Advanced Ferritic Steels Research Articles

Characterization of high temperature long-term fatigue properties for advanced ferritic heat-resisting steels

Characterization of high temperature long-term fatigue properties for advanced ferritic heat-resisting steels

Effect of precipitates on long-term creep deformation properties of P92 and P122 type advanced ferritic steels for USC power plants

Long-term creep rupture strengths and the microstructural stability of ASME P92 and P122 pipes have been studied using creep testing at the temperatures from 550 to 700 °C and detailed scanning transmission electron microscopy. Creep rupture strength of P92 is found to be more stable than that of P122 at temperatures over 600 °C, which is mainly due to the difference in their Cr content. P122 type model steel with reduced Cr content, 9%Cr, has been prepared to explore the effect of Cr on the stability of MX and formation of Z-phase during creep deformation. MX in 9%Cr steel is found to be stable even after prolonged exposure at 650 °C, while Cr and Fe concentration to MX without marked coarsening has been observed in 10.5%Cr steel after aging for 10,000 h at 650 °C. This seems to lead to the transition of MX carbonitride into the Z-phase after aging for 23,000 h, which requires ordering in a M 2N lattice to achieve a tetragonal Z-phase to be stable. Creep deformation behavior in the transient creep region of the steels is almost same up to about 7000 h, while in the acceleration creep region the creep rate of 10.5%Cr steel becomes much faster than that of 9%Cr steel, resulting in shorter rupture life. It is obvious that the creep rupture strength degradation starts prior to the formation of Z-phase in 10.5%Cr steel. It is thus concluded that Z-phase is not a necessary factor for degradation of creep rupture strength but the instability of the fine precipitates such as Cr 2(C, N) caused by the compositions change like Cr supply to MX carbonitride is more essential.

Creep – fatigue damage accumulation and interaction diagram based on metallographic interpretation of mechanisms

The deleterious effect of creep damage upon fatigue endurance in elevated temperature low cycle fatigue is well established and has become known as the ‘creep – fatigue interaction’. This has been identified as a narrow region where a transgranular crack becomes intergranular as creep damage develops. In other cases transgranular fatigue cracks and intergranular creep cavitation damage to some extent can take place separately (‘competitive’ mode) so that interaction is less pronounced. These phenomena have been incorporated into ‘damage interaction diagrams’ which feature in many high temperature Code cases (ASME, R5, RCC-MR etc.). Fractional creep and fatigue damage are considered separately, and failure is conceded when their sum is unity (‘additive’ mode). For some materials the sum is set to a value <1.It appears that purely empirical arguments have been used to arrive at total damage values <1. It is shown that such deviations from linearity can be simply predicted, assuming that creep damage accumulates uniformly during a given test. For maximum interaction, the resulting assessment curve is shown to pass through the points (total fatigue damage = 0.4; total creep damage = 0.4), or very nearly. The influence of fatigue upon creep damage is less well known. Examples are provided, and when included in the calculation, produce a more damaging curve passing through the point (0.33, 0.33). By use of ‘interaction coefficients’ less damaging curves for the ‘competitive’ model can be produced. The calculation may be simplified by use of an effective fatigue damage cycle number for use as a reference.Based on metallographic examples from power plant operation and laboratory testing, the ‘competitive’, ‘additive’ and ‘true interaction’ modes of creep and fatigue damage are identified. These can be related to practical applications of base load operation, fast start-up and shut-down procedures, slow start-up and shut-down procedures etc. Components might operate under base load conditions (creep) followed by temperature cycling (fatigue). These sequential events may not be interactive (as has been demonstrated in the laboratory) and thus use of the current assessment diagram would be pessimistic. Comment is made on a proposed damage diagram for the advanced ferritic steels, which is highly conservative and does not reflect their good performance in service and laboratory tests.

Creep Property Evaluation of Heat-Resistant Steels by Small Punch Creep Test

The recently developed small punch (SP) creep test was applied to four different heatresistant ferritic steels, namely, two kinds of conventional ferritic steels which had been actually used in the high-temperature components for long periods and two advanced high chromium ferritic steels for fusion reactor materials to investigate the applicability of the SP creep test. The ratio of the load of SP creep test to the stress of standard uniaxial creep test was calculated so that both the creep rupture curves (load/stress versus Larson-Miller parameter curves) were overlapped to convert the results of SP creep test into those of standard test. As a result, the ratio was determined to be 2.4, irrespective of the kind of ferritic steel. This result indicates that the creep rupture strength of heat-resistance ferritic steels can be estimated using a miniaturized plate-type specimen and this conversion coefficient 2.4 independent of the kind of ferritic steel.

Long-term creep deformation characteristics of advanced ferritic steels for USC power plants

Creep rupture and deformation behavior of T122-type steels with different matrix phases such as α′ (martensite) and α ′ + δ (martensite and delta-ferrite) at different stress levels has been studied using creep testing and a detailed FE-STEM microscopy. Long-term creep rupture strength of the dual phase steel is found to be lower than that expected from the short-term creep testing. Fine grain microstructure enhances the creep deformation of the dual phase steel at lower stress region, but is not the major factor on the degradation. Decrease in fine MX and unequal distribution of MX in the ferrite matrix are found to be the major causes of the strength degradation in the dual phase steel with the higher Cr content. It is thus concluded that the heterogeneous creep deformation is much more pronounced at lower stress level in the dual phase steel, which is due to inhomogeneous microstructure consisting of α ′ + δ phase matrix and the relevant heterogeneous distribution of fine precipitates such as MX and M 23C 6 in the δ-ferrite matrix and near the interface between δ / γ .

Alloy design and creep strength of advanced 9%Cr USC boiler steels containing high concentration of boron

The research and development project of National Institute for Materials Science (NIMS) for advanced ferritic heat resistant steels for 650°C ultra super critical (USC) plants, revealed that the addition of >0·01 mass% boron to a 0·08C–9Cr–3W–3Co–V–Nb–,0·003N steel remarkably improves the long term creep strength. Boron enriched in M23C6 carbides near prior austenite grain boundaries suppresses the coarsening of carbides during creep deformation, leading to excellent microstructural stability and creep strength. If creep strength was further improved by the addition of nitrogen, it was found to enhance precipitation of fine MX. Addition of excess nitrogen to the high boron containing steel was found to reduce creep rupture lives and ductility. This results from a decrease in the amount of effective boron, which is dissolved in M23C6 and suppresses its coarsening, resulting from the formation of coarse BN at normalising temperature. The highest creep strength is obtained with steel of the following composition: 0·08C–9Cr–3W–3Co–0·2V–0·05Nb–0·008N–0·014B (mass%), which has an improved creep strength compared to P92. The 105 h extrapolated creep rupture strength at 650°C is ∼100 MPa. This steel also shows good creep ductility even in the long term. In conclusion, high boron bearing 9Cr–3W–3Co–V–Nb steel combined with the addition of 0·008 mass% nitrogen is a promising candidate for thick section components in the 650°C USC plants as it shows superior creep strength without impaired creep ductility.

Improvement of compatibility of advanced ferritic steels with super critical pressurized water toward a higher thermally efficient water-cooled blanket system

Various oxide dispersion strengthened (ODS) steels which have high Cr concentrations with and without Al addition were made by mechanical alloying method. Corrosion measurement was done in the closed system of super critical pressurized water (SCPW) at 783 K with the pressure of 25 MPa. In order to evaluate the mechanical properties and the effect of thermal aging, a miniaturized Charpy V notch (MCVN) test and tensile test were performed before and after aging at 773 K up to 1000 h. The susceptibility to stress corrosion cracking (SCC) of high Cr-ODS steels was evaluated by slow strain rate test (SSRT). The SSRT was performed in a simulated boiling water reactor (BWR) condition with high temperature oxygenated (8–10 ppm) water at 561 K and at a pressure of 7.8 MPa. Strain rates were ranging from 1 × 10 −4 to 3 × 10 −7 s −1. The high Cr-ODS steels showed a better resistance to corrosion than SUS316L stainless steel in SCPW. The 14Cr and 16Cr-ODS steel did not suffer from aging embrittlement at 773 K for 1000 h. The ODS steels showed no susceptibility to SCC at strain rates from 1 × 10 −4 to 3 × 10 −7 s −1.

05/01526 Molten salt self-cooled solid first wall and blanketdesign based on advanced ferritic steel

05/01526 Molten salt self-cooled solid first wall and blanketdesign based on advanced ferritic steel

05/01516 Engineering and geometric aspects of the solidwall re-circulating fluid blanket based on advanced ferritic steel

05/01516 Engineering and geometric aspects of the solidwall re-circulating fluid blanket based on advanced ferritic steel

Improvement of Creep Strength of Advanced Ferritic Steel Welded Joints

The microstructure and creep strength of simulated HAZ specimens and welded joints have been investigated for advanced 9–12 %Cr steels in order to make clear the mechanisms responsible for Type IV crack and to improve the creep strength of welded joints at high temperature. Creep and creep rupture tests were carried out at 650 °C (923 K) for up to about 104 h. Creep crack growth tests were also carried out using CT specimens. The creep rupture time of simulated HAZ specimens has its minimum after heating to AC3 temperature, which produces fine-grained microstructure. Reducing the width of HAZ by means of EB welding is effective for the extension of creep life but the brittle Type IV fracture appears even in the EB welded joints at low stress and long time conditions. The welded joint specimens were fractured in fine-grained HAZ and resulted in shorter creep life than those of base metal due to the formation of creep voids and cracks. Using a specially-designed FEM code for Type IV crack growth behaviour, the effect of creep ductility and void formation ahead of the crack tip on creep crack growth rate is successfully simulated. The Type IV fracture is suppressed by the addition of boron, which suppresses the formation of fine-grained region in HAZ.

Thermal helium desorption behavior in advanced ferritic steels

Thermal helium desorption measurements were performed to investigate the difference in the helium trapping and accumulation behavior among a reduced activation ferritic (RAF) steel and oxide dispersion strengthening (ODS) steels after implantation of He + ions at room temperature. Thermal helium desorption spectra (THDS) were obtained during annealing to 1200 °C at a heating rate of 1 °C/s. The THDS of the ODS steels are very similar to that of the RAF steel, except for the presence of the peak in the temperature range from 800 to 1000 °C, where the α–γ transformation related helium desorption from the γ-phase is considered to occur in the 9Cr-ODS martensitic steels. The fraction of helium desorption becomes larger at higher temperatures, and this trend is increased with the amount of implanted helium. In the 9Cr-ODS steels, the fraction of helium desorption by bubble migration mechanism was smaller than that in the RAF steel. This suggests that the bubble formation was suppressed in the ODS steels. In the 12Cr-ODS steel, the fraction of helium desorption by bubble migration reached more than 90%, suggesting that the trapping capacity of martensite phase in the 9Cr-ODS steel is rather large.

Safety assessment of two advanced ferritic steel molten salt blanket design concepts

In this article, we explore some of the safety issues associated with two advanced ferritic steel (AFS) molten salt blanket designs from the Advanced Power Extraction (APEX) design study [M.A. Abdou, The APEX Team, On the exploration of innovative concepts for fusion chamber technology, Fus. Eng. Des. 54 (2000) 181]. In particular, we examine radiological inventories, decay heat, waste disposal ratings, and toxic chemical inventories of these design concepts. In addition, we predict the thermal response of these blanket designs during accident conditions, and the mobilization of the radiological inventories and site boundary dose from the release of this mobilized material during a worst-case confinement-boundary-bypass accident. The molten salts being proposed for these blanket concepts are Flibe and Flinabe, and the structural material is a nano-composite strengthened ferritic steel alloy called 12YWT. The estimated dose at the site boundary is less than the no-evacuation limit of 10 mSv for a ground level release during conservative weather conditions if plant isolation occurs within 5 days.

Molten salt self-cooled solid first wall and blanket design based on advanced ferritic steel

Abstract As an element in the U.S. Advanced Power Extraction (APEX) program, the solid first wall and blanket design team assessed innovative design configurations with the use of advanced nano-composite ferritic steel (AFS) as the structural material and FLiBe as the tritium breeder and coolant. The goal for the assessment is to search for designs that can have high volumetric power density and surface heat-flux handling capability, with assurance of fuel self-sufficiency, high thermal efficiency and passive safety for a tokamak power reactor. We selected the re-circulating flow configuration as our reference design. Based on the recommended material properties of AFS we found that the reference design can handle a maximum surface heat flux of 1 MW/m2, and a maximum neutron wall loading of 5.4 MW/m2, with a gross thermal efficiency of 47%, while meeting all the tritium breeding, structural design and passive safety requirements. This paper will cover the results of the following areas of assessment: material design properties, FW/blanket design configuration, materials compatibility, components fabrication, neutronics analysis, thermal-hydraulics analysis including MHD effects, structural analysis; molten salt and helium closed cycle power conversion system; and safety and waste disposal of the re-circulating coolant first wall and blanket design.

Engineering and geometric aspects of the solid wall re-circulating fluid blanket based on advanced ferritic steel

Abstract The Advanced Power Extraction (APEX) project has been exploring concepts for power producing blankets that can enhance the potential of fusion energy. The pursued concepts cover both liquid and solid wall designs. The solid wall blanket branch of the project has been concentrating on the use of nano-composited ferritic (NCF) steel structure coupled with Flibe (Li 2 BeF 4 ) coolant. The present blanket is a solid wall design with an innovative coolant scheme, which allows part of the coolant to be re-circulated in order to enhance the outlet temperature, and thus improve the power cycle efficiency. The structure is 12YWT, an oxide dispersion strengthened (ODS) ferritic steel, which has a maximum operating temperature of 800 °C and a compatibility with Flibe up to 700 °C. Several methods for fabricating the complex geometry of the first wall (FW) and blanket are presented. Preliminary coolant routing is proposed with solutions offered for minimizing heat losses and simplifying assembly and maintenance. Even though this blanket is somewhat complicated, its forward looking aims, that of maximizing nuclear performance to achieve a high thermal conversion efficiency, are well worth striving for.

APEX advanced ferritic steel, Flibe self-cooled first wall and blanket design

As an element in the US Advanced Power Extraction (APEX) program, we evaluated the design option of using advanced nanocomposite ferritic steel (AFS) as the structural material and Flibe as the tritium breeder and coolant. We selected the recirculating flow configuration as our reference design. Based on the material properties of AFS, we found that the reference design can handle a maximum surface heat flux of 1 MW/m 2, and a maximum neutron wall loading of 5.4 MW/m 2, with a gross thermal efficiency of 47%, while meeting all the tritium breeding and structural design requirements. This paper covers the results of the following areas of evaluation: materials selection, first wall and blanket design configuration, materials compatibility, components fabrication, neutronics analysis, thermal hydraulics analysis including MHD effects, structural analysis, molten salt and helium closed cycle power conversion system, and safety and waste disposal of the recirculating coolant design.

Microstructure and creep strength of welds in advanced ferritic power plant steels

The microstructure and creep strength of simulated heat affected zone (HAZ) specimens and welded joints have been investigated for advanced 9-12%Cr steels in order to understand the mechanisms responsible for Type IV cracks and to improve the creep strength of welded joints at high temperature. The creep and creep rupture tests were carried out at 650° C (923 K) for up to about 104 h. The creep crack growth tests were also carried out for welded joints, base metal and simulated HAZ specimens using the CT specimens. The creep rupture time of simulated HAZ specimens has its minimum after heating to AC3 temperature, which produces fine-grained martensitic microstructure. Decreasing the width of HAZ by means of electron beam (EB) welding is effective for the extension of creep life but the brittle Type IV fracture appears even in the EB welded joints at low stress and long time conditions. Most of the welded joint specimens were fractured in fine-grained HAZ and resulted in shorter creep life than those of base metals as a result of the formation of creep voids and cracks. It should also be noted that in the fine-grained zone, the recovery of martensitic microstructure during creep is inhomogeneous as shown by the formation of coarse subgrains in the region of fine subgrains. Using a specially designed FEM code for Type IV crack growth behaviour, the vacancy diffusion under multi-axial stress conditions of welded joints in HAZ is analysed. The effect of creep ductility and void formation ahead of the crack tip on creep crack growth rate is successfully simulated.

Microstructure Aspects of Creep Resistant Welded Joints

A soft zone in the welded joint, negatively influencing the creep behaviour, was detected and localised in the intercritical zone of the heat affected zone (HAZ), limiting the long-time creep strength of welds. Energy-filtering transmission electron microscopy (EFTEM) has been used for imaging precipitates and grain boundary phases in the soft zone of the HAZ and in the weld deposit of advanced ferritic martensitic Cr steels. Due to different creep duration times at a temperature of 600°C, many different precipitates and grain boundary phases and a new type of Cr-V-Nb-N rich precipitation, the so-called modified Z phase emerge in the joint. The appearance of the Cr and V rich Z phase was followed by a decrease of the population of M23C6 and MX precipitates. According to these results a decrease of the creep strength in the soft zone of the HAZ beginning after about 10,000 h was found, which seems to be in connection with the observed microstructural changes. This effect must be taken into account in the design of welded components made from this type of materials. Basic investigations using mainly the Gleeble HAZ simulation technique corroborate a drop in the hardness (soft zone) and creep resistance that occurred in the inter critical zone of the HAZ, where the peak temperature reached a level of about 900–1,000°C (zone of α/γ-transformation). This and previous investigations have shown a drop in the creep resistance for the HAZ. Compared with the base material, reductions of the creep resistance in weld samples of about 20–25% have to be taken into account. At lower stress values, the fracture location shifts from the base material towards the softened intercritical zone of the HAZ (type IV cracking). The microstructure of the HAZ shows after stressed condition pores and micro-cracks in the softened intercritical zone (temperature between Ac1 and Ac3). A correlation between the creep resistance and the size distribution of particular particle populations could not be clearly proven. For the total evaluation of the microstructural development the reduction of the dislocation density and the recovery processes (formation and growth of subgrain boundaries) must be taken into account. The tests were made on cast G-X12 CrMoWVNbN 10.1.1 parent material and Cromocord 10M weld deposit

A Helical Coolant Channel Design for the Solid Wall Blanket

ABSTRACTA helical coolant channel scheme is proposed for the APEX solid wall blanket module. The self-coolant breeder in this system is FLIBE (LiF)2(BeF2). The structural material is the nanocomposited alloy 12YWT. The neutron multiplier used in the current design is either stationary or slow moving liquid lead. The purpose of this study is to design a blanket that can handle a high wall loading (5 MW/m2). In the mean time the design provides means to attain the maximum possible blanket outlet temperature and meet all engineering limits on temperature of structural material and liquids. An important issue for such a design is to optimize the system for minimum pressure loss. For advanced ferritic steel (12YWT) an upper temperature limit of 800°C is expected, and a limit of 700°C at the steel/FLIBE interface is recommended.The blanket module is composed of two main continuous routes. The first route is three helical rectangular channels side-by-side that surround a central box. The helical channels are fed from the bottom and exit at the top to feed the central channels in the central box. The coolant helical channels have a cross sectional area with a length of about 10 cm and a width that changes according to the position around the central box. For instance: the width of the coolant channels facing the plasma is the narrowest while it is the widest in the back (farthest from the plasma).In this design the coolant runs around the central box for only 5 turns to cover the total height of the first wall (6.8 m). The design is optimized with the FW channel width as a parameter with the heat transfer requirements at the first wall as the constraints.

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

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.

Progress in the United States programme to develop low activation structural materials for fusion

It has long been recognized that attainment of the safety and environmental potential of fusion energy requires the successful development of low activation materials for the first wall, blanket and other high heat flux structural components. Only a limited number of materials potentially possess the physical, mechanical and low activation characteristics required for this application. The current US structural materials research effort is focused on three candidate materials: advanced ferritic steels, vanadium alloys and silicon carbide composites. Recent progress has been made in understanding the response of these materials to neutron irradiation.