Creep and tensile behaviour of austenitic Fe–Cr–Ni stainless steels

The control of creep behaviour during service of reformer tubes made of HP-40 austenitic stainless steels is still limited by the knowledge of creep mechanisms in these alloys. Two different HP-40 alloys modified with a low-level addition ofNbwere studied. Creep testswere carried out at 980 and 1050 ◦C with different stress levels, in the range of 20–50MPa, and their resultswere plotted in a Norton-type diagram. Also, low strain rate tensile tests were performed at temperature of 950, 980 or 1000 ◦C. As low strain rate tensile tests showed a plateau at nearly constant stress for a given strain rate, they could be somehow linked with creep tests. Accordingly, tensile and creep results were plotted together on a Larson–Miller (LMP) diagram. The fracture modes of tensile and creep samples were investigated and the effect of different parameters such as sample dimensions, temperature and atmosphere, was also studied

Microstructure evolution of HP40-Nb alloys during aging under air at 1000 °C

Two as-cast HP 40 alloys provided by different manufacturers were aged at 1000 °C under laboratory air. They had the same as-cast microstructure consisting of austenite dendrites delineated by a network of eutectic Nb-rich MC and Cr-rich M7C3 carbides. After aging for several months, they showed similar microstructures in the bulk materials, though M7C3 carbides have been replaced by M23C6 carbides. As expected, a sub-surface zone depleted in chromium has appeared where a tetragonal CrNbC could be identified in both materials. However, the composition of the transition zones between the surface and the bulk materials differed, mainly because one of the materials underwent significant nitrogen pick-up with associated precipitation of M6(C,N) and M2(C,N) phases. On the contrary, the other alloy did show only one intermediate zone with a mix of CrNbC, M23C6 and MC carbides. A full account of the microstructures observed in the aged materials is given

Piezomagnetic behavior : experimental approach and multiscale modeling

International audienceThis work deals with the study of magnetoelastic coupling in the framework of non-destructive testing. Experimental hysteretic and cyclic piezomagnetic measurements carried out on a dual-phase steel submitted to different magnetic field and stress conditions are reported. The effect of concomitant magnetic field and stress, considering static or variable amplitudes, is discussed. A new multiscale modeling of piezomagnetic hysteresis is finally proposed

High temperature corrosion of cast heat resisting steels in CO+CO2 gas mixtures

Two commercial variants of the cast heat resistant grade HP40Nb (Fe–25Cr–35Ni, Nb modified) were exposed to CO/CO2 gases at 982 and 1080°C in order to simulate exposure to the carbon and oxygen potentials realised in steam reformers under normal and overheated conditions. Both alloys developed external chromium-rich oxide scales, intradendritic silica precipitates and interdendritic oxide protrusions where primary, interdendritic carbides were oxidised in situ. Surprisingly, the lower silicon content alloy developed a more continuous internal silica layer, thereby slowing external scaling. Intradendritic oxidation was fast in both alloys, and is attributed to interfacial oxygen diffusion. Both alloys underwent rapid internal carburisation, indicating that their oxide scales failed to prevent carbon access to the underlying alloys under these reaction conditions

Mitigation of hydrogen embrittlement by carbon monoxide impurity in gaseous H2

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Densidade da jaguatirica em um ambiente semiárido no nordeste do Brasil

Jaguatiricas possuem um papel fundamental em comunidades ecológicas como mesopredadores, afetando níveis tróficos inferiores e também outros mesopredadores. Esta espécie possui uma grande variabilidade em suas características ecológicas em toda a sua distribuição, no entanto, o conhecimento desta espécie possui lacunas em vários locais onde ela ocorre. Neste trabalho, nós apresentamos o primeiro estudo desta espécie no semiárido brasileiro da Caatinga. Ambientes áridos podem afetar negativamente as espécies carnívoras e, aliado a alterações antrópicas, esta espécie pode ser levada a extinção local se sua densidade populacional é baixa. Portanto, para verificar o nível populacional da jaguatirica em uma região protegida da Caatinga, instalamos armadilhas fotográficas, entre setembro de 2009 e janeiro de 2010. Com os dados obtidos, calculamos a densidade desta espécie através de métodos espacialmente explícitos (SECR). A densidade estimada da jaguatirica foi de 3.16 ± 0.46 indivíduos por 100 km2. Esta estimativa é muito baixa para esta espécie, o que pode ser um reflexo das condições áridas deste ambiente. Um estudo populacional de maior duração pode ajudar a responder se esta baixa densidade é o suficiente para a persistência desta espécie a longo prazo tanto neste, quanto em outros ambientes áridos onde ela ocorre.Ocelots play a key role in ecological communities as mesopredators affecting the lower trophic level and other mesopredators. They show great variability in ecological traits across their distribution, but knowledge of this species is missing in several regions where it occurs. Here, we present the first study of ocelot in the Brazilian semiarid of Caatinga. Arid habitats might keep carnivore population density low and therefore vulnerable to environmental shocks and to human-induced changes, at risk of local extinction. To assess their population status, we used camera traps between September 2009 and January 2010. We estimated the density of ocelots using a spatially explicit capture-recapture method (SECR) to be 3.16 ± 0.46 individuals per 100 km2. This is a low-density estimate for ocelots, which might reflect the harsh conditions of the arid habitat. A longer population study of the ocelot can answer if this low population density is enough for a long-term persistence of this species in this and other arid environments

Piezomagnetic behavior: experimental observations and multiscale modeling

This work deals with the study of magnetoelastic coupling in the framework of non-destructive testing. Experimental hysteretic and cyclic piezomagnetic measurements carried out on a dual-phase steel submitted to different magnetic field and stress conditions are reported. The effect of concomitant magnetic field and stress, considering static or variable amplitudes, is discussed. A new multiscale modeling of piezomagnetic hysteresis is finally proposed

Phase transformations in Fe–Ni–Cr heat-resistant alloys for reformer tube applications

International audienc

Fatigue crack initiation and propagation in Cr-Mo Steel hydrogen storage vessels:Research on design for safe life

International standards and codes dedicated to design of pressure vessels are still unable to competitively ensure safe design and fitness for service of steel vessels for high pressure gaseous hydrogen. Emptying and shallow pressure cycles subject the material to hydrogen enhanced fatigue. A pre-normative project, MATHRYCE under the EU joint research program focused in this subject through material and component testing, analytical work, review of design methodologies and international collaboration. An easy to implement, safe and economically competitive vessel design methodology is targeted. Steps towards this goal were taken by deepening our understanding on hydrogen enhanced fatigue in different kinds of laboratory specimens and real vessels designed for hydrogen service at maximum 45 MPa pressure. This included cyclic pressure testing of artificially notched vessels both in hydrogen and inert environment. The effect of hydrogen pressure, frequency and mechanical loading parameters (ΔK, Sa) on fatigue crack initiation and propagation was analyzed. Attention was paid on the definition of “initiation” and influence of hydrogen on the relative parts of initiation and propagation on the fatigue life of a component. A good correlation between results with various test types was found. Particularly promising was the match between the measured — and estimated — crack growth rates in laboratory specimens and vessels. This supports our proposal for a safe design procedure based on crack growth and defect tolerant approach. Recommendations for implementation in a new international standard, on how to properly address hydrogen enhanced fatigue based on laboratory tests, were given and will be summarized in this presentation. Our results indicate that crack initiation from inclusions or other small microstructural features is not necessarily affected by hydrogen to a similar extent as crack growth, but when initiated, the remaining life may be short due to fast growth. This is challenging for design and inspection rules to allow economically competitive construction of hydrogen equipment without compromising safety.</jats:p