The effect of carbon on 2.25 Cr-1 Mo steel: (II). Creep—rupture properties

The creep-rupture properties of normalized-and-tempered 2.25 Cr-1 Mo steel with 0.009, 0.030, 0.120, and 0.135 wt % C were determined at 454, 510, and 565°C; the results show an effect of carbon that depends on temperature. At all three temperatures, the 0.120 and 0.135 wt % C steels have similar strengths, and at 565°C are considerably stronger than the 0.009 and 0.030 wt % C steels, which also have similar strengths. At 454 and 510°C, however, the properties for the 0.030 wt % C steel are superior to those of the 0.009 wt % C steel and approach those for the 0.120 and 0.135 wt % C steels. These observations can be explained in terms of the type of carbide precipitation reactions that occur in 2.25 Cr-1 Mo steel.     

Heat treatment effects on the tensile properties of annealed 2.25 Cr-1 Mo steel

The effect of heat treatment on the tensile properties of annealed 2.25 Cr-1 Mo steel was investigated. Detailed tensile properties were determined from 25 to 593°C and strain rates between 2.67 × 10^?6 and 6.67 × 10^?3/s on steel plates (from a single heat) given three different heat treatments. For all heat treatments, dynamic strain-aging peaks were observed between 200 and 400°C. The peak height for the annealed material that was cooled fastest was largest and occurred at the highest temperature. The dynamic strain aging was concluded to be the result of interaction solid solution hardening in the proeutectoid ferrite and involves interactions between molybdenum and carbon atoms or atom clusters with dislocations. The difference in the dynamic strain-aging effects for the different heat treatments was explained in terms of the precipitation reactions that occur during the heat treatment.     

Elevated-temperature tensile properties of irradiated 214 Cr-1 Mo steel

The effect of neutron irradiation on the tensile properties of normalized-and-tempered $\text{2}\text{1}\text{4}$ Cr-1 Mo steel was determined for specimens irradiated in Experimental Breeder Reactor II (EBR-II) at 390 to 550°C. Two types of unirradiated control specimens were tested: as-heat-treated specimens and as-heat-treated specimens aged for 5000 h at the irradiation temperatures. Irradiation to approximately 9 dpa at 390° C increased the strength and decreased the ductility compared to the control specimens. Softening occurred in samples irradiated and tested at temperatures of 450, 500, and 550 °C; the amount of softening increased with increasing temperature. The tensile results were explained in terms of the displacement damage caused by the irradiation and changes in carbide precipitates that occur during elevated-temperature exposure.     

Elevated-temperature tensile properties of irradiated 9 Cr-1 MoVNb steel

Normalized-and-tempered 9 Cr-1 MoVNb steel tensile specimens were irradiated in the Experimental Breeder Reactor-11 (EBR-11) at 390, 450, 500, and 550°C to ～2.1 and 2.5 × 10²⁶ neutrons/m² (> 0.1 MeV), which produced displacement damage levels of ～10 and 12 dpa, respectively. Tensile tests were conducted at the irradiation temperature and at room temperature. In addition to the irradiated specimens, as-heat-treated specimens and as-heat-treated specimens thermally aged at the irradiation for 5000 h were also tested.Thermal aging had no effect on the unirradiated tensile properties. Irradiation at 390°C increased the 0.2% yield stress and the ultimate tensile strength above those of the unirradiated control specimens. The ductility decreased slightly. After irradiation at 450, 500, and 550°C, the tensile properties were essentially the same as the unirradiated values. The hardening at 390°C was attributed to the dislocation and precipitate structure formed during the irradiation. The lack of hardening at 450°C and higher correlates with an absence of an irradiation-induced damage structure.     

High-temperature mechanical properties improvement on modified 9Cr-1Mo martensitic steel through thermomechanical treatments

In the framework of the development of generation IV nuclear reactors and fusion nuclear reactors, materials with an improved high temperature (≅650 °C) mechanical strength are required for specific components. The 9-12%Cr martensitic steels are candidate for these applications. Thermomechanical treatments including normalisation at elevated temperature (1150 °C), followed by warm-rolling in metastable austenitic phase and tempering, have been applied on the commercial Grade 91 martensitic steel in order to refine its microstructure and to improve its precipitation state. The temperature of the warm-rolling was set at 600 °C, and those of the tempering heat-treatment at 650 °C and 700 °C thanks to MatCalc software calculations. Microstructural observations proved that the warm-rolling and the following tempering heat-treatment lead to a finer martensitic microstructure pinned with numerous small carbide and nitride particles. The hardness values of thermomechanically treated Grade 91 steel are higher than those of the as-received Grade 91. It is also shown that the yield stress and the ductility of the thermomechanically treated Grade 91 steel are significantly improved compared to the as-received material. Preliminary creep results showed that these thermomechanical treatments improve the creep lifetime by at least a factor 14.     

Tertiary creep behavior of annealed 214 Cr-1 Mo steel

Under various creep conditions for annealed $\text{2}\text{1}\text{4}$ Cr-1 Mo steel, nonclassical creep curves that contain two steady-state stages were observed. The transition from the first to second steady-state stage involves a quasi-tertiary (increasing creep rate) stage, thus complicating the definition of tertiary creep. Tertiary creep is important because it is often associated with the formation of gross structural instability (i.e., the formation of cracks, voids, or a neck). The present studies indicated a consistent correlation between the onset of tertiary creep and rupture life was obtained when the end of the second steady-state stage was used as the onset of tertiary creep for the nonclassical curves. The creep strains to the end of the second steady-state stage were similar to those to the end of the secondary stage of the classical curves. These results along with previous work indicate that the creep rate during the second steady-state stage of the nonclassical curves is controlled by the same process that controls creep during the secondary stage of a classical curve.     

Creep behaviour of modified 9Cr-1Mo ferritic steel

Creep deformation and fracture behaviour of indigenously developed modified 9Cr-1Mo steel for steam generator (SG) tube application has been examined at 823, 848 and 873 K. Creep tests were performed on flat creep specimens machined from normalised and tempered SG tubes at stresses ranging from 125 to 275 MPa. The stress dependence of minimum creep rate obeyed Norton’s power law. Similarly, the rupture life dependence on stress obeyed a power law. The fracture mode remained transgranular at all test conditions examined. The analysis of creep data indicated that the steel obey Monkman-Grant and modified Monkman-Grant relationships and display high creep damage tolerance factor. The tertiary creep was examined in terms of the variations of time to onset of tertiary creep with rupture life, and a recently proposed concept of time to reach Monkman-Grant ductility, and its relationship with rupture life that depends only on damage tolerance factor. SG tube steel exhibited creep-rupture strength comparable to those reported in literature and specified in the nuclear design code RCC-MR.     

Effect of sodium environment on the low cycle fatigue properties of modified 9Cr-1Mo ferritic martensitic steel

Modified 9Cr-1Mo ferritic steel is the material of current interest for the steam generator components of liquid metal cooled fast breeder reactors (LMFBRs). The steam generator has been designed to operate for 30-40 years. It is important to accurately determine the life of the components in the actual environment in order to consider the extension of life beyond the design life. With this objective in view, a programme has been initiated at our laboratory to evaluate the effects of flowing sodium on the LCF behaviour of modified 9Cr-1Mo steel. LCF tests conducted in flowing sodium environment at 823 K and 873 K exhibited cyclic softening behaviour both in air and sodium environments. The fatigue lives are significantly improved in sodium environment when compared to the data obtained in air environment under identical testing conditions. The lack of oxidation in sodium environment is considered to be responsible for the delayed crack initiation and consequent increase in fatigue life. Comparison of experimental lifetimes with RCC-MR design code predictions indicated that the design curve based on air tests is too conservative.     

Development of thick wall 2.25Cr-1MoNiNb steel forgings for steam generators of fast breeder reactors

The large and thick forgings made of 2.25Cr-1MoNiNb steel are required for vessel material of steam generator (SG) of the fast breeder reactor SNR 300.In order to study the feasibility of 2.25Cr-1MoNiNb steel heavy section forging, chemical composition, melting practice and ingot making, hot working and heat treatment conditions were investigated. The following recommendations were derived: (1) 0.04% C, 0.10% ΔNb, (2) application of electro-slag remelting process, (3) grain refining by hot working, (4) two-step austenitizing at 1020°C.Based on these recommendations, the actual products such as hollow cylinders with maximum 290 mm thickness, solid bars with 420 mm diameter and forged plates with maximum 185 mm thickness could be supplied for application in the helical coiled SG of the SNR 300. Statistical analysis of the products showed the sufficient and isotropic material properties, which fulfill the requirements of the basic safety rules.     

Influence of dynamic sodium environment on the creep-fatigue behaviour of Modified 9Cr-1Mo ferritic-martensitic steel

The use of liquid sodium as a heat transfer medium for sodium-cooled fast reactors (SFRs) necessitates a clear understanding of the effects of dynamic sodium on low cycle fatigue (LCF), creep and creep-fatigue interaction (CFI) behaviour of reactor structural materials. Mod. 9Cr-1Mo ferritic steel is the material of current interest for the steam generator components of sodium cooled fast reactors. The steam generator has a design life of 30-40 years. The effects of dynamic sodium on the LCF and CFI behaviour of Mod. 9Cr-1Mo steel have been investigated at 823 and 873 K. The CFI life of the steel showed marginal increase under flowing sodium environment when compared to air environment. Hence, the design rules for creep-fatigue interaction based on air tests can be safely applied for components operating in sodium environment. This paper attempts to explain the observed LCF and CFI results based on the detailed metallography and fractography conducted on the failed samples.     

Presentation of the properties of the ferritic alloy 2.25 Cr-1 MoNiNb for steam generator application in FBR systems

2.25 Cr-1 MoNiNb steel, used for the construction of the steam generator of a fast breeder reactor, is subjected to operation at elevated temperature in the creep range. Although this operation condition is a limiting factor for allowable loads during normal operation, it is necessary to have sufficient knowledge of the strength-properties of this kind of steel after thermal aging. Experiences with this steel are described. It is shown that this stabilized ferritic steel reveals the common behaviour of all ferritic steels at elevated temperature. The change of mechanical properties can easily be quantified. Special attention was given to the analysis of strain-controlled tensile testing and the uniform elongation of s.g.-tubing.     

Fatigue-creep life prediction of Cr-1Mo steel under combined tension-torsion at 600°C

The results of fatigue-creep life prediction for Cr-1Mo steel under multiaxial stress conditions of combined tension-compression and cyclic torsion are summarized. This work was performed as the second task A-II of the cooperative project of phase 3 by the Subcommittee on Inelastic Analysis and Life Prediction of High Temperature Materials, JSMS. The ratios of axial to torsional strain range, Δε/Δ(γ/3^1/2), were set to be non-unity in these phase 3 tests for both in-phase and out-of-phase (with 90° phase difference) strain-controlled wave patterns, including pure torsion, while the ratios were unity in phase 2. By comparing the evaluated failure lives with the corresponding data of phase 2, some discussion on the effect of the wave patterns is presented and the validity of the life prediction methods is evaluated.     

Inelastic stress-strain response for notched specimen of Cr-1Mo steel at 600°C

Following a series of cooperative studies A-I and A-II (phase III) concerning the inelastic behaviour of high temperature materials under uniform state of stress, finite element analyses were carried out on circumferential notched cylinders subjected to plasticity-creep interaction conditions. Using an electric capacitance type extensometer “Strain-Pecker”, which is capable of measuring a local strain response with a gauge length of 0.5 mm under high temperature conditions, stress-strain responses for both global and local regions near the notch root were evaluated. Ten kinds of inelastic constitutive model were introduced into a finite element code, and the responses for four kinds of loading pattern were examined for two types of notch shape.     

Microstructure and its influence on mechanical properties of CLAM steel

The microstructure of China Low Activation Martensitic steel (CLAM) and its influence on mechanical properties were investigated. The tensile test showed that the strength of CLAM (HEAT 0603A) was higher than that of HEAT 0408B at room temperature, and the reverse results were obtained at elevated temperatures. The results indicated that the microstructure was composed of dispersived carbide particles and lath martensite with high dislocation density. The main precipitation phases were Cr-rich M₂₃C₆ carbides precipitated mainly along the lath boundaries and prior-austenite grain boundaries and Ta-rich MX particles precipitated mainly in the laths and lath boundaries. The finer lath was the main reason for the higher strength of HEAT 0603A compared with HEAT 0408B at room temperature; contrasted with the lower strength at high temperature. Heavier hot forging deformation degree was considered as the main possible reason for the decrease of martensite lath width in HEAT 0603A.     

Microstructure changes and properties of TiC-coated carbon fiber-reinforced carbon composites

In the present paper, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to study distortion of TiC crystals after thermal cycles in plasma environment. Scanning electron microscopy (SEM) was used to observe morphology changes and penetrating cracks in TiC/C coatings. To avoid the cracks and enhance properties of coated carbon fiber-reinforced carbon (C/C) composites, TiC/C composites were prepared as buffer layer to relieve thermal stresses. Thermal cycles indicated that the buffer layer could effectively improve thermal shock resistance of pure TiC coated C/C composites. To study the reason, transmission electron microscopy (TEM) results suggested that TiC particles were uniformly imbedded in pyrocarbon in the buffer layer, which was advantageous to relieve mismatch of coefficient of thermal expansion (CTE) between pure TiC and C/C. Moreover, thermal conductivity tests showed that the buffer layer was in favor of transferring heat loading.     

The effect of thermal history on the tensile properties of polycrystalline zircaloy-2

The effect of heating rate, prior to tensile testing, on the elevated temperature flow stress of zircaloy-2 was investigated in the temperature interval 473 to 923 K. An increase in the heating time from 30 to 120 min caused a decrease in the 0.2% flow stress of approximately 4 MPa (3%) which is within the experimental error involved in the measurement of this property by a single test. Similarly, the holding time, prior to testing, was found to have only a minor influence on the flow stress. Recently published work has reported that thermal history had a much greater effect than this and reasons for the discrepancy are proposed.     

Effect of tensile test conditions on the fracture behaviour of 20Cr-25Ni-Nb stabilised stainless steel after high-temperature irradiation

Tensile specimens were cut from 20 Cr-25 Ni-Nb stabilised austenitic steel tubes irradiated at 650–700 °C, in contact with UO₂ fuel at their inner surfaces and CO₂ at their outer surfaces, to a thermal neutron dose between 1 and 10 × 10²⁰ nvt. Tensile tests were made at temperatures between 450 °C and 800 °C using three strain-rates in thp range 1.4 × 10⁻³to 1.4 × 10⁻⁶/sec. The results were correlated using the expression σtⁿ = Kεenp (Q/RT), where σt is the flow stress and n is an index which proves to be close to 7. Under test conditions giving full recovery-creep an activation energy Qof ∼ 68.5 kcal/mole was obtained for the irradiated specimens and ∼ 80 kcal/mole for unirradiated thermal control specimens. The tensile elongation and rupture mode of the specimens were also related to the Zener-Holloman parameter Z = log₁₀ [εexp (Q/RT)]. The intergranular fracture mode of the irradiated specimens varied from development of micro-cracks at the higher values of Z to cavitation at lower values. The reduced ductility observed on the irradiated tubing is discussed in terms of the effect of helium from the ¹⁰B(n, < α)⁷Li reaction on intergranular crack and cavity development. The fission-fragment-bombarded zone at the tube bore was ∼ 10 μm thick and contained numerous gas bubbles of 20–100 Å dia. This zone apparently had little overall effect on the ductility of the tubing.     

The Influence of lithium environment on tensile behavior and microstructure of V-4Cr-4Ti

Thermal tests exposed V-4Cr-4Ti in static liquid lithium at 700 and 800 °C for 250, 500, and 1000 h. Post-exposure examination included chemical analysis of interstitial impurities in V-4Cr-4Ti to monitor impurity transfer, and tensile tests at room temperature and 500 °C. Microstructures were characterized by room-temperature electrical resistivity measurements and transmission electron microscopy. Oxygen was not depleted from V-4Cr-4Ti when nitrogen pickup occurred during lithium exposures. In spite of a significant increase in interstitial impurity concentration, the matrix interstitial solute content was reduced due to precipitation. Plate-shaped precipitates in the matrix and globular precipitates at grain boundaries were formed during lithium exposures at 700 °C, while only globular precipitates were observed at grain boundaries at 800 °C. Increases in strength, decreases in ductility, and reduced dynamic strain aging resulted. Ductility remained high after 1000 h exposures at both 700 and 800 °C.     

钇对9Cr-2WVTa低活化马氏体钢力学性能的影响

研究了稀土合金元素钇对聚变堆用低活化马氏体钢9Cr-2WVTa力学性能的影响,分析了钇在低活化马氏体钢中的冶金行为.研究结果表明,钇元素容易在钢中聚集形成富钇的块状夹杂,割裂了基体的连续性,容易破裂形成微孔;轧板中这些富钇块夹杂沿着轧制方向成条线状分布,使得在拉伸和冲击断裂时出现断口分层现象,降低了钢的力学性能.     

The effect of tensile stress on the growth of helium bubbles in an austenitic stainless steel

Foils of the ternary alloy, Fe-17 wt% Cr-17 wt% Ni, were cyclotron-injected with ~160 at ppm helium and annealed at 1023 K for times up to 1.74 Ms (482 h). Some foils were subjected to tensile stresses which ranged from 9.8 to 27.5 MPa during annealing while others were unstressed. Helium bubbles grew in unstressed specimens at different rates depending on their location in the microstructure; the largest bubbles were found at triple grain junctions, smaller ones on the grain boundaries, and the smallest in the grain matrices. Bubble size in the matrix was approximately proportional to $\text{t}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ for annealing times greater than 2.88 × 10⁴s (8 h). Applied tensile stress accelerated the growth of bubbles, especially at triple grain junctions and grain boundaries. The enhanced growth in these areas appear to be associated with grain-boundary sliding. The number of bubbles in the matrix and on grain boundaries generally decreased with annealing time, suggesting that a migration-coalescence mechanism was also operating.