted in terms of multiphase oxidation theory.« less
  • The high-temperature oxidation of Cr-Cr{sub 2}Nb alloys is being studied as part of a program to develop ductile, corrosion-resistant versions of this very-high-strength material. The kinetics, scale development, and spallation tendencies of Cr {minus}6 and {minus}12 at. % Nb alloys oxidized in air at 900 and 950{degrees}C were characterized. The alloy with the higher niobium content performed significantly better in terms of overall weight gain and resistance to spallation. This behavior is ascribed to an increased volume fraction of the Cr{sub 2}Nb-Cr eutectic mixture at the expense of the Cr-rich phase. The study suggested some general approaches to improving themore » oxidation resistance of Cr-Nb alloys.« less
  • The high-temperature oxidation of Cr-Cr{sub 2}Nb alloys is being studied as part of a program to develop ductile, corrosion-resistant versions of this very-high-strength material. The kinetics, scale development, and spallation tendencies of Cr {minus}6 and {minus}12 at. % Nb alloys oxidized in air at 900 and 950{degrees}C were characterized. The alloy with the higher niobium content performed significantly better in terms of overall weight gain and resistance to spallation. This behavior is ascribed to an increased volume fraction of the Cr{sub 2}Nb-Cr eutectic mixture at the expense of the Cr-rich phase. The study suggested some general approaches to improving themore » oxidation resistance of Cr-Nb alloys.« less
  • Search for new high temperature materials for energy applications continues. This presentation will focus on degradation of Cr alloys containing 0-30%W by weight in a flowing gas mixture containing 30%CO, 8%CO2, 20%H2, 2%CH4, 0.8%H2S, 0.02%HCl, and 40%N2 by volume at temperatures up to 1000ºC. A pseudo-cyclic test involving heating the specimens, holding them at temperature for varying periods, and cooling them to room temperature was employed. Mass change of the specimens was determined after each cycle. Corrosion scale on the specimens was characterized using SEM, WDX, and XRD. Various sulfides, oxides, carbides, and nitrides were determined in different layers ofmore » the scale.« less
  • An investigation was carried out to study the kinetics and products of oxidation of a wrought Ni-Cr-W-Mn-Si-La alloy at temperatures in the range of 950 to 1150{degrees}C. Oxidation kinetics were evaluated from measurements of weight change, metal loss, and internal penetration. Analytical electron microscopy, scanning electron microscopy, electron probe microanalysis, and X-ray diffraction were used to characterize the scale microstructure. Initially, La was observed to segregate within a surface layer of about 5 {mu}m thick, which promoted selective oxidation of Cr and Mn. Oxidation kinetics were found to follow a parabolic-rate law with an activation energy of about 232 kJ/mol.more » During steady-state oxidation, the scale consisted of an inner adherent layer of {alpha}-Cr{sub 2}O{sub 3} modified by the presence of La and Si, and shielded by an outer layer of MnCr{sub 2}O{sub 4}. Most of the La was segregated to grain boundaries of the {alpha}-Cr{sub 2}O{sub 3} scale, however, Si was homogeneously distributed. It was concluded that the characteristic oxidation resistance of the alloy was related to the synergistic effects of Ni and Cr and to the effective minor additions of La, Si, and Mn; however, the useful life of the scale was limited by rupture and surface depletion in Cr, leading to accelerated internal oxidation.« less