Isothermal Grain Growth Studies on Nanostructured 9Cr-1Mo and 9Cr-1W Ferritic Steels Containing Nano-sized Oxide Dispersoids

Analysis of isothermal grain growth kinetics of nanocrystalline Fe-9Cr-1Mo and Fe-9Cr-1W-based ferritic oxide dispersion strengthened alloys is reported. Fe-9Cr-1Mo-0.25Ti-0.5Y₂O₃ alloy exhibited ~900 and ~250 pct enhancement in grain-coarsening resistance at 1073 K (800 °C) in comparison with Fe-9Cr-1Mo-0.5Y₂O₃ alloy and Fe-9Cr-1W-0.5Y₂O₃ alloy, respectively. Comparison of grain growth time exponents also revealed that addition of Ti and Y₂O₃ to nanocrystalline Fe-9Cr alloy has significantly enhanced the grain growth resistance. This is attributed to the possible presence of Y-Ti-O-based nanoclusters (<5 nm).     

Analysis of Tensile Stress-Strain and Work-Hardening Behavior in 9Cr-1Mo Ferritic Steel

Detailed analysis on tensile true stress (??)-true plastic strain (??) and work-hardening behavior of 9Cr-1Mo steel have been performed in the framework of the Voce relationship and Kocks-Mecking approach for wide range of temperatures, 300 K to 873 K (27 °C to 600 °C) and strain rates (6.33 × 10^?5 to 6.33 × 10^?3 s^?1). At all test conditions, ??-?? data were adequately described by the Voce equation. 9Cr-1Mo steel exhibited two-stage work-hardening behavior characterized by a rapid decrease in instantaneous work-hardening rate (?? = d??/d??) with stress at low stresses (transient stage) followed by a gradual decrease in ?? at high stresses (stage III). The variations of work-hardening parameters and ??-?? as a function of temperature and strain rate exhibited three distinct temperature regimes. Both work-hardening parameters and ??-?? displayed signatures of dynamic strain aging at intermediate temperatures and dominance of dynamic recovery at high temperatures. Excellent correlations have been obtained between work-hardening parameters evaluated using the Voce relationship and the respective tensile properties. A comparison of work-hardening parameters obtained using the Voce equation and Kocks-Mecking approach suggested an analogy between the two for the steel.     

Evaluation of Interface Boundaries in 9Cr-1Mo Steel After Thermal and Thermomechanical Treatments

The grain boundary character distribution (GBCD) and microstructure in 9Cr-1Mo ferritic/martensitic steel subjected to different heat treatments and thermomechanical treatments (TMTs) have been evaluated using electron backscatter diffraction (EBSD) technique. Microstructures obtained through displacive transformation of high-temperature austenite yielded higher amounts of Σ1-29 coincidence site lattice (CSL) boundaries (from 29 to 38 pct) compared with the ferrite grains obtained by diffusional transformation (~16 pct) or by recrystallization process (~14 pct). Specifically, the low-angle (Σ1), Σ3, Σ11, and Σ25b boundaries were enhanced in the tempered martensite substructure, whereas the prior austenite grain boundaries were largely of random type. Misorientation between the product ferrite variants for ideal orientation relationships during austenite transformation was calculated and compared with CSL misorientation to find its proximity based on Brandon’s criteria. The observed enhancements in Σ1, Σ3, and Σ11 could be interpreted based on Kurdjumov–Sachs (K–S) relation, but Nishiyama–Wassermann (N–W) relation was needed to understand Σ25b formation. The amounts of CSL boundaries in the tempered martensite structure were not significantly influenced by austenite grain size or the kinetics of martensitic transformation. In mixed microstructures of “polygonal ferrite + tempered martensite”, the frequencies of CSL boundaries were found to systematically decrease with increasing amounts of diffusional/recrystallized ferrite.     

Modification in the Microstructure of Mod. 9Cr-1Mo Ferritic Martensitic Steel Exposed to Sodium

Mod. 9Cr-1Mo is used as the structural material in the steam generator circuit of liquid metal-cooled fast breeder reactors. Microstructural modifications on the surface of this steel are investigated after exposing to flowing sodium at a temperature of 798 K (525 °C) for 16000 hours. Sodium exposure results in the carburization of the ferritic steel up to a depth of ~218 µm from the surface. Electron microprobe analysis revealed the existence of two separate zones with appreciable difference in microchemistry within the carburized layer. Differences in the type, morphology, volume fraction, and microchemistry of the carbides present in the two zones are investigated using analytical transmission electron microscopy. Formation of separate zones within the carburized layer is understood as a combined effect of leaching, diffusion of the alloying elements, and thermal aging. Chromium concentration on the surface in the α-phase suggested possible degradation in the corrosion resistance of the steel. Further, concentration-dependent diffusivities for carbon are determined in the base material and carburized zones using Hall’s and den Broeder’s methods, respectively. These are given as inputs for simulating the concentration profiles for carbon using numerical computation technique based on finite difference method. Predicted thickness of the carburized zone agrees reasonably well with that of experiment.     

钒对18Cr-2Mo铁素体不锈钢性能的影响

 利用Thermo-Calc热力学软件、扫描电子显微镜、X射线衍射仪及透射电镜等分析方法,研究了不同钒含量对单铌稳定18Cr-2Mo铁素体不锈钢微观组织、力学性能和点腐蚀性能的影响规律。结果表明：添加钒后的析出相主要是NbC和VN,弥散分布于晶内和晶界,其中NbC的尺寸为100~250nm;随着钒含量的增加,试验钢的晶粒尺寸及屈服强度变化不明显,而冲击韧性及点腐蚀性能逐步得到改善和提高,当wV=0.36%时,钢的韧脆?湮露扔稍嫉?10℃下降至-30℃,点腐蚀电位由0.38V提高至0.42V。     

Effect of Constraint on Creep Behavior of 9Cr-1Mo Steel

The effect of constraint on creep rupture behavior of 9Cr-1Mo steel has been investigated. The constraint was introduced by incorporating a circumferential U-notch in a plain cylindrical creep specimen of 5 mm diameter. The degree of constraint was increased by decreasing the notch root radius from 5 to 0.25 mm. Creep tests were conducted on plain and notched specimens at stresses in the range of 110 to 210 MPa at 873 K (600 °C). The creep rupture life of the steel was found to increase under constrained conditions, which increased with the increase in degree of constraint and applied stress, and tended to saturate at a higher degree of constraint. The creep rupture ductility (pct reduction in area) of the steel was found to be lower under constrained conditions. The decrease in creep ductility was more pronounced at a higher degree of constraint and lower applied stresses. Scanning electron microscopic studies revealed a change in fracture behavior with stress and degree of constraint. The fracture surface appearance for relatively lower constrained specimens at higher stresses was predominantly transgranular dimple. Creep cavitation-induced intergranular brittle fracture near the notch root was observed for specimens having a higher degree of constraint at relatively lower stresses. The creep rupture life of the steel under constrained conditions has been predicted based on the estimation of damage evolution by continuum damage mechanics coupled with finite element analysis of the triaxial state of stress across the notch. It was found that the creep rupture life of the steel under constrained conditions was predominantly governed by the von-Mises stress and the principal stress became progressively important with increase in the degree of constraint and decrease in applied stress.     

Phase-Dependent Tensile Properties of 9Cr-1Mo(V, Nb) Ferritic/Martensitic Steel

Phase-dependent tensile properties of 9Cr-1Mo(V, Nb) ferritic/martensitic steel were evaluated in the temperature range 300 K to 1073 K (27 °C to 800 °C) to quantify differences in the tensile behavior of different phases of this material. The results showed considerable difference in the tensile properties such as yield strength, ultimate tensile strength, and elongation of the three phases of this material—tempered martensite, metastable austenite, and martensite—which can exist at a common temperature. This has been discussed on the basis of structural hysteresis in this material when subjected to thermal cycles causing excursions across various phase fields.     

Effect of Compositional Changes of Laves Phase Precipitate on Grain Boundary Embrittlement in Long-Term Annealed 9 Pct Cr Ferritic Steel

In 9 to 12 pct chromium steels, the high-temperature mechanical properties are known to be strongly dependent on the formation and coarsening of Laves phase precipitates at boundaries. During high-temperature deformation, the Laves phase precipitate coarsening to over a critical size has been considered to trigger cavity formation at the precipitate-matrix interfaces. This coarsening, accompanied by the diffusion of W, Mo, and Cr, should change the mechanical properties and chemical composition of both Laves phase precipitates and the matrix. In this study, we aimed to clarify the effects of compositional changes of Laves phase precipitates on cavity formation during coarsening. The values of the Fe/Cr and W/Mo ratios in Laves phase precipitates were shown to induce different levels of strain energy in the vicinity of the Laves phase precipitate, consequently promoting the formation of cavities. Therefore, the compositional change of Laves phase precipitates was found to play a critical role in the grain boundary embrittlement of high Cr steel at elevated temperature.

     

铜对19Cr-1.6Mo铁素体不锈钢力学性能和腐蚀行为的影响

 采用力学性能测试、时效处理、电化学测试、显微硬度以及TEM微观分析等分析手段,研究了19Cr-1.6Mo和19Cr-1.6Mo-0.5Cu两种超纯铁素体不锈钢的力学性能和在3.5%NaCl腐蚀介质中的耐腐蚀性能。试验结果表明：合金元素铜的添加,提高了试验用钢的强度,同时降低了Δr值;随着时效时间的增加,铜析出相尺寸在不断的增加且均匀分布,基体的显微硬度由HV148增加到HV162;合金元素铜的添加降低了试验用钢在氯离子溶液下的耐点蚀能力,尤其是随着时效时间的增加,点蚀电位值由390mV降低到290mV,耐点蚀能力呈明显的下降趋势。     

Effect of Microalloy Precipitates on the Microstructure and Texture of Hot-Deformed Modified 9Cr-1Mo Steel

Microalloying elements like Nb and V are added to modified 9Cr-1Mo steel to ensure excellent creep resistance by the formation of fine MX precipitates during tempering treatment. The effect of those elements on the evolution of microstructure (and texture) in hot-deformed steel has hardly been studied. Industrial processing of modified 9Cr-1Mo steel often develops deformed and elongated prior-austenite grain structure, which can be detrimental from property point of view. The present study shows that the formation of such structure can primarily be attributed to the pinning effect from strain-induced Nb(C,N) precipitation, which can effectively retard the static recrystallization of deformed-γ at high-deformation temperature and short inter-pass times (~10 seconds). Based on the results, the application of either heavy deformation pass at high-temperature or multiple-lighter passes maintaining sufficient inter-pass interval (30 to 50 seconds) is recommended to achieve fine and equiaxed γ-grain structure by dynamic recrystallization and static recrystallization, respectively.

     

Effect of Y2O3 on Spark Plasma Sintering Kinetics of Nanocrystalline 9Cr-1Mo Ferritic Oxide Dispersion-Strengthened Steels

Nanocrystalline mechanically alloyed powders of 9Cr-1Mo ferritic steels with and without yttria dispersoids were densified using spark plasma sintering (SPS) to near-theoretical density at a temperature of 1073 K (800 °C). Studies on densification behaviour revealed that steels with dispersoids densified faster when compared to Fe-9Cr-1Mo steel. The evaluation of densification mechanisms during SPS reveals that grain boundary and lattice diffusion to be predominant at relative densities ranging from >0.7 to 0.9 in both the alloys.     

Effect of Thermomechanical Parameters on Σ3~n Grain Boundaries and Grain Boundary Networks of a New Superaustenitic Stainless Steel

Hot compression tests were conducted in a temperature range of 800-1 100℃and strain rate range of 0.1-10s-1 using a Gleeble 3500thermomechanical simulator to investigate the influence of hot deformation parameters(temperatures,strain rates and strains)on the grain boundary network evolution of a new grade Fe-Cr-Ni superaustenitic stainless steel.The results showed that a dominant effect of deformed temperature isΣ3n(n=0,1,2,3) boundaries population increased with decreasing temperature,while they first increased and then reduced with increasing strain and strain rate.Interestingly,besidesΣ3n(n=1,2,3)twin grain boundaries,someΣ1boundaries could interrupt grain boundaries network effectively,which enhance material performances.But they are scarcely reported.The misorientation of some segments LAGBs in the deformed microstructure(pancaked grains)increased and slid to high angle grain boundaries with increasing the fraction of recrystallized grains during hot deformation.     

Understanding the Effect of Grain Boundary Character on Dynamic Recrystallization in Stainless Steel 316L

Dynamic recrystallization (DRX) occurs during high-temperature deformation in metals and alloys with low to medium stacking fault energies. Previous simulations and experimental research have shown the effect of temperature and grain size on DRX behavior, but not the effect of the grain boundary character distribution. To investigate the effects of the distribution of grain boundary types, experimental testing was performed on stainless steel 316L specimens with different initial special boundary fractions (SBF). This work was completed in conjunction with computer simulations that used a modified Monte Carlo method which allowed for the addition of anisotropic grain boundary energies using orientation data from electron backscatter diffraction (EBSD). The correlation of the experimental and simulation work allows for a better understanding of how the input parameters in the simulations correspond to what occurs experimentally. Results from both simulations and experiments showed that a higher fraction of so-called “special” boundaries (e.g., Σ3 twin boundaries) delayed the onset of recrystallization to larger strains and that it is energetically favorable for nuclei to form on triple junctions without these so-called “special” boundaries.     

Magnetic Nondestructive Evaluation of Creep Behavior in Water-Quenched Modified 9Cr-1Mo Steel

Water-quenched modified 9Cr-1Mo steel was creep tested in the laboratory at a temperature of 600 °C and 125 MPa stress. Magnetic Barkhausen emissions (MBE) measurements were carried out by interrupting the test at different lengths of time. Creep damage in such steel was observed by an increase in root mean square (RMS) voltage of the MBE. The magnetic softening was corroborated with the decrease in pinning density in the material for the coarsening of carbides (M₂₃C₆) and formation of massive phases (Fe₂Mo), which comes at the expense of a large number of finer carbides. Before failure, the rate of increase in RMS voltage of the MBE decreased due to the demagnetizing field offered by the massive phases. The microstructural analysis was carried out using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) study.     

Grain Cluster Microstructure and Grain Boundary Character Distribution in Alloy 690

The effects of thermal-mechanical processing (TMP) on microstructure evolution during recrystallization and grain boundary character distribution (GBCD) in aged Alloy 690 were investigated by the electron backscatter diffraction (EBSD) technique and optical microscopy. The original grain boundaries of the deformed microstructure did not play an important role in the manipulation of the proportion of the Σ3 ⁿ (n = 1, 2, 3…) type boundaries. Instead, the grain cluster formed by multiple twinning starting from a single nucleus during recrystallization was the key microstructural feature affecting the GBCD. All of the grains in this kind of cluster had Σ3 ⁿ mutual misorientations regardless of whether they were adjacent. A large grain cluster containing 91 grains was found in the sample after a small-strain (5 pct) and a high-temperature (1100 °C) recrystallization anneal, and twin relationships up to the ninth generation (Σ3⁹) were found in this cluster. The ratio of cluster size over grain size (including all types of boundaries as defining individual grains) dictated the proportion of Σ3 ⁿ boundaries.     

抗氢2.25Cr-1Mo钢热处理工艺与性能研究

本文主要研究了2.25C r-1M o钢正火处理后显微组织和回火过程中碳化物相对钢的强韧性的影响,奥氏体化处理后进行冷却(加速冷却和空冷),得到的显微组织为粒状贝氏体和先共析铁素体。对于2.25C r-1M o厚钢板,显微组织和碳化物相的变化是造成2.25C r-1M o钢强韧性能变化的主要原因。