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Metallurgical and Materials Transactions A

, Volume 46, Issue 8, pp 3460–3469 | Cite as

Effect of Laves Phase on High-Temperature Deformation and Microstructure Evolution in an 18Cr-2Mo-0.5Nb Ferritic Stainless Steel

  • Ken-ichi Ikeda
  • Nana Kwame Gyan Yamoah
  • William T. ReynoldsJr.
  • Jun-ichi Hamada
  • Mitsuhiro MurayamaEmail author
Article

Abstract

Niobium-containing ferritic stainless steels are finding new applications in automotive exhaust components because of their oxidation resistance, thermal fatigue resistance, and high-temperature strength. The mechanical behavior of Nb-containing ferritic steels at service temperatures of 973 K (700 °C) and higher results from the convolution of dynamic microstructural changes including precipitation, precipitate coarsening, strain hardening, recovery, and recrystallization. The relative contributions of these competing processes have yet to be clarified. In this study, the high-temperature flow strength of an 18Cr-2Mo-0.5Nb ferritic stainless steel (SUS 444) was correlated with microstructure under different strain and initial precipitate distributions to clarify the relative role of the strengthening and softening processes. High-temperature tensile tests at 1023 K (750 °C) of un-aged (initial microstructure is precipitate-free) and pre-aged (initial microstructure contains precipitates) samples were carried out and transmission electron microscopy was used to assess dislocation distributions and precipitate morphology. The difference in the stress–strain curves between un-aged and pre-aged samples was drastic; the yield strength of the un-aged sample was twice that of the pre-aged sample, and the un-aged sample exhibits a noticeable yield drop. Transmission electron microscopy revealed a Laves phase nucleated and grew during the high-temperature tensile test in the un-aged sample and the majority of the precipitates in the pre-aged sample were the same Laves phase. Furthermore, a strain effect on precipitate growth was recognized in un-aged and pre-aged conditions by comparing grip (no strain) and gage (strained) sections of tensile samples. The dominant strengthening contribution in un-aged samples is initially the precipitate shearing mechanism and it changes to Orowan strengthening beyond the ultimate tensile strength, whereas the dominant contribution in the pre-aged samples appears to be Orowan strengthening throughout the stress–strain curve.

Keywords

Flow Stress Lave Phase Ferritic Stainless Steel Orowan Strengthen Precipitate Volume Fraction 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Notes

Acknowledgments

The authors are grateful to the Nanoscale Characterization and Fabrication Laboratory of the Institute for Critical Technology and Applied Science (NCFL-ICTAS) at Virginia Tech for the use of its facilities. K.I. thanks Professor Hideharu Nakashima in Kyushu University for fruitful discussions. K.I. was partly supported by Strategic Young Researcher Overseas Visits Program (JSPS #R2408, Japan).

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Copyright information

© The Minerals, Metals & Materials Society and ASM International 2015

Authors and Affiliations

  • Ken-ichi Ikeda
    • 1
    • 2
  • Nana Kwame Gyan Yamoah
    • 3
    • 4
  • William T. ReynoldsJr.
    • 3
  • Jun-ichi Hamada
    • 5
  • Mitsuhiro Murayama
    • 3
    Email author
  1. 1.Faculty of Engineering SciencesKyushu UniversityKasugaJapan
  2. 2.Faculty of EngineeringHokkaido UniversitySapporoJapan
  3. 3.Department of Materials Science and EngineeringVirginia TechBlacksburgUSA
  4. 4.Department of Mechanical EngineeringNorth Carolina A&T State UniversityGreensboroUSA
  5. 5.Nippon Steel & Sumikin Stainless Steel CorporationHikariYamaguchiJapan

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