Study of the microstructure of the grade 91 steel after more than 100.000h of creep exposure at 600°C

International audienceThis paper presents results on the evolution of microstructure (both matrix and precipitates) of an ASME Grade 91 steel that has been creep tested for 113,431 h at 600 °C under a load of 80 MPa. The microstructure was investigated using transmission electron microscopy (TEM) and revealed chromium rich M23C6 carbides, MX-type precipitates, Laves phases and modified Z-phases. Only a small amount of modified Z-phase was found. In order to quantify coarsening of precipitates and growth of new phases during creep, the size distributions of the identified precipitates were determined by analysis of TEM images. In addition to this, the size distribution of Laves phases was determined by image analysis of scanning electron micrographs. Substructure modifications and creep damage were investigated on cross sections of the creep specimen using Electron Backscatter Diffraction and Scanning Electron Microscopy

Evolution of dislocation density, size of subgrains and MX-type precipitates in a P91 steel during creep and during thermal ageing at 600 °C for more than 100,000 h

International audienceThere are rather few quantitative data on the microstructure of the 9–12%Cr heat resistant steels after long-term creep. This paper presents results of the quantitative measurement of the size of MX precipitates, subgrain size and dislocation density in a P91 steel that had been creep tested for 113,431 h at 600 °C. The same measurements were conducted in the same P91 steel in the as received conditions. Transmission electron microscopy investigations were conducted using thin foils and revealed a decrease in dislocation density and an increase in subgrain size after creep exposure. MX carbonitrides are very stable during thermal and creep exposure of P91 steel at 600 °C up to 113,431 h. Electron backscatter diffraction (EBSD) investigations also revealed a significant change in the substructure of the steel after creep exposure

Long term aging effect on the creep strength of the T92 steel

International audienceCreep strength loss of T92 steel after long-term creep exposure at 600°C and 650°C is partially due to a thermal aging of the steel during the first part of the test. In order to quantify the effect of long-term aging on the creep strength loss, creep tests were conducted at 600 and 650°C on T92 steel thermally aged for 10,000h at the same temperature and on as-received T92 steel. Laves phases precipitates were found after thermal aging at 600°C and 650°C with an average equivalent diameter of about 200nm and of about 350nm, respectively. No significant change in hardness and in the matrix substructure as revealed by electron backscatter diffraction occurred during aging. For stresses higher than 170MPa at 600°C and higher than 110MPa at 650°C the time to rupture is four times lower in the aged steels compared to the as-received steel, this is correlated to a secondary creep rate four times higher for the aged specimens compared to that of the as-received steel. Creep tests conducted at 650°C under lower stresses revealed a creep lifetime only twice lower after aging

Évolution métallurgique et résistance en fluage à 600°C et 650°C d'aciers à 9-12% Cr

9-12%Cr tempered martensite steels are used for high temperature (400-600°C) especially applications in components of fossil power plants, such as tubes, pipes, heaters. These components are exposed to creep, thermal aging, fatigue, oxidation, corrosion. The development of advanced heat resistant steels with improved long-term creep strength relies on a better understanding of the long-term microstructural evolution and of its influence on the creep strength. This study aims at better understanding of the effect of microstructural evolution on long-term creep strength at 600°C and 650°C of a Grade 92 steel. There are rather few published data on the microstructure of the Grade 92 steel after long-term creep or thermal aging exposure (times higher than 10,000h) both at 600°C and 650°C. Thus, in a first part, P92 steel specimens that had been creep tested for times up to 50,000h at 600°C and 650°C were investigated using transmission electron microscopy on extractive replicas of precipitates, scanning electron microscopy and electron backscatter diffraction to get data on the microstructure of the Grade 92 steel after long-term exposure. These investigations revealed significant precipitation of Laves phases, recovery of the matrix and little precipitation of modified Z-phase. A quantification of the Laves phases and creep damage were realized by image analysis of scanning electron micrographs. Significant precipitation of Laves phase and recovery of the matrix seem to be the most prominent microstructural degradation mechanisms. To separately investigate the effect of each of these two mechanisms on creep strength, creep tests were conducted on thermally aged and thermo-mechanically prepared creep specimens. Creep tests were also conducted on notched specimens. Thermally aged creep specimens enable to study the effect of large Laves phases on the creep strength. A time to rupture four times lower was observed on the aged creep specimens compared to a standard creep test for same testing conditions. This seems not be confirmed, however, at low stresses. Two creep specimens thermo-mechanically prepared by creep-fatigue prestraining at 550°C were used to study the effect of the matrix substructure on the creep strength. A time to rupture twice lower was observed at 600°C (short-term creep) on a thermo-mechanically prepared creep specimen compared to a standard one for the same testing conditions. No such effect was detected at 650°C in the low stress regime. Creep tests were also conducted on notched specimens with various notch shapes to study the effect of stress triaxiality on creep damage. In notched specimens higher amounts of creep damage were observed compared to smooth specimens for similar testing time. A finite element mechanical model coupling microstructural evolution and creep damage was proposed to estimate the long-term creep strength of the Grade 92 steel at 600°C and 650°C. The model taking stress triaxiality into account might also be used to estimate the remaining life of service components and to analyze service components with complex geometry more sensitive to creep damage due to stress triaxiality.Les aciers martensitiques à 9-12% Cr sont utilisés pour applications à haute température notamment comme composants dans des centrales thermiques de production de l'électricité. Ces composants sont soumis à des phénomènes de fluage, vieillissement thermique, fatigue, oxydation, corrosion. Cette étude vise une meilleure compréhension de l'évolution métallurgique à long terme des aciers martensitiques à 9-12% Cr, ainsi que de son influence sur la résistance en fluage à 600 et 650°C. Le matériau de l'étude est l'acier Grade 92. La littérature manque de données quantitatives sur la microstructure de l'acier Grade 92 après des temps prolongés d'exposition au fluage ou au vieillissement thermique (temps supérieurs à 10.000h) à 600°C et 650°C. Par conséquent, dans un premier temps une expertise d'éprouvettes de fluage testées à 600°C et 650°C pendant des temps allant jusqu'à 50.000h a été réalisée par microscopie électronique en transmission (MET) sur des répliques extractives, microscopie électronique à balayage (MEB) et par diffraction des électrons rétrodiffusés. Cette expertise a révélé une précipitation significative de phase de Laves, une restauration de la matrice et une très faible précipitation de la phase Z-modifiée. Une quantification de l'endommagement et des phases de Laves a été réalisée à partir des micrographies MEB. La précipitation significative des phases de Laves et la restauration de la matrice semblent être les mécanismes prédominants de l'évolution métallurgique à 600°C et 650°C de l'acier Grade 92. Afin d'étudier séparément l'influence de chacun de ces deux mécanismes sur la résistance au fluage, des essais de fluage ont été réalisés sur des éprouvettes pré-vieillies et sur des éprouvettes pré-fatiguées. Des éprouvettes entaillées ont été également utilisées. L'intérêt des éprouvettes pré-vieillies est d'étudier l'influence de grosses phases de Laves sur la résistance au fluage. Le temps à rupture des éprouvettes pré-vieillies est quatre fois plus faible que celui des éprouvettes standard pour les mêmes niveaux de contraintes. Ceci n'est toutefois pas confirmé pour les contraintes les plus faibles. Deux essais de fluage ont été réalisés sur des éprouvettes pré-fatiguées à 550°C pour étudier l'effet de l'état de la matrice sur la résistance au fluage. Un temps à rupture deux fois plus faible a été observé à 600°C sur une éprouvette pré-fatiguée comparée à une éprouvette standard pour le même niveau de contrainte. Cet effet n'a pas été observé à 650°C pour les contraintes faibles. Des éprouvettes entaillées ont été également utilisées pour étudier l'influence du taux de triaxialité des contraintes sur le développement de l'endommagement par fluage. Pour des temps de fluage comparables, l'endommagement est plus développé dans une éprouvette entaillée que dans une éprouvette lisse. Un modèle mécanique qui prend en compte l'évolution métallurgique de l'acier a été développé pour estimer la résistance au fluage " à long terme " de l'acier Grade 92 à 600°C et 650°C. Le modèle prend également en compte le taux de triaxialité des contraintes. A ce titre, il peut être utilisé pour estimer la durée de vie des composants en service ou pour analyser les composants avec une géométrie complexe qui serait plus sensible au développement de l'endommagement par fluage, du fait d'une triaxialité des contraintes plus élevée

Microstructure evolution in base metal and welded joint of Grade 91 martensitic steels after creep at 500-600°C

2-5 septembre 2008International audienceThe microstructure of grade 91 base metal after long term creep exposure at 600°C and after short-term exposure at 500°C was examined...

Creep strength and microstructural evolution of 9-12%Cr heat resistant steels during creep exposure at 600°C and 650°C

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Nutritional status at age 1 year in patients born with esophageal atresia: A population-based, prospective cohort study

International audienceObjective Despite recent progress in caring for patients born with esophageal atresia (EA), undernutrition and stunting remain common. Our study objective was to assess nutritional status in the first year after birth with EA and to identify factors associated with growth failure. Study design We conducted a population-based study of all infants born in France with EA between 2010 and 2016. Through the national EA register, we collected prenatal to 1 year follow-up data. We used body mass index and length-for-age ratio Z scores to define patients who were undernourished and stunted, respectively. Factors with P < 0.20 in univariate analyses were retained in a logistic regression model. Results Among 1,154 patients born with EA, body mass index and length-for-age ratio Z scores at 1 year were available for about 61%. Among these, 15.2% were undernourished and 19% were stunted at the age of 1 year. There was no significant catch-up between ages 6 months and 1 year. Patients born preterm (41%), small for gestational age (17%), or with associated abnormalities (55%) were at higher risk of undernutrition and stunting at age 1 year ( P < 0.05). Neither EA type nor surgical treatment was associated with growth failure. Conclusion Undernutrition and stunting are common during the first year after birth in patients born with EA. These outcomes are significantly influenced by early factors, regardless of EA type or surgical management. Identifying high-risk patient groups with EA (i.e., those born preterm, small for gestational age, and/or with associated abnormalities) may guide early nutritional support strategies

Nutritional status at age 1 year in patients born with esophageal atresia: A population-based, prospective cohort study

International audienceObjective Despite recent progress in caring for patients born with esophageal atresia (EA), undernutrition and stunting remain common. Our study objective was to assess nutritional status in the first year after birth with EA and to identify factors associated with growth failure. Study design We conducted a population-based study of all infants born in France with EA between 2010 and 2016. Through the national EA register, we collected prenatal to 1 year follow-up data. We used body mass index and length-for-age ratio Z scores to define patients who were undernourished and stunted, respectively. Factors with P < 0.20 in univariate analyses were retained in a logistic regression model.Results Among 1,154 patients born with EA, body mass index and length-for-age ratio Z scores at 1 year were available for about 61%. Among these, 15.2% were undernourished and 19% were stunted at the age of 1 year. There was no significant catch-up between ages 6 months and 1 year. Patients born preterm (41%), small for gestational age (17%), or with associated abnormalities (55%) were at higher risk of undernutrition and stunting at age 1 year ( P < 0.05). Neither EA type nor surgical treatment was associated with growth failure.Conclusion Undernutrition and stunting are common during the first year after birth in patients born with EA. These outcomes are significantly influenced by early factors, regardless of EA type or surgical management. Identifying high-risk patient groups with EA (i.e., those born preterm, small for gestational age, and/or with associated abnormalities) may guide early nutritional support strategies

Global economic burden of unmet surgical need for appendicitis

Background There is a substantial gap in provision of adequate surgical care in many low- and middle-income countries. This study aimed to identify the economic burden of unmet surgical need for the common condition of appendicitis. Methods Data on the incidence of appendicitis from 170 countries and two different approaches were used to estimate numbers of patients who do not receive surgery: as a fixed proportion of the total unmet surgical need per country (approach 1); and based on country income status (approach 2). Indirect costs with current levels of access and local quality, and those if quality were at the standards of high-income countries, were estimated. A human capital approach was applied, focusing on the economic burden resulting from premature death and absenteeism. Results Excess mortality was 4185 per 100 000 cases of appendicitis using approach 1 and 3448 per 100 000 using approach 2. The economic burden of continuing current levels of access and local quality was US $92 492 million using approach 1 and$73 141 million using approach 2. The economic burden of not providing surgical care to the standards of high-income countries was $95 004 million using approach 1 and$75 666 million using approach 2. The largest share of these costs resulted from premature death (97.7 per cent) and lack of access (97.0 per cent) in contrast to lack of quality. Conclusion For a comparatively non-complex emergency condition such as appendicitis, increasing access to care should be prioritized. Although improving quality of care should not be neglected, increasing provision of care at current standards could reduce societal costs substantially