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Title: MULTI-PHASE HIGH TEMPERATURE ALLOYS: EXPLORATION OF LAVES-STRENGTHENED STEELS

Abstract

Exploratory effort was initiated for the development of Fe-base alloys strengthened by intermetallic Laves phase combined with MC (M: metals) carbide for improved elevated-temperature strength in fossil energy system components such as super-heater tubes and industrial gas turbines. Work in FY 2006 was focused on strengthening of Fe-Cr-Ni base austenitic stainless alloys by Fe2Nb Laves-phase precipitates with/without MC carbides, in combination with the improvement of oxidation resistance via Al-modification to promote alumina scale formation. A series of Fe-Cr-Ni-Nb base austenitic alloys with additions of Mo, Al, Si, C, B, etc. were cast and thermomechanically processed, and then tensile creep-rupture tested at the conditions of 750-850oC/70-170 MPa. The Al-modified alloys strengthened by Laves + MC show superior creep strength to that of conventional type 347 stainless steels, and their creep life-limit reaches up to 500 h at 750 oC/100 MPa. These alloys also show an excellent oxidation resistance from 650-800oC in air and air + 10% water vapor environments due to formation of a protective Al2O3 scale. Microstructural analysis of alloys strengthened by only Laves phase revealed that the Laves phase was effective to pin dislocations when the particle size is less than 0.5 m, but the resultant creep rupture livesmore » were relatively short. The Al-modification was also applied to an advanced carbide-strengthened austenitic stainless steel, and it yielded creep resistance comparable to state-of-the-art austenitic alloys such as NF709, together with protective alumina scale formation. Modification of this alloy composition for its creep strength and oxidation resistance will be pursued in FY2007. Preliminary results suggest that the developed alloys with Al-modification combined with MC carbide strengthening are promising as a new class of high-temperature austenitic stainless steels.« less

Authors:
 [1];  [1];  [1];  [1]
  1. ORNL
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Shared Research Equipment Collaborative Research Center
Sponsoring Org.:
FE USDOE - Office of Fossil Energy (FE)
OSTI Identifier:
931670
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Conference
Resource Relation:
Conference: 20 th Annual Fossil ARM Program Review, Knoxville, TN, USA, 20060612, 20060614
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ALLOYS; CREEP; ENERGY SYSTEMS; EXPLORATION; GAS TURBINES; LAVES PHASES; OXIDATION; PARTICLE SIZE; STAINLESS STEELS; STEELS; WATER VAPOR

Citation Formats

Yamamoto, Yukinori, Brady, Michael P, Lu, Zhao Ping, and Liu, Chain T. MULTI-PHASE HIGH TEMPERATURE ALLOYS: EXPLORATION OF LAVES-STRENGTHENED STEELS. United States: N. p., 2007. Web.
Yamamoto, Yukinori, Brady, Michael P, Lu, Zhao Ping, & Liu, Chain T. MULTI-PHASE HIGH TEMPERATURE ALLOYS: EXPLORATION OF LAVES-STRENGTHENED STEELS. United States.
Yamamoto, Yukinori, Brady, Michael P, Lu, Zhao Ping, and Liu, Chain T. Mon . "MULTI-PHASE HIGH TEMPERATURE ALLOYS: EXPLORATION OF LAVES-STRENGTHENED STEELS". United States. doi:.
@article{osti_931670,
title = {MULTI-PHASE HIGH TEMPERATURE ALLOYS: EXPLORATION OF LAVES-STRENGTHENED STEELS},
author = {Yamamoto, Yukinori and Brady, Michael P and Lu, Zhao Ping and Liu, Chain T},
abstractNote = {Exploratory effort was initiated for the development of Fe-base alloys strengthened by intermetallic Laves phase combined with MC (M: metals) carbide for improved elevated-temperature strength in fossil energy system components such as super-heater tubes and industrial gas turbines. Work in FY 2006 was focused on strengthening of Fe-Cr-Ni base austenitic stainless alloys by Fe2Nb Laves-phase precipitates with/without MC carbides, in combination with the improvement of oxidation resistance via Al-modification to promote alumina scale formation. A series of Fe-Cr-Ni-Nb base austenitic alloys with additions of Mo, Al, Si, C, B, etc. were cast and thermomechanically processed, and then tensile creep-rupture tested at the conditions of 750-850oC/70-170 MPa. The Al-modified alloys strengthened by Laves + MC show superior creep strength to that of conventional type 347 stainless steels, and their creep life-limit reaches up to 500 h at 750 oC/100 MPa. These alloys also show an excellent oxidation resistance from 650-800oC in air and air + 10% water vapor environments due to formation of a protective Al2O3 scale. Microstructural analysis of alloys strengthened by only Laves phase revealed that the Laves phase was effective to pin dislocations when the particle size is less than 0.5 m, but the resultant creep rupture lives were relatively short. The Al-modification was also applied to an advanced carbide-strengthened austenitic stainless steel, and it yielded creep resistance comparable to state-of-the-art austenitic alloys such as NF709, together with protective alumina scale formation. Modification of this alloy composition for its creep strength and oxidation resistance will be pursued in FY2007. Preliminary results suggest that the developed alloys with Al-modification combined with MC carbide strengthening are promising as a new class of high-temperature austenitic stainless steels.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}

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  • Work in 2007 focused on the development of a new class of heat-resistant austenitic stainless steel alloys which achieved a unique combination of high-temperature creep strength and excellent oxidation resistance via protective Al{sub 2}O{sub 3} scale formation. Strengthening is achieved via the formation of stable nano NbC carbides with/without Fe{sub 2}Nb and related intermetallic phase dispersions, with controlled levels of Al to enable Al{sub 2}O{sub 3} scale formation in both air and air + water vapor environments up to {approx}800-900 C. The developed alloys exhibit comparable creep resistance to that of the best commercial heat-resistant austenitic stainless steels, and themore » protective Al{sub 2}O{sub 3} scale formation provides oxidation resistance superior to that of advanced Cr{sub 2}O{sub 3}-forming heat-resistant austenitic alloys. Preliminary screening also indicated that the developed Al-modified alloys were amenable to welding.« less
  • Attractive high-temperature mechanical properties and oxidation/hot corrosion resistance have been achieved in a new family of Cr{sub 2}Ta-reinforced Cr alloys. However, inadequate room-temperature toughness remains a key challenge, with the best Cr-Cr{sub 2}Ta alloys exhibiting only modest toughness in the range of 12-14 MPa m{sup 1/2}. The addition of MgO has been shown to significantly improve the room-temperature mechanical properties of unalloyed Cr and was investigated as a means for improving the room-temperature mechanical properties of the Cr-Cr{sub 2}Ta alloys. Microstructural analysis of a series of Cr and Cr-6MgO base alloys was used to investigate the proposed ductilization mechanism ofmore » nitrogen gettering by a MgCr{sub 2}O{sub 4} spinel phase, which forms during consolidation of Cr and MgO powders. Nitride and related impurity precipitates have been linked to reduced ductility in Cr at room-temperature. Surprisingly, nitride (and carbide) impurity precipitates were found i n hot-pressed Cr-6 MgO base alloys despite room-temperature tensile ductility of 5%. These precipitates were found adjacent to MgO/MgCr{sub 2}O{sub 4} particles and were somewhat more blunt than those observed in unalloyed Cr. The addition of TiO{sub 2} to unalloyed Cr resulted in similar morphological changes to the nitride and carbide impurity precipitates; however, the TiO{sub 2} dispersed alloy was brittle at room-temperature. Why MgO dispersions are effective in ductilizing Cr, but others such as TiO{sub 2} are not, is not clear and is the subject of ongoing study. Efforts to introduce the effect in Cr-Cr{sub 2}Ta-MgO alloys were not successful, and it was concluded that significant modification of the Cr matrix phase in the Cr-Cr{sub 2}Ta alloys by macroalloying is necessary to improve room-temperature mechanical properties. Preliminary attempts at macroalloying with Fe were quite successful and resulted in an increase in room-temperature toughness to 18-20 MPa m{sup 1/2} in Cr-C r{sub 2}Ta + Fe alloys.« less
  • The objective of this work is to develop and characterize a new family of Cr-based alloys for structural use in aggressive 900-1300 C corrosion environments. The potential advantages of Cr are high melting point, moderate density, and good high-temperature corrosion resistance in many environments [1]. However, these are currently negated by inadequate high-temperature strength, ambient-temperature brittleness, and susceptibility to environmental embrittlement at elevated-temperatures by rapid nitride subscale formation [1]. Over the course of this effort, two distinct approaches to overcoming these problems have been pursued: Cr{sub 2}Ta-reinforced Cr, and MgO-dispersed Cr. The Cr{sub 2}Ta-reinforced Cr alloys are based on themore » Cr-Cr{sub 2}Ta eutectic structure and contain a Cr solid solution matrix phase reinforced with lamellar Cr{sub 2}Ta Laves phase. They exhibit an attractive combination of high-temperature strength (tensile fracture strengths of 340-550 MPa at 1200 C), high-temperature ductility (15-40% tensile elongation above 1000 C), creep resistance (creep rupture life in excess of 1000 hours at 138 MPa loading at 1000 C in air), and oxidation resistance (comparable to that of commercial chromia-forming alloys in 1100 C, 1000 h cyclic oxidation screenings in air) [2]. However, no room-temperature ductility has been achieved and extensive microalloying and microstructural control efforts have 1ed to only modest room-temperature fracture toughness of 12-14 MPa {radical}m.« less