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Title: Microstructural changes caused by yttrium addition to NbTi-modified centrifugally cast HP-type stainless steels

Abstract

Centrifugally cast heat-resistant HP stainless steels are particularly suitable for applications where service conditions comprise high temperatures and aggressive environments; thus, they are extensively used in reformer furnaces, in which hydrogen production takes place. The demand for better performance has motivated developments in these steels. The addition of Nb and Ti as microstructural modifiers has proved successful in providing a more stable microstructure. In this work yttrium was added to centrifugally cast NbTi-modified HP steels. It was observed that its presence increased the level of fragmentation of the chromium carbides, a positive aspect for creep resistance. The main cause of the fragmentation is the formation of yttrium carbides, which serve as heterogeneous nucleation sites for the other carbides. One of the tubes, with a lower titanium content, showed the best creep performance among those tubes studied owing to the presence of a smaller volume fraction of the deleterious G phase.

Authors:
 [1];  [2];  [3];  [3];  [4]
  1. Federal University of Rio de Janeiro (UFRJ), C.P. 68505, Ilha do Fundao, Rio de Janeiro (Brazil). E-mail: fcnunes@gmail.com
  2. Federal University of Rio de Janeiro (UFRJ), C.P. 68505, Ilha do Fundao, Rio de Janeiro (Brazil)
  3. Free University of Brussels, C.P. 194/03, Av. F. D. Rooselvelt, 50, B-1050, Brussels (Belgium)
  4. Metallurgical Consulting Services, P.O. Box 5006, Saskatoon, SK, S7K 4E3 (Canada)
Publication Date:
OSTI Identifier:
21003528
Resource Type:
Journal Article
Resource Relation:
Journal Name: Materials Characterization; Journal Volume: 58; Journal Issue: 2; Other Information: DOI: 10.1016/j.matchar.2006.04.007; PII: S1044-5803(06)00104-5; Copyright (c) 2006 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; CASTING; CASTINGS; CHROMIUM CARBIDES; CREEP; FRAGMENTATION; FURNACES; HEAT; HYDROGEN PRODUCTION; MICROSTRUCTURE; NUCLEATION; PERFORMANCE; STAINLESS STEELS; TITANIUM; YTTRIUM; YTTRIUM ADDITIONS; YTTRIUM CARBIDES

Citation Formats

Nunes, F.C., Almeida, L.H. de, Dille, J., Delplancke, J.-L., and Le May, I. Microstructural changes caused by yttrium addition to NbTi-modified centrifugally cast HP-type stainless steels. United States: N. p., 2007. Web. doi:10.1016/j.matchar.2006.04.007.
Nunes, F.C., Almeida, L.H. de, Dille, J., Delplancke, J.-L., & Le May, I. Microstructural changes caused by yttrium addition to NbTi-modified centrifugally cast HP-type stainless steels. United States. doi:10.1016/j.matchar.2006.04.007.
Nunes, F.C., Almeida, L.H. de, Dille, J., Delplancke, J.-L., and Le May, I. Thu . "Microstructural changes caused by yttrium addition to NbTi-modified centrifugally cast HP-type stainless steels". United States. doi:10.1016/j.matchar.2006.04.007.
@article{osti_21003528,
title = {Microstructural changes caused by yttrium addition to NbTi-modified centrifugally cast HP-type stainless steels},
author = {Nunes, F.C. and Almeida, L.H. de and Dille, J. and Delplancke, J.-L. and Le May, I.},
abstractNote = {Centrifugally cast heat-resistant HP stainless steels are particularly suitable for applications where service conditions comprise high temperatures and aggressive environments; thus, they are extensively used in reformer furnaces, in which hydrogen production takes place. The demand for better performance has motivated developments in these steels. The addition of Nb and Ti as microstructural modifiers has proved successful in providing a more stable microstructure. In this work yttrium was added to centrifugally cast NbTi-modified HP steels. It was observed that its presence increased the level of fragmentation of the chromium carbides, a positive aspect for creep resistance. The main cause of the fragmentation is the formation of yttrium carbides, which serve as heterogeneous nucleation sites for the other carbides. One of the tubes, with a lower titanium content, showed the best creep performance among those tubes studied owing to the presence of a smaller volume fraction of the deleterious G phase.},
doi = {10.1016/j.matchar.2006.04.007},
journal = {Materials Characterization},
number = 2,
volume = 58,
place = {United States},
year = {Thu Feb 15 00:00:00 EST 2007},
month = {Thu Feb 15 00:00:00 EST 2007}
}
  • The repair weldability of two types of heat-resistant austenitic stainless steel castings, HP-Nb modified alloys and 20-32Nb alloys, has been evaluated after extended service exposure of these materials at temperatures on the order of 815 deg. C (1500 deg. F). The initial part of this investigation has focused on characterization of the microstructure of these alloys after service exposure times of up to 12 years. Microstructure evolution from the as-cast condition to the service-exposed condition is described. In the as-cast condition, both eutectic NbC and Cr-rich M{sub 7}C{sub 3} carbides were observed in the HP-Nb alloys. In contrast, only eutecticmore » NbC carbides were present in the as-cast 20-32Nb alloys due to a significantly higher atomic ratio of Nb/C as compared to the HP-Nb alloys. In the service-exposed condition, an Ni-Nb silicide and Cr-rich M{sub 23}C{sub 6} were identified in both alloys. The HP-Nb alloys exhibited a much higher total volume fraction of microconstituents than the 20-32Nb alloys and the M{sub 23}C{sub 6} phase was more prevalent than Ni-Nb silicide. In contrast, the Ni-Nb silicide was dominant in the service-exposed 20-32Nb alloys. This difference resulted in a significant increase in the repair welding liquation cracking susceptibility of the 20-32Nb service-exposed alloy.« less
  • HP steels modified with Nb and Nb-plus-Ti, produced in the form of centrifugally cast tubes, were characterized by means of light optical microscopy, scanning electron microscopy (SEM) with secondary and backscattered electron imaging, and transmission electron microscopy (TEM) with energy-dispersive spectroscopy (EDS) and selected area diffraction. A complete description of the phases present in the as-cast, welded, and aged condition was made as a function of the modifying elements. The chromium carbide stoichiometry, secondary precipitation, NbC instability at elevated temperatures, the G-phase transformation, and the effect of Ti on this reaction are discussed. It is indicated that partial transformation ofmore » G-phase in the alloy modified with Ti is one of the explanations for the superior creep properties observed in this material.« less
  • No abstract prepared.
  • Mechanical testing and microstructural characterization were performed on short-term thermally aged cast austenitic stainless steels (CASS) to understand the severity and mechanisms of thermal-aging degradation experienced during extended operation of light water reactor (LWR) coolant systems. Four CASS materials – CF3, CF3M, CF8, and CF8M – were thermally aged for 1500 hours at 290 °C, 330 °C, 360 °C, and 400 °C. All four alloys experienced insignificant change in strength and ductility properties but a significant reduction in absorbed impact energy. The primary microstructural and compositional changes during thermal aging were spinodal decomposition of the δ-ferrite into α/ α`, precipitationmore » of G-phase in the δ-ferrite, segregation of solute to the austenite/ ferrite interphase boundary, and growth of M23C6 carbides on the austenite/ferrite interphase boundary. These changes were shown to be highly dependent on chemical composition, particularly the concentration of C and Mo, and aging temperature. A comprehensive model is being developed to correlate the microstructural evolution with mechanical behavior and simulation for predictive evaluations of LWR coolant system components.« less
  • We performed mechanical testing and microstructural characterization on short-term thermally aged cast austenitic stainless steels (CASS) to understand the severity and mechanisms of thermal-aging degradation experienced during extended operation of light water reactor (LWR) coolant systems. Four CASS materials–CF3, CF3M, CF8, and CF8M–were thermally aged for 1500 h at 290 °C, 330 °C, 360 °C, and 400 °C. All four alloys experienced insignificant change in strength and ductility properties but a significant reduction in absorbed impact energy. The primary microstructural and compositional changes during thermal aging were spinodal decomposition of the δ-ferrite into α/α', precipitation of G-phase in the δ-ferrite,more » segregation of solute to the austenite/ferrite interphase boundary, and growth of M 23C 6 carbides on the austenite/ferrite interphase boundary. These changes were shown to be highly dependent on chemical composition, particularly the concentration of C and Mo, and aging temperature. Finally, the low C, high Mo CF3M alloys experienced the most spinodal decomposition and G-phase precipitation coinciding the largest reduction in impact properties.« less