| 1. |
- Erneman, J., et al.
(författare)
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Comparison between quantitative metallography and modeling of sigma-phase particle growth in AISI 347 stainless steel
- 2005
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Ingår i: Metallurgical and Materials Transactions. A. - : Springer Science and Business Media LLC. - 1073-5623 .- 1543-1940. ; 36A:10, s. 2595-2600
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Tidskriftsartikel (refereegranskat)abstract
- A comparison was made between two experimental methods to determine the (T-phase volume fraction and three methods to model a-phase growth in a niobium-stabilized stainless steel (AISI 347). The a-phase volume fraction and precipitate size were determined in material statically aged and creep deformed at 700 degrees C with both KOH etched specimens using bright field optical microscopy (OM/BF) (conventional method) and specimens etched with oxalic acid using scanning electron microscopy and backscattered electrons (SEM/BSE) (new method). Both experimental methods used manual thresholding together with digital image analysis. The calculations were made with DICTRA software, using both the TCFE database and the SSOL database with some modification concerning the effect of silicon on the stability of sigma-phase particles. The modeled sigma-phase volume- fractions showed rather good agreement with the measured results from statically aged material using the new method. It was found that the stabilizing effect of silicon on sigma phase should be included in the thermodynamic database used for modeling.
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| 2. |
- Erneman, J., et al.
(författare)
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Precipitation reactions caused by nitrogen uptake during service at high temperatures of a niobium stabilised austenitic steel
- 2004
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 52:14, s. 4337-4350
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Tidskriftsartikel (refereegranskat)abstract
- Precipitation phenomena in type 347 austenitic stainless steels have been investigated after long-term heat treatment and creep in air at 700 and 800 degreesC. Nitrogen uptake was observed during long-term creep testing at 800 degreesC. No such effect was observed at 700 degreesC although times up to about 70,000 h were used. The major phases precipitated after long time exposure at 800 degreesC were primary Nb(C,N), Z-phase, Cr2N and M23C6, while primary Nb(C,N), secondary Nb(C,N) and sigma-phase were the major phases at 700 degreesC. Z-phase precipitated in both intragranular and intergranular form at 800 degreesC. Large precipitates exhibiting a core/rim structure showed a rim of Z-phase surrounding undissolved primary Nb(C,N). The microstructural evolution during creep deformation in air at 800 degreesC was modelled thermodynamically. The model satisfactorily predicts nitrogen uptake and the essential features of the evolution of the microstructure with time. The precipitation sequence could be qualitatively described, although it was not possible to model the formation of all precipitates.
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| 3. |
- Erneman, J., et al.
(författare)
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The evolution of primary and secondary niobium carbonitrides in AISI 347 stainless steel during manufacturing and long-term ageing
- 2006
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 54:1, s. 67-76
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Tidskriftsartikel (refereegranskat)abstract
- Nb(C,N) precipitates were studied in a niobium-stabilised stainless steel (AISI 347) statically aged at 700 degrees C. Scanning electron microscopy and energy filtered transmission electron microscopy were used to determine the volume fraction and precipitate size of primary and secondary Nb(C,N) after ageing times between 0 and 70,000 h. The experimental data were correlated with simulations of Nb(C,N) formation based on the assumption that the process is controlled by diffusion. These simulations provide a rationale for the existence of two sets of mobium carbonitrides in commercial tubes of AISI 347. Growth of primary Nb(C,N) occurred essentially during manufacturing, with no significant growth at 700 degrees C. Rapid dissolution and re-precipitation of secondary Nb(C,N) occurred during manufacturing. Coarsening at 700 degrees C of secondary particles was modelled using the Lifshitz-Slyozov-Wagner theory, which overestimated the coarsening rate. These problems were overcome with a model developed by the authors. This model takes both growth and coarsening into account.
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| 4. |
- Schwind, M., et al.
(författare)
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sigma-phase precipitation in stabilized austenitic stainless steels
- 2000
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 48:10, s. 2473-2481
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Tidskriftsartikel (refereegranskat)abstract
- Experimental observations of sigma-phase precipitation in two stabilized austenitic stainless steers, AISI 321 and AISI 347, aged up to 80,000 h at temperatures between 500 and 800 degrees C, are compared with the results obtained from a simple model of the process. The model is based on the assumption that diffusion of alloying elements from inside austenite grains to sigma-phase particles at grain boundaries limits the rate of formation, and this is supported by the presence of zones depleted in chromium at grain boundaries revealed by TEM/EDS. The full multicomponent thermodynamic behaviour of the system is taken into account using the DICTRA software and the influence of grain shape is discussed. The predictions of the model are in fair agreement with experimentally estimated Volume fractions of sigma-phase.
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| 5. |
- Ekroth, M., et al.
(författare)
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Gradient zones in WC-Ti(C,N)-Co-based cemented carbides : Experimental study and computer simulations
- 2000
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 48:9, s. 2177-2185
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Tidskriftsartikel (refereegranskat)abstract
- The formation of surface zones with a composition gradient during sintering of WC-Ti(C,N)-Co cemented carbides has been studied experimentally and by computer simulations. The microstructure has been investigated with SEM and EPMA. The simulations are based on a solution of the multicomponent diffusion equations, coupled with thermodynamic calculations using thermodynamic descriptions of the individual phases. The results from the simulations are in good agreement with the experimental results, indicating that diffusion and the thermodynamic properties are the two major factors that control the gradient structure formation.
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| 6. |
- Frykholm, R., et al.
(författare)
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A new labyrinth factor for modelling the effect of binder volume fraction on gradient sintering of cemented carbides
- 2003
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 51:4, s. 1115-1121
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Tidskriftsartikel (refereegranskat)abstract
- To increase the cutting performance of WC-MC-Co cemented carbide tools, it is common to use a high temperature CVD process to coat them with thin wear resistant layers. During the process cracks are unavoidably introduced in the coating. To prevent crack propagation it is of interest to create a tough surface zone in the substrate, enriched in WC and binder phase. A way to create such a zone is to sinter a nitrogen-containing cemented carbide in a nitrogen free atmosphere. This formation of gradient structures has been extensively studied using microscopy and simulations, and it has been shown that the process is driven by diffusion in the binder phase. However, the diffusion paths are partly blocked by the dispersed particles. This effect can be formally handled by considering effective diffusivities by introducing a so-called labyrinth factor, lambda. In prior work it has been assumed that lambda =f(2) where f is the volume fraction of the binder. The validity of this assumption has been studied by simulations and experimental analysis of gradient sintered WC-Ti(C,N)-Co cemented carbides containing 5.0, 6.7, 10.0 and 20.0 vol% binder phase. It was found that by using the labyrinth factor lambda = f(2) instead of a better correspondence between experiments and simulations can be achieved.
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| 7. |
- Frykholm, R., et al.
(författare)
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Effect of cubic phase composition on gradient zone formation in cemented carbides
- 2001
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Ingår i: International journal of refractory metals & hard materials. - 0263-4368. ; 19:06-apr, s. 527-538
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Tidskriftsartikel (refereegranskat)abstract
- Cemented carbide cutting tool inserts with a surface zone depleted of hard cubic carbides and enriched in ductile binder phase have been studied experimentally using scanning electron microscopy (SEM), electron probe micro analysis (EPMA) and transmission electron microscopy (TEM). The results are compared with simulations based on a solution of the multicomponent diffusion equations, coupled with calculations using thermodynamic descriptions of the individual phases. The materials in the study are based on WC-Ti(C,N)-Co, WC-Ti(C,N)-NbC-Co and WC-Ti(C,N)-TaC-Co. The surface zone is formed by creating a gradient in nitrogen activity in the material, leading to an outward diffusion of N. Due to thermodynamical coupling between N and Ti, the outward diffusion of N will lead to an inward diffusion of Ti, and a surface zone depleted in cubic carbides is formed. Additions of elements like Ta or Nb will affect the width of the surface zone. A material with a Ta-containing cubic phase will have a narrower surface zone than a material with a Nb-containing cubic phase. Ta or Nb additions also affect the distribution of the different phases in and adjacent to the surface zone.
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| 8. |
- Katterer, T., et al.
(författare)
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Pedotransfer functions for estimating plant available water and bulk density in Swedish agricultural soils
- 2006
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Ingår i: Acta Agriculturae Scandinavica - Section B. - : Informa UK Limited. - 0906-4710 .- 1651-1913. ; 56:4, s. 263-276
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Tidskriftsartikel (refereegranskat)abstract
- Pedotransfer functions (PTFs) to estimate plant available water were developed from a database of arable soils in Sweden. The PTFs were developed to fulfil the minimum requirements of any agro-hydrological application, i.e., soil water content at wilting point (theta(wp)) and field capacity (theta(fc)),from information that frequently is available from soil surveys such as texture and soil organic carbon content (SOC). From the same variables we also estimated bulk density (rho) and porosity (epsilon), which seldom are included in surveys, but are needed for calculating element mass balances. The seven particle-size classes given in the data set were aggregated in different ways to match information commonly gained from surveys. Analysis of covariance and stepwise multiple linear regression were used for quantifying the influence of depth, particle size class, textural class and soil organic carbon on the characteristic variables. PTFs developed from other data sets were also tested and their goodness-of-fit and bias was evaluated. These functions and those developed for the Swedish database were also tested on an independent data set and finally ranked according to their goodness of fit. Among single independent variables, clay was the best predictor for theta(wp), sand ( or the sum of clay and silt) for theta(fc) and SOC for rho and epsilon. A large fraction of the variation in theta(wp) and theta(fc) is explained by soil texture and SOC ( up to 90%) and root mean square errors (RMSEs) were as small as 0.03 m(3) water m(-3) soil in the best models. For the prediction of rho and epsilon in the test data set, the best PTF could only explain 40-43% of the total variance with corresponding RMSEs of 0.14 g cm(-3) and 5.3% by volume, respectively. Recently presented PTFs derived from a North American database performed very well for estimating theta(wp) ( low error and bias) and could be recommended for Swedish soils if measurements of clay, sand and SOC were available. Although somewhat less accurately, also theta(fc) could be estimated satisfactorily. This indicates that the determination of plant available water by texture and SOC is rather independent of soil genesis and that certain PTFs are transferable between continents.
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