Papers by Keyword: M23C6 Carbide

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Authors: Fujio Abe
Abstract: The effect of fine precipitates, excess dislocations and sub-boundary hardening on creep strain behavior in the transient region has been investigated for tempered martensitic 9%Cr steel at 600 and 650oC. The fine precipitates that form during tempering or during creep decrease the creep rate in the transient region, while excess dislocations produced by cold rolling promote the recovery of dislocations during creep, resulting in higher creep rates. The sub-boundary hardening is enhanced by fine precipitates along lath and block boundaries, which retards the onset of acceleration creep. The movement and annihilation process of dislocations in the transient region is controlled by not only the movement of dislocations in the matrix but also the absorption of dislocations at boundaries. The minimum creep rate is basically determined by the time to minimum creep rate.
Authors: Fujio Abe
Abstract: The effect of boron on microstructure evolution and creep deformation behavior has been investigated for a tempered martensitic 9Cr-3W-3Co-0.2V-0.05Nb steel at 650oC. Creep tests were carried out at 650oC for up to about 6 x 104 h. The addition of boron retards the onset of acceleration creep at low stress and long time conditions, which results in lower minimum creep rate and longer time to rupture. The addition of boron also retards the Ostwald ripening of M23C6 carbides near prior austenite grain boundaries (PAGBs) during creep. The retardation of the onset of acceleration creep results from the retardation of the recovery of martensitic microstructure near PAGBs by pinning effects due to fine M23C6 carbides. The main effect due to boron is considered to occupy vacancies near growing M23C6 carbides, which makes it difficult to accommodate local volume change around the growing carbides. This reduces the rate of Ostwald ripening of M23C6 carbides.
Authors: Fujio Abe, H. Semba, T. Sakuraya
Abstract: The effect of boron on microstructure and creep deformation behavior has been investigated for a tempered martensitic 9Cr-3WVNb steel with emphasis on the role of boron free from boron nitrides. Creep tests were carried out at 650oC for up to about 3 x 104 h, using specimens of 10 mm in gauge diameter and 50 mm in gauge length. The addition of boron in combination with no nitrogen addition effectively reduces the coarsening rate of M23C6 carbides by an enrichment of boron in M23C6 particles in the vicinity of prior austenite grain boundaries during creep at 650oC. This stabilizes martensitic microstructure during creep and retards the onset of acceleration creep, resulting in a decrease in minimum creep rate and an increase in creep life. Excess addition of boron and nitrogen causes the formation of boron nitrides during normalizing at 1050-1150oC, which reduces dissolved boron and nitrogen. The dissolved boron enriches in M23C6 carbides, while the dissolved nitrogen causes the precipitation of fine MX carbonitrides. The variation of creep rates in transient region and of the onset time of acceleration creep with various combinations of boron and nitrogen contents can be explained by the dissolved boron and nitrogen concentrations after normalizing into account.
Authors: Alexandra Fedoseeva, Nadezhda Dudova, Rustam Kaibyshev
Abstract: Microstructural evolution in a 9Cr-3Co-3W-0.2V-0.06Nb-0.05N-0.005B steel crept at T=650°C under an applied stress of 140 MPa up to strains of 1, 3, 5.75 and 12%, which represent primary, secondary and tertiary creep stages and rupture, respectively, was studied. The steel was initially normalized from 1050°C, and finally tempered at 750°C for 3h. After tempering the boundaries of tempered martensite lath structure (TMLS) were decorated by M23C6 carbides, M6C carbides and Laves phase particles. The 3% W additives provide the narrow size distribution of the boundary particles excepting M6C carbides. The depletion of thermodynamically none-equilibrium content of W from the solid solution during creep leads to following events. (i) Continuous precipitation of small Laves phase particles occurs during all creep stages and results in the formation of bimodal size distribution. As a result, the average size of Laves phase particles remains unchanged during creep. (ii) Coarsening of M23C6 carbides starts to occur only at the transition to tertiary creep. (iii) Transformation of laths to subgrains followed by their growth is observed during all stages of creep. The density of particle located at lath/subgrain boundaries decreases from 5.6 to 2.6 μm-1 during creep up to rupture. However, no full transformation of TMLS into subgrain structure has been revealed.
Authors: M. Murata, Hisao Tanaka, G.R. Booker, Hiroshi Irie
Authors: Xian Chao Hao, Bo Chen, Ying Che Ma, Kui Liu
Abstract: A series of heat treatments were preformed to study the evolution of microstructure in an austenitic nickel-base Alloy 690 using optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the pre-existed chromium carbides of cold-rolled Alloy 690 were dissolved into the austenitic matrix after high temperature mill annealing (MA) at 1353K for 5min. Mill-annealed and thermally treated Alloy 690 had intergranular M23C6-type carbides. The morphology of intergranular M23C6 varied from continuous, semi-continuous to discrete with the temperature of thermal-treatment (TT) increasing from 873K to 1073K. The holding time at temperature of 988K had less influence on the intergranular carbide appearance than that of temperature. The minimum chromium concentration in the vicinity of intergranular carbides was 22.3 wt% in the sample thermally treated at 715°C for 2h. With the holding time of TT increasing from 2h to 27h, Cr concentration increased gradually and the chromium depletion became wider.
Authors: Nadezhda Dudova, Roman Mishnev, Rustam Kaibyshev
Abstract: A 10%Cr martensitic steel with 3%Co and 0.008%B tempered at 770°C exhibits no creep strength breakdown at a temperature of 650°C up to an extremely high rupture time of ∼4×104 h under an applied stress of 120 MPa. The minimum creep rate was ∼3×10-11 s-1. Microstructural characterization showed that superior creep resistance associated with a high stability of tempered martensite lath structure. Boundary M23(B⋅C)6 phase particles are highly stable against coarsening under long-term aging and creep conditions. These particles retain their orientation relationship with ferritic matrix unchanged under creep at a temperature of 650°C. As a result, no migration of lath boundaries and their transformation to subboundaries diminishing the long-range elastic stress fields take place. The role of M(C,N) carbonitrides in achieving extraordinary high creep strength consists in hindering the knitting reaction between mobile lattice dislocations and lath boundaries.
Authors: Ana Isabel Martinez-Ubeda, A.D. Warren, Ian Griffiths, Peter E.J. Flewitt
Abstract: A significant factor that influences the creep life of AISI Type 316H austenitic stainless steel components such as headers, and tubes is the initial microstructure. These components typically have a comparable specified composition but different thermo-mechanical fabrication histories. The variations in composition within the nominal range result in initial microstructures which become increasingly divergent during ageing. In this paper we explore effect of these contributions on the long term service aged microstructure and discuss the resulting impact on the overall creep life of these components. The microstructure of specific regions has been characterised with a range of techniques, including high resolution transmission electron microscopy imaging and chemical analyses undertaken using a JEOL ARM instrument operating at 200 KeV fitted with an energy dispersive spectrometer. This provides a unique identification of the service aged precipitates and the distribution of alloying and impurity elements. The results are discussed with respect to the initiation of creep cavities and the associated creep damage accumulation in the context of lifetime assessment of these AISI Type 316H austenitic stainless steel boiler components.
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