Effect of Ta Rich Inclusions and Microstructure Change During Precracking on Bimodal Fracture of Reduced Activation Ferritic/Martensitic Steels Observed in Transition Range

Volume 6, Issue 5 (May 2009)

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ISSN: 1546-962X
CODEN: JAIOAD
Published Online: 7 May 2009
Page Count: 10

Effect of Ta Rich Inclusions and Microstructure Change During Precracking on Bimodal Fracture of Reduced-Activation Ferritic/Martensitic Steels Observed in Transition Range

Tanigawa, Hiroyasu
Japan Atomic Energy Agency, Tokai, Ibaraki

Sokolov, Mikhail A.
Metals & Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN

Sawahata, Atsushi
Ibaraki Univ., Hitachi, Ibaraki

Hashimoto, Naoyuki
Hokkaido Univ., Sapporo, Hokkaido

Ando, Masami
Japan Atomic Energy Agency, Tokai, Ibaraki

Shiba, Kiyoyuki
Japan Atomic Energy Agency, Tokai, Ibaraki

Enomoto, Masato
Ibaraki Univ., Hitachi, Ibaraki

Klueh, Ronald L.
Metals & Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN

(Received 29 February 2008; accepted 18 March 2009)

Abstract

The master curve method for analyzing fracture toughness data depends on the assumption that the fracture initiation points are homogeneously distributed and fracture initiation is independent of test temperature. The reduced-activation ferritic/martensitic steels, such as F82H (Fe-8Cr-2W-0.2V-0.04Ta), form Al₂O₃-Ta(V,Ti)O composite inclusions and its distribution is not homogeneous throughout one heat, and this microstructural inhomogeneity appears to be correlated with bimodal facture of F82H in the transition range. To investigate this possibility, 1TCT fracture toughness specimens of F82H-IEA steel were fatigue precracked and sliced through the specimen thickness for microstructure analysis around the crack. It was found that the crack penetration was straight in the beginning, and then tended to follow a prior austenite grain boundary and finally to branch into two or three different directions. In addition, the microstructures around the crack and ahead of the crack formed a cell structure and became softer than nearby regions, which is typical for fatigue-loaded F82H. Possible mechanisms for how this cell structure ahead of cracks affects fracture toughness are suggested.

Keywords:
Ta rich inclusion, precracking, RAFM, bimodal fracture

Paper ID: JAI101728
DOI: 10.1520/JAI101728
ASTM International is a member of CrossRef.

Author Title Effect of Ta Rich Inclusions and Microstructure Change During Precracking on Bimodal Fracture of Reduced-Activation Ferritic/Martensitic Steels Observed in Transition Range Symposium Fifth Symposium on Small Specimen Test Techniques, 2007-02-01 Committee E10

		SEDL / Journals / Journal of ASTM International (JAI) / Citation Page Volume 6, Issue 5 (May 2009) Click here to download this paper now for $25 PDF (548K) View License Agreement ISSN: 1546-962X CODEN: JAIOAD Published Online: 7 May 2009 Page Count: 10 Effect of Ta Rich Inclusions and Microstructure Change During Precracking on Bimodal Fracture of Reduced-Activation Ferritic/Martensitic Steels Observed in Transition Range Tanigawa, Hiroyasu Japan Atomic Energy Agency, Tokai, Ibaraki Sokolov, Mikhail A. Metals & Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN Sawahata, Atsushi Ibaraki Univ., Hitachi, Ibaraki Hashimoto, Naoyuki Hokkaido Univ., Sapporo, Hokkaido Ando, Masami Japan Atomic Energy Agency, Tokai, Ibaraki Shiba, Kiyoyuki Japan Atomic Energy Agency, Tokai, Ibaraki Enomoto, Masato Ibaraki Univ., Hitachi, Ibaraki Klueh, Ronald L. Metals & Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN (Received 29 February 2008; accepted 18 March 2009) Abstract The master curve method for analyzing fracture toughness data depends on the assumption that the fracture initiation points are homogeneously distributed and fracture initiation is independent of test temperature. The reduced-activation ferritic/martensitic steels, such as F82H (Fe-8Cr-2W-0.2V-0.04Ta), form Al₂O₃-Ta(V,Ti)O composite inclusions and its distribution is not homogeneous throughout one heat, and this microstructural inhomogeneity appears to be correlated with bimodal facture of F82H in the transition range. To investigate this possibility, 1TCT fracture toughness specimens of F82H-IEA steel were fatigue precracked and sliced through the specimen thickness for microstructure analysis around the crack. It was found that the crack penetration was straight in the beginning, and then tended to follow a prior austenite grain boundary and finally to branch into two or three different directions. In addition, the microstructures around the crack and ahead of the crack formed a cell structure and became softer than nearby regions, which is typical for fatigue-loaded F82H. Possible mechanisms for how this cell structure ahead of cracks affects fracture toughness are suggested. Keywords: Ta rich inclusion, precracking, RAFM, bimodal fracture Paper ID: JAI101728 DOI: 10.1520/JAI101728 ASTM International is a member of CrossRef. Author Title Effect of Ta Rich Inclusions and Microstructure Change During Precracking on Bimodal Fracture of Reduced-Activation Ferritic/Martensitic Steels Observed in Transition Range Symposium Fifth Symposium on Small Specimen Test Techniques, 2007-02-01 Committee E10