Microstructure Of Ferritic Stainless Steel Research Articles

Change in the Microstructure of Ferritic Stainless Steel with Surface Roughness and the Number of Thermal Cycles

One of the candidates for metallic interconnects of solid oxide fuel cells is ferritic stainless steel, Crofer 22 APU. Ferritic stainless steel Crofer 22 APU specimens with different surface roughness were prepared by grinding with SiC powder papers of various grits and then thermally cycled in air. Variation in the microstructure of the samples having different roughness with thermal cycling was investigated. Polished Crofer 22 APU specimens after three and five thermal cycles had relatively flat oxide layers with thicknesses of about 13.8 and 17.9 μm, respectively. Micrographs of a trench made by milling with FIB (focused ion beam) for a Crofer 22 APU specimen ground with grit 80 SiC powder paper after 8 thermal cycles (total oxygen exposure time of 200 h at 1073 K), captured by ESB (energy selective back-scattering) and SE2 (type II secondary electrons), showed that the surface of the sample was very coarse and its oxide layer was undulated. In the oxide layer, the phase of the sublayer was Cr2O3, and that of the top layer was (Cr, Mn)3O4 spinel. The surface of the sample ground with grit 80 SiC powder paper was very rough after 60 thermal cycles (total oxygen exposure time of 1500 h at 1073 K). The polished Crofer 22 APU is a better applicant to an interconnect of SOFC than those with rougher surfaces.

Effect of Initial Grain Size on Thermal Fatigue Behavior and Changes in Microstructure of Ferritic Stainless Steel

Effect of Initial Grain Size on Thermal Fatigue Behavior and Changes in Microstructure of Ferritic Stainless Steel

The effect of strong carbide-forming elements such as Mo, Ti, V and Nb on the microstructure of ferritic stainless steel

Abstract The effects of both stabilizing elements such as Mo, Ti, V and Nb and of homogenization on microstructure, thermal properties, and some mechanical properties of ferritic stainless steels containing 18 wt.% Cr were investigated using scanning electron microscopy, energy-dispersive spectrometry, X-ray diffraction, differential thermal analysis, microhardness measurements and Charpy V notch impact tests. These stabilizing elements formed hard MC carbides, but decreased the amount of M23C6 carbides that were forming to too great a degree in the ferritic stainless steel considered. In addition, they also reduced the dissolving temperature of the M23C6 carbides, the grain coarsening temperature of the ferrite phase (except for V), and the toughness of the samples, but increased the microhardness. Homogenization increased the microhardness, but it did not affect the toughness considerably.

The effect of niobium on the microstructure of ferritic stainless steel

The effect of different amounts of Nb and of homogenization on the ferritic stainless steels containing 17–18 wt.% Cr was investigated with scanning electron microscopy (SEM), optical microscopy, energy-dispersive spectrometry (EDS), X-ray diffraction (XRD) and differential thermal analysis (DTA). It was observed that M 23C 6, NbC and sigma phase formed in these steels. In addition, the formation of Nb 2C was observed in the sample containing 3.0 wt.% Nb. While the amount of Nb increased from 0.5 to 3.0 wt.% Nb, the microhardness of the matrix and the amount of M 23C 6 decreased and the toughness of the samples increased. After homogenization, the increase in the toughness of the samples containing 1.5–3.0 wt.%Nb was considerable and impressive.