Abstract
ABSTRACT Applications of neutron diffraction to microstructure evaluation of steel investigated by a project commissioned by the Innovative Structural Materials Association are summarized. The volume fraction of austenite (γ) for a 1.5Mn-1.5Si-0.2C steel was measured by various techniques including backscatter electron diffraction (EBSD) and X-ray diffraction. It is recommended to measure volume fraction and texture simultaneously using neutron diffraction. The γ reverse transformation was in situ monitored using dilatometry, EBSD, X-ray diffraction and neutron diffraction. The γ reversion kinetics showed excellent agreements between dilatometry and neutron diffraction, whereas the γ formation started at higher temperatures in EBSD and X-ray diffraction measurements. Such discrepancy is attributed to the change in chemical compositions at the specimen surface by heating; Mn and C concentrations were decreased with heating. Phase transformations from γ upon cooling were monitored, which enabled us to elucidate the changes in lattice parameters of ferrite (α) and γ affected by not only thermal contraction but also transformation strains, thermal misfit strains and carbon enrichment in γ in the above hypoeutectoid steel. Pearlitic transformation started after the carbon enrichment reached approximately 0.76 mass% and contributed to diffraction line broadening. Martensitic transformation with or without ausforming at 700°C was monitored for a medium carbon low alloyed steel. Dislocation density after ausforming was determined using the convolutional multiple whole profile fitting method for 10 s time-sliced data. The changes in γ and martensite lattice parameters upon quenching were tracked and new insights on internal stresses and the axial ratio of martensite were obtained.
Highlights
Industrial applications of neutron scattering and diffraction measurements have recently been progressed with achieving high-intensity neutron beam at a spallation neutron source like Materials and Life Science Experiment Facility (MLF) at Japan Proton Accelerator Research Complex (J-PARC)
It is strongly expected directly to monitor microstructural evolution and internal stresses during steel production process, in particular, thermomechanically controlled processing (TMCP), which has been challenged using in situ neutron diffraction
With respect to the obtained neutron diffraction profiles, the convolutional multiple whole profile (CMWP) fitting method [55,56] was applied resulting in the dislocation densities of 0.982 × 1014 m−2 for TR1 and 1.13 × 1014 m−2 for TR2, roughly supporting the transmission electron microscopy (TEM) observations described above
Summary
Industrial applications of neutron scattering and diffraction measurements have recently been progressed with achieving high-intensity neutron beam at a spallation neutron source like Materials and Life Science Experiment Facility (MLF) at Japan Proton Accelerator Research Complex (J-PARC). The γ in advanced steels is metastable in general, so that the method to measure the surface layer like electron backscatter diffraction (EBSD) or X-ray diffraction often results in smaller fγ in comparison with the result by neutron diffraction that evaluates the interior or global average value [23,24] This is because martensitic transformation is easy to occur near the free surface. It is strongly expected directly to monitor microstructural evolution and internal stresses during steel production process, in particular, thermomechanically controlled processing (TMCP), which has been challenged using in situ neutron diffraction. This topic will be briefly reviewed, the details of which will be reported in another paper [9]
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