Development of oxide dispersion strengthened ferritic steels with and without aluminum

Pure Fe,Cr,Al,Ti elemental powders and prealloyed Y2O3 powder were processed by high energy mechanical milling.The compositions of the mixed powders are designed as Fe-18Cr-0.2Ti-0.35Y2O3 and Fe-18Cr-5...     

Oxidation resistance of novel ferritic stainless steels alloyed with titanium for SOFC interconnect applications

Chromia (Cr₂O₃) forming ferritic stainless steels are being developed for interconnect application in Solid Oxide Fuel Cells (SOFC). A problem with these alloys is that in the SOFC environment chrome in the surface oxide can evaporate and deposit on the electrochemically active sites within the fuel cell. This poisons and degrades the performance of the fuel cell. The development of steels that can form conductive outer protective oxide layers other than Cr₂O₃ or (CrMn)₃O₄ such as TiO₂ may be attractive for SOFC application. This study was undertaken to assess the oxidation behavior of ferritic stainless steel containing 1 weight percent (wt.%) Ti, in an effort to develop alloys that form protective outer TiO₂ scales. The effect of Cr content (6–22 wt.%) and the application of a Ce-based surface treatment on the oxidation behavior (at 800 °C in air + 3% H₂O) of the alloys was investigated. The alloys themselves failed to form an outer TiO₂ scale even though the large negative ΔG of this compound favors its formation over other species. It was found that in conjunction with the Ce-surface treatment, a continuous outer TiO₂ oxide layer could be formed on the alloys, and in fact the alloy with 12 wt.% Cr behaved in an identical manner as the alloy with 22 wt.% Cr.     

Applicability of extra low interstitials ferritic stainless steels for bipolar plates of proton exchange membrane fuel cells

Ferritic stainless steels can be attractive bipolar plate materials of proton exchange membrane fuel cells (PEMFC), provided that the stainless steels show sufficient corrosion resistance, for instance, by eliminating interstitial elements such as carbon and nitrogen. In the present study, thus, ferritic stainless steels (19Cr2Mo and 22Cr2Mo) with extra low interstitials (ELI) are evaluated to determine the required level of chromium content to apply them for PEMFC bipolar plates. In a simulated PEMFC environment (0.05 M SO₄²⁻ (pH 3.3) + 2 ppm F⁻ solution at 353 K), the 22Cr2Mo stainless steel showed lower current density during the polarization in comparison with the 19Cr2Mo one. The polarization behavior of the 22Cr2Mo stainless steel resembles that of the type 316 one (17Cr12Ni2Mo). Similar values of interfacial contact resistance (ICR) are observed for both ferritic stainless steels. The 22Cr2Mo stainless steel bipolar plate is found to be stable throughout the cell operation, while the 19Cr2Mo stainless steel corroded within 1000 h. After the cell operation, the 22Cr2Mo stainless steel retains the chromium enriched passive film, while the chromium enriched surface film is not found for the 19Cr2Mo one, showing iron oxide/hydroxide based film. X-ray fluorescence (XRF) analysis of the membrane electrode assemblies (MEAs) after the cell operation indicates that the 22Cr2Mo stainless steel was less contaminated with iron species. The above results suggest that the 22Cr2Mo stainless steel can be applicable to bipolar plates for PEMFC, especially 22 mass% of chromium content in ferritic stainless steel with ELI system is, at least, demanded to ensure stable cell performance.     

Potential suitability of ferritic and austenitic steels as interconnect materials for solid oxide fuel cells operating at 600 °C

The oxidation behavior of a number of commercially available ferritic and austenitic steels was tested in air and in two simulated anode gases of a solid oxide fuel cell (SOFC) to evaluate the potential suitability as construction materials for interconnects in SOFC's operating at 600 °C. During air exposure all studied materials showed excellent oxidation resistance due to formation of a protective, double layered chromia/spinel surface scale even if the steel Cr content was as low as 17%. However, in the anode side gases the presence of water vapour (and possibly CO/CO₂) increased the tendency to form poorly protective Fe-base oxide scales, in combination with internal oxidation of Cr. The occurrence of this adverse effect could be suppressed not only by increased Cr contents of the alloy but also by a small alloy grain size either in the bulk of the material or in the specimen/component surface. The latter can be promoted by cold work e.g. introduced by specimen/component grinding. As high Cr contents may lead to undesired σ-phase formation and defined surface treatments of an interconnect will not be possible in all designs, the relatively low operating temperature of 600 °C, resulting in low Cr diffusivity in the alloy grains, may require the use of a fine grained interconnect material to obtain and sustain protective chromia base surface scale formation during long-term operation.     

Microstructure evolution in HAZ and suppression of Type IV fracture in advanced ferritic power plant steels

The effect of boron and nitrogen on the microstructure evolution in heat affected zone (HAZ) of 9Cr steel during simulated heating and on the Type IV fracture in welded joints has been investigated at 650 °C. Gr.92 exhibits a significant decrease in time to rupture after thermal cycle to a peak temperature near A_C3, while the creep life of Gr.92N, subjected to only normalizing but no tempering, and 9Cr-boron steel is substantially the same as that of the base metals. In Gr.92 after A_C3 thermal cycle, very few precipitates are formed along PAGBs in the fine-grained microstructure. In the P92N and 9Cr-boron steel after A_C3 heat cycle, on the other hand, not only PAGBs but also lath and block boundaries are covered by M₂₃C₆ carbides in the coarse-grained microstructure. It is concluded that the degradation in creep life in Gr.92 after the A_C3 thermal cycle is not caused by grain refinement but that the reduction of boundary and sub-boundary hardening is the most important. Soluble boron is essential for the change in α/γ transformation behavior during heating and also for the suppression of Type IV fracture in welded joints. Newly alloy-designed 9Cr steel with 160 ppm boron and 85 ppm nitrogen exhibits much higher creep rupture strength of base metal than P92 and also no Type IV fracture in welded joints at 650 °C.     

Microstructure evolution and mechanical properties of Co coated AISI 441 ferritic stainless steel/ YSZ reactive air brazed joint

This study investigates the microstructure evolution and mechanical properties of bare and Co coated AISI 441 ferritic stainless steel/YSZ ceramic reactive air brazed joints achieved by Ag–CuO braze for Solid Oxide Fuel/Electrolysis Cells applications. Interfacial microstructure of steel/YSZ joints is analyzed by SEM and TEM with EDS. A thick and porous oxide layer rich in Fe, Cr, and Cu is found in bare steel/YSZ ceramic joints, which is induced by the severe oxidation and its intense reaction with CuO during the brazing process. For the coated steel/YSZ ceramic joint, a comparably dense and uniform (Co, Fe, Cu)₃O₄ spinel layer is formed on the steel surface, which is tightly bonded with Ag–CuO braze without visible bonding defects. Meanwhile, the oxidation of steel substrates and its interaction with CuO is significantly suppressed. Co coated steel/YSZ joints possess reliable mechanical properties with the shear strength of 51 MPa, which is 54.5% higher than that of bare steel/YSZ ceramic joints (33 MPa). Besides, the microstructure evolution of coated steel/YSZ ceramic joints during brazing is schematically illustrated by a physical model.     

国产S30432钢和T92异种钢焊接接头力学性能和显微组织分析

研究了国产奥氏体S30432钢和马氏体T92异种钢焊接接头的力学性能、各微区的显微组织、硬度分布以及T92钢化学成分变化对焊接接头微观组织的影响,并与进口S30432钢和T92异种钢焊接接头的性能进行了对比.结果表明:国产S30432钢和T92异种钢焊接接头与进口同类型钢焊接接头的力学性能相当,硬度分布情况相同,T92钢侧熔合区的硬度最高,但国产S30432钢和T92异种钢焊接接头的硬度稍低于进口钢.在异种钢焊接接头的T92钢侧熔合区出现块状铁素体带,且发现T92钢不同的Creq当量会影响块状铁素体生成的数量和尺寸.