Showing papers by "Fujio Abe published in 2001"

Creep behavior and stability of mx precipitates at high temperature in 9cr–0.5mo–1.8w–vnb steel

Abstract: The growth behavior of MX carbonitrides during aging and creep was investigated for 9Cr–0.5Mo–1.8W–VNb steel (ASME-P92). The stress exponent of minimum creep rate decreases with increasing testing temperature. The value of stress exponent is 5.7 at 1023 K over a wide range of stress examined, while the value is 12.7 at 923 K. The low stress exponent at 1023 K may be due to the growth of MX carbonitrides during creep. The MX carbonitrides grow to a size of 60 nm during aging at 1023 K for 400 h. The size is much larger than the critical value over which the coherent strain between the MX and matrix decreases. The growth rate of the MX carbonitrides is larger in gauge portion than in head portion of crept specimens, implying a stress or strain effect.

Creep rates and strengthening mechanisms in tungsten-strengthened 9Cr steels

Abstract: The creep deformation behavior has been investigated for simple 9Cr–W and solute modified 9Cr–WVTa steels containing high W, where Fe 2 W Laves phase precipitates during creep. Creep tests were carried out at 823, 873 and 923 K for up to 15 000 h. The precipitation of Fe 2 W effectively decreases a minimum creep rate, but the large coarsening of Fe 2 W promotes the acceleration of creep rate after reaching a minimum creep rate. As a result, the effect of Fe 2 W on the extension of creep rupture time is rather small. The fine MC carbides are significantly effective for the stabilization of lath martensitic microstructure.

Degradation of Creep Strength in Welded Joint of 9%Cr Steel

Abstract: The degradation of creep strength in the heat affected zone (HAZ) of welded joint has been investigated for a tungsten-strengthened 9Cr steel, 9Cr-0.5Mo-1.8W-VNb. The creep test was carried out for the simulated HAZ specimens and the welded joint at 923 K. The creep rupture strength of the welded joint is almost the same as that of the base metal at high stresses but it decreases rapidly and then it becomes almost the same as that of the Ac3 simulated HAZ specimen at low stresses. The creep fracture of the welded joint occurs at the fine-grained zone of HAZ, corresponding to the Ac3 heating, at low stresses. The fine-grained zone of HAZ contains higher density of dislocations than the base metal. The recovery of higher density of dislocations and the sparse distribution of large M23C6 carbides promote the formation of coarse subgrains near prior austenite grain boundaries. This results in the concentration of creep deformation in the coarse subgrains, which accelerates eventual creep fracture.

Microstructural evolution and change in hardness in type 304H stainless steel during long-term creep

Abstract: The microstructural evolution and the change in hardness have been investigated for 18Cr–8Ni (type 304H) stainless steel during long-term creep. Creep and creep-rupture tests were carried out at temperatures between 550 (823) and 750°C (1023 K) for up to 180 000 h. The hardening behavior during creep depends on the stress level, as well as the precipitation of M 23 C 6 carbides and σ phase. At a high stress of 177 MPa, the hardening during creep is much larger than the age hardening, indicating that the hardening during creep is mainly caused by the strain hardening due to creep deformation. At a later stage of creep, the softening occurs due to the recovery of excess dislocations, which becomes more significant with decreasing stress and increasing test duration. The strain hardening disappears with decreasing stress level and increasing test duration. At a low stress of 61 MPa or less, the hardening during creep can be approximately given by the age hardening under no stress, except for the final stage of creep.

Creep Properties affected by Morphology of MX in High-Cr Ferritic Steels

Abstract: MX-type carbonitrides such as (Nb, V)(C, N) are well known to be effective to improve creep properties of the heat resisting steels at elevated temperatures. The control of precipitation behavior of MX is essential to obtain the stable microstructure that is suitable for heat resisting steels in the Ultra Super Critical plants. In this study, strong relationship between creep properties and precipitation behavior of MX is investigated by using the high-Cr steels with different C/N-balance. MX is categorized to three types from their characteristic morphology, type I, II and III. It is found that the C/N-balance of steel is one of factors to determine which MX is dominant in the tempered steels. Presence of type III-MX that is formed by secondary precipitation of VN adhering to Nb(C, N) seems to be unfavorable to long-term creep resistance by considering relatively decreasing in distribution density of fine VN, type II-MX. Precipitation sites such as Nb(C, N), type I-MX and excess solute nitrogen promote growth of type III-MX at the tempering stage. We conclude that the control of precipitation kinetics of MX is quite important to achieve good creep properties at high temperatures.

High Temperature Strength. Microstructures and Creep Strength of Welded Joints for W Strengthened High Cr Ferritic Steel.

Abstract: On an increasing demand to reduce CO2 emissions and save energy, high strength ferritic heat resisting steels of 9-12%Cr have been developed with a view to elevating metal temperatures of thermal power plants. Type IV creep cracking in heat affected zone (HAZ) of welded joints is a serious problem for these high strengthened steels. The present paper investigates the microstructures and creep properties of welded joints for W strengthened 11Cr-0.4Mo-2W-CuVNb steel. Most of the welded joint specimens were ruptured at HAZ and creep lives decreased than that of base metals. The comparison of creep properties of simulated HAZ specimens showed that fine grains produced by heating around Ac3 temperature was responsible for such degradation of creep strength. The growth of precipitates on grain boundaries and recovery of dislocation structures during creep was faster for fine-grained HAZ. The electron beam (EB) welded joints indicated two times longer creep life than gas tungsten arc (GTA) welded joints, however the brittle Type IV fracture was occurred even in the EB welded joints for long-term creep test. The FEM creep analysis by using the creep data of simulated HAZ specimens could explain the experimental results for creep properties of welded joints.

Development of High Strength 15Cr Ferritic Creep Resistant Steel with Addition of Tungsten and Cobalt

Abstract: Effects of alloying elements of W and Co on a creep strength of full annealed 15Cr ferritic steel whose chemical composition is Fe-01C-15Cr-1Mo-3W-02V-005Nb-007N-0003B has been investigated Increases in creep strength with increase in W content and addition of Co have been obviously observed About 10 times and 30 times longer time to rupture than that of the base steel have been obtained at 923K-100MPa by increase in W content from 3 mass% to 6 mass% and addition of 3 mass% of Co, respectively Moreover, significant improvement of creep strength has been observed by the combination of increase in W content and addition of Co Time to rupture of the steel strengthened by 6 mass% of W and 3 mass% of Co is 100 times longer than that of the base steel at 923K-100MPa Creep rupture strength of the steel containing 6 mass % of W and 3 mass% of Co is almost the same as that of ASME T92 steel Since Cr content of conventional ferritic creep resistant steels is less than 12 mass%, oxidation resistance of the 15Cr ferritic steel investigated in this study is better than those of conventional ferritic creep resistant steels It has been concluded that a full annealed 15Cr ferritic creep resistant steel should be one of the candidate materials for new Ultra Supercritical (USC) power plant in the 21st century

Creep Deformation and the Corresponding Microstructural Evolution in High-Cr Ferritic Steels

Abstract: Creep deformation and the corresponding microstructural evolution in high-Cr ferritic steels has been studied using model steels with the initial microstructures consisting of the various combination of a, MX and M 23 C 6 to clarify the role of each precipitate on creep deformation and the resultant creep strength of the steels. A carbon free steel strengthened by the a phase has exhibited high creep resistance at the initial stage of transient creep region but a quick transition to the acceleration creep at a small strain, due to less precipitation along boundaries such as lath, block, packet and the prior austenite grain boundary. Precipitation of M 23 C 6 along these boundaries has been found to delay the transition to the acceleration creep, which gives a larger offset creep strain. Fine dispersion of MX has reduced much the creep rate in the transient creep region but enhances the heterogeneous creep deformation in the acceleration creep region. It is thus concluded that an optimum microstructure consists of fine dispersion of the a phase and MX carbonitride inside lath grain decorated with M 23 C 6 along lath, block, packet and the prior austenite grain boundary.

L10-disorder phase equilibria in the Fe-Pd system investigated by phenomenological calculation

Abstract: The creep strength of advanced ferritic heat-resistant steels for ultra super critical (USC) power plants is significantly improved by the addition of Pd. It has been found by a detailed transmission electron microscopy (TEM) observation that fine precipitation of an Fe-Pd based L10-ordered phase within a lath-martensite grain possessing a certain crystallographic orientation relationship with the matrix is reponsible for the strengthening mechanism. Phenomenological calculation by combining the cluster variation method (CVM) with a Lennard-Jones (L-J) type pair interaction energy is herein attempted for the Fe-Pd binary system to evaluate L10-disorder phase equilibria as an initial step to investigate ternary systems. Some of the unknown parameters in the Lennard-Jones pair interaction energies are determined with the help of computations based on a combination of a thermodynamic database and experimental measurement of the latent heat for the L10-disorder transition, the latter from differential scanning calorimetry. The experimental L10-disorder transition temperature is well reproduced by incorporating a tetragonal distortion of the L10-ordered phase into the calculation. In addition, detailed atomistic information, which is indispensable in designing and controlling the morphology of the L10-ordered phase, is obtained by the present calculation.

High chromium ferritic heat resisting steel and method of heat treatment for the same

Abstract: High chromium ferritic steel containing chromium (Cr) in amount of 7˜12 weight %, one or two elements of molybdenum (Mu) and tungsten (W) as a solid-solution strengthening element, and one or more of elements for forming a MX carbonitride(s) is heat-treated at a temperature higher than both solid-solution temperature of the element(s) for forming the MX carbonitride(s) and a precipitation starting temperature of 6 ferrite for 5 seconds or longer, cooled at a rate of 05° C/s or faster, and subsequently tempered

Development of high strength 15Cr creep resistant steel for low emission power plant.

Abstract: Creep strength property of a full-annealed 0.1C-15Cr-1Mo-3W 0.2V-0.05Nb-0.07N-0.003B steel and the effects of tungsten and cobalt on that have been investigated. Contrary to a tempered martensite microstructure of the conventional ferritic creep resistant steels, microstructure of full-annealed 15Cr steel is mainly ferrite. Increases in creep strength at 923K with increase in tungsten content and addition of cobalt have been obviously observed, especially with a combination of increase in tungsten content and addition of cobalt. Higher Cr content of full-annealed 15Cr steel than those of the conventional ferritic creep resistant steels is desirable for better oxidation resistance. It has been concluded that full-annealed 15Cr ferritic creep resistant steel should be one of the candidate materials for a low emission power plant.

Long-Term Creep Life Prediction Based on Understanding of Creep Deformation Behavior of Ferritic Heat Resistant Steels

Abstract: An understanding of long-term creep properties is important for safe designing and reliable assessment of structural components operating at high temperature. However, it is difficult to predict the long-term creep rupture life by extrapolating the short-term rupture data. The rupture life is a reflex of creep deformation behavior. The shape of creep curves is dependent on tested material, stress and temperature conditions [1], because the creep deformation is affected by the change in microstructure during creep. Therefore, the long-term creep life must be predicted based on the understanding of creep deformation behavior.

"In situ" Preparation of Replica for Microstructural Observation of Plant Components by Micro-sputtering Technique.

Abstract: Metallic microstructure inspection is essential in assuring long-life operation of steam power stations. In common chemical etching methods for microstructure observation, the etching conditions are not readily specifiable and delicate handling techniques are required. To overcome this difficulty a physical etching method is employed with micro-sputtering technique where an inner-magnet is provided with a portable evacuation system and a movable power supply. A specimen taken from the main Y-shaped steam pipe of a disassembled power plant was etched using this new apparatus and a replica of the pipe surface was observed by optical microscopy. The result of observation was as good as that obtained by the chemical etching performed under the most favorable condition.