Authors: Kenichi Ikeda Nana Kwame Gyan Yamoah William T Reynolds Junichi Hamada Mitsuhiro Murayama
Publish Date: 2015/05/05
Volume: 46, Issue: 8, Pages: 3460-3469
Abstract
Niobiumcontaining ferritic stainless steels are finding new applications in automotive exhaust components because of their oxidation resistance thermal fatigue resistance and hightemperature strength The mechanical behavior of Nbcontaining 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 hightemperature flow strength of an 18Cr2Mo05Nb 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 Hightemperature tensile tests at 1023 K 750 °C of unaged initial microstructure is precipitatefree and preaged 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 unaged and preaged samples was drastic the yield strength of the unaged sample was twice that of the preaged sample and the unaged sample exhibits a noticeable yield drop Transmission electron microscopy revealed a Laves phase nucleated and grew during the hightemperature tensile test in the unaged sample and the majority of the precipitates in the preaged sample were the same Laves phase Furthermore a strain effect on precipitate growth was recognized in unaged and preaged conditions by comparing grip no strain and gage strained sections of tensile samples The dominant strengthening contribution in unaged samples is initially the precipitate shearing mechanism and it changes to Orowan strengthening beyond the ultimate tensile strength whereas the dominant contribution in the preaged samples appears to be Orowan strengthening throughout the stress–strain curveThe authors are grateful to the Nanoscale Characterization and Fabrication Laboratory of the Institute for Critical Technology and Applied Science NCFLICTAS at Virginia Tech for the use of its facilities KI thanks Professor Hideharu Nakashima in Kyushu University for fruitful discussions KI was partly supported by Strategic Young Researcher Overseas Visits Program JSPS R2408 Japan
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