Dislocation damping and microstructural evolutions during creep of 2.25Cr-1Mo steels

We studied the microstructural evolution of 2.25Cr-1Mo steels subjected to tensile creep at 923 K through monitoring of shear-wave attenuation and velocity, using electromagnetic acoustic resonance (EMAR). Contactless transduction based on the magnetostrictive mechanism is the key to establishing a monitor for microstructural change in the bulk of the metals with a high sensitivity. In the short interval, 50 to 60 pct of the creep life, attenuation experiences a peak, being independent of the applied stress. This novel phenomenon is interpreted in terms of the drastic change in dislocation mobility and rearrangement, which is supported by transmission electron microscopy (TEM) observations for dislocation structure. At this particular period, the dense dislocation structure starts to transform to subgrain boundaries, which temporally accompanies long, free dislocation, absorbing much ultrasonic energy to produce the attenuation peak. The EMAR has the potential to assess the damage advance and to predict the remaining creep life of metals.     

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.     

The effect of titanium on creep strength in 2.25 Pct Cr-1 Pct Mo steels

The effect of a low level of titanium on the microstructure and creep properties of 2.25 pct Cr-1 pct Mo steels has been examined as a function of carbon content and austenitizing temperature. The addition of 0.04 wt pct titanium resulted in a dramatic increase in creep strength at 565 °C, and this was found to be associated with the presence in the microstructure of very small (50 to 100 Å) titanium-bearing precipitates based upon both TiC and Mo₂C. The variation of the minimum creep rate with carbon content and austenitizing treatment was explained in terms of the solubility of TiC in austenite. The titanium-bearing carbides have an important effect on microstructural stability and on the maintenance of creep strength, but it is also apparent that solid solution strengthening by molybdenum can make a significant contribution to creep strength at low carbon levels (0.02 wt pct).     

The effect of phosphorus on the ductility of 9Cr-1Mo steels

The ductile fracture characteristics of three special casts of 9Cr-1Mo steel with different phosphorus contents have been studied using plain tensile, notched tensile and Charpy impact specimens. A commercial cast of this steel has also been investigated for comparative purposes. The materials have been tested in the normalised and tempered condition and after ageing at 550°C for 1000 h and 5000 h. The results show that the ductility, DBTT and upper shelf energy (USE) of the low phosphorus material is unaffected by the 1000 h age, in contrast to the higher phosphorus casts which experience a deterioration in these properties. All the materials exhibit a deterioration in these properties after the 5000 h ageing treatment. The results support the view that the decrease in ductility associated with the 1000 h age in the higher P materials is caused by the segregation of phosphorus to the carbide/matrix interfaces and that the decrease in USE also arises from this source. The decrease in ductility and USE and the increase in DBTT after long term ageing is associated principally with the precipitation of Laves phase. Each material exhibits a decrease in ductility with increasing stress triaxiality and it is shown that a contribution to this effect may arise from the dependence of void nucleation on stress state.     

Mod 9Cr-1Mo耐热钢析出相的热力学计算及应用

利用热力学方法建立了耐热钢Ｍｏｄ９Ｃｒ１Ｍｏ中析出相ＭＸ、Ｍ２３Ｃ６以及ＡｌＮ数量和组成与钢的化学成分和热处理温度之间关系的数学模型。采用相分析验证了模型的准确性，并介绍了该模型的用途。     

Evaluation of fatigue properties for aged 9Cr-1Mo steel

Low cycle fatigue tests of 9Cr-1Mo steel aged at 600°C for 5000 h and 10000 h were conducted at RT-600°C. Fatigue life was not decreased with aging. Cyclic softening was seen with cycles in both unaged and aged specimens. Aging decreased the amount of softening at RT and 400°C but increased it at 600°C. The precipitate species were not changed with aging upto 5000 h but Laves phase was precipitated after 10000 h aging. Fatigue life of 9Cr-1Mo steel aged to 10000 h is dependent on dislocation slip behavior rather than the amount and coarsening of precipitate.     

Effect of Ti and Ti+B additions on the creep properties of 1.25 Cr-0.5 Mo steels

An investigation of the effect of Ti additions with and without B on the mechanical properties of normalized and tempered 1.25 Cr-0.5 Mo Steels has been completed. Concentrations of impurities P, Sb and Sn were also varied to obtain insight into possible interactions between alloy additions and impurities. Results show that by a combined addition of Ti and B, appreciable improvements can be achieved in tensile strength, creep strength and rupture strength at 538°C, regardless of the nature and the amount of other impurity species present in the steel. Effects of Ti in the absence of B are complex. With P as the major impurity Ti additions reduced the tensile, creep and rupture strength. In presence of large amounts of P+Sb+Sn, however, improved strength levels are achieved due to Ti additions. Rupture ductilities are higher for all the steels containing Ti, independent of the presence or absence of B and the nature of the impurity species. The work reported here was conducted at the Westinghouse Research Laboratories, Pittsburgh, PA.     

Effect of normalization temperature on the creep strength of modified 9Cr-1Mo steel

The effect of normalization temperature from 850 °C to 1050 °C on the structure and creep-rupture properties of modified 9Cr-1Mo steel was studied. Normalization at temperatures below 925 °C resulted in structures containing significant polygonized, recovered ferrite. The ferrite structures had poor creep-rupture strength: roughly two orders of magnitude increase in minimum creep rate or decrease in rupture life for 850 °C compared to 1050 °C normalization at test conditions of 600 °C and 145 MPa. Room-temperature strength and hardness were also reduced. The microstructure after normalization at the standard 1050 °C temperature consisted of tempered martensite with fine M₂₃C₆ carbide along prior austenite and lath boundaries and fine MX carbonitride precipitates within the laths. Normalization at temperatures between 925 °C and 1000 °C also resulted in reduced creep strength in comparison with 1050 °C normalization, even though tempered martensite microstructures were formed and little change in room-temperature strength was observed; the reduction was attributed to subtle differences in the MX precipitates. The effect of reduced normalization temperature was more pronounced for higher-temperature, lower-stress creep-rupture conditions.     

Ball-indentation test technique for evaluating thermal and creep damage of modified 9Cr-1Mo steel

The Ball-Indentation (BI) testing based on multiple cycles of loading-unloading using a spherical indenter is a useful technique for evaluating tensile properties from a very small volume of material. In this study, the BI technique has been used in a novel way to evaluate the changes in mechanical properties of Modified (Mod) 9Cr-1Mo caused by creep exposures. Microstructural degradation of varying degrees in Mod 9Cr-1Mo steel is simulated by conventional creep test terminated at various strains. By carrying out BI tests on unstressed head and stressed gage portions of the creep specimens, the changes in the strength and ductility are evaluated. Microstructural evolution in the creep exposed conditions studied using transmission electron microscopy is related to the strength changes caused by the stressed exposures.     

Heat treatment effects on creep and rupture behavior of annealed 2.25 Cr-1 Mo steel

The strength of 2 1/4 Cr-1 Mo steel depends on the microstructure, which, in turn, de-pends on the heat treatment. In the fully annealed and isothermally annealed conditions, the microstructure is primarily proeutectoid ferrite with varying amounts of bainite and pearlite. The relative amounts of the latter constituents depend on the cooling rates during the anneal. The creep and rupture properties were determined for steel plates (from a single heat) given three different annealing treatments: two were fully annealed, but cooled at different rates from the austenitizing temperature, and the third was iso-thermally annealed. Properties were determined at 454, 510, and 566°C. At 454 and 510°C, the cooling rate had a significant effect on the creep and rupture properties, with the ma-terial cooled fastest being the strongest. Although at 510°C strengths at short rupture times differed widely, the properties approached a common value at longer rupture times. The properties differed very little at 566°C, even for short rupture times. The effect of heat treatment was concluded to be the result of interaction solid solution hardening, a dislocation-drag process. This process gave rise to nonclassical creep curves (as op-posed to classical curves with single primary, secondary, and tertiary stages). By examining the creep-curve shape, it was possible to interpret the heat treatment effects on the creep-rupture properties.     

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.     

Microstructural evolution of modified 9Cr-1Mo steel

The tempering and subsequent annealing of modified 9Cr-lMo steel have been investigated to determine the influence of trace amounts of V and Nb on the sequence of precipitation processes and to identify the basis for the enhanced high-temperature strength compared to the standard 9Cr-lMo composition. Air cooling (normalizing) from 1045 °C results in the precipitation of fine (Fe, Cr)₃C particles within the martensite laths. Additional carbide precipitation and changes in the dislocation structure occur during the tempering of martensite at 700 °C and 760 °C after normalizing. The precipitation of M₂₃C₆ carbides occurs preferentially at lath interfaces and dislocations. The formation of Cr₂C was detected during the first hour of tempering over the range of 650 °C to 760 °C but was replaced by V₄C₃ within 1 hour at 760 °C. During prolonged annealing at 550 °C to 650 °C, following tempering, the lath morphology remains relatively stable; partitioning of the laths into subgrains and some carbide coarsening are evident after 400 hours of annealing at 650 °C, but the lath morphology persists. The enhanced martensite lath stability is attributed primarily to the V₄C₃ precipitates distributed along the lath interfaces and is suggested as the basis for the improved performance of the modified 9Cr-lMo alloy under elevated temperature tensile and creep conditions.     

Interesting relationships for creep deformation and damage and their applicability for 9Cr-1Mo ferritic steel

The paper presents the validity of several interesting relationships examined for better understanding of creep behaviour of 9Cr-1Mo ferritic steel. Creep rate-rupture time relationships of Monkman-Grant type have been found to be valid. Like stress dependence of creep rate and rupture life, both Monkman-Grant and modified Monkman-Grant relations (MGR and MMGR) exhibited distinct constant values of C _MG and C _MMG , respectively for low and high stress regimes. The validity of MGR and MMGR is a consequence of the creep deformation behaviour of 9Cr-1Mo ferritic steel obeying first order kinetics. On the basis of creep rate-rupture time relationships of Monkman-Grant type, several other relationships involving transient and tertiary creep parameters have been evolved and their applicability have been examined for the steel. Analogous to MGR and MMGR, a relationship involving transient creep parameters and the other involving tertiary creep parameters were found to be valid. Further, 9Cr-1Mo steel obeyed a recently introduced critical damage criterion interrelating time to reach Monkman-Grant ductility with rupture life, and the criterion depends only on creep damage tolerance factor. This unique relationship is evolved based on the seminal concept of time to reach Monkman-Grant ductility as the time at which the useful safe creep life is exhausted and damage attains a critical level. The important implications of this concept have been discussed.     

Magnetic Evaluation of Microstructure Changes in 9Cr-1Mo and 2.25Cr-1Mo Steels Using Electromagnetic Sensors

This paper presents results from a multi-frequency electromagnetic sensor used to evaluate the microstructural changes in 9Cr-1Mo and 2.25Cr-1Mo power generation steels after tempering and elevated temperature service exposure. Electromagnetic sensors detect microstructural changes in steels due to changes in the relative permeability and resistivity. It was found that the low frequency inductance value is particularly sensitive to the different relative permeability values of both steels in the different microstructural conditions. The changes in relative permeability have been quantitatively correlated with the microstructural changes due to tempering and long-term thermal exposure, in particular to changes in martensitic/bainitic lath size and number density of carbide precipitates that determine the mean free path to reversible domain wall motion. The role of these microstructural features on pinning of magnetic domain wall motion is discussed.     

Improving the creep properties of 9Cr-3W-3Co-NbV steels and their weld joints by the addition of boron

New ferritic steels with a controlled addition of boron have been developed recently for ultrasuper-critical fossil power plants. These steels possess excellent creep resistance compared to conventional steels like P91, P92, P122, etc., and this has been attributed to the delay in coarsening of the carbides during creep owing to partial replacement of carbon by boron in these carbides. However, the susceptibility of the weld joints of the boron-containing ferritic steels to type IV cracking, which significantly brings down the rupture life of the weld joints, has not been investigated so far. In the present work, the creep properties of recently developed 9Cr-3W-3Co-NbV steels with boron contents varying from 47 to 180 ppm and of their weld joints have been studied. Creep tests were carried out at 923 K in the stress range of 140 to 80 MPa. Specimens were examined for particle coarsening using field-emission scanning electron microscopy, and the boron content in the precipitates was estimated using field-emission auger electron spectroscopy (FE-AES). The grain size of the parent metal and the heat-affected zone (HAZ) were estimated using electron backscattered pattern (EBSP) imaging. Results showed that the creep properties of the steels with 90 and 130 ppm boron and of their weld joints are superior to those of the P92 steels and its weld joints. Further, no weld joints exhibited type IV cracking. No significant coarsening of the carbides was observed, not only in the parent metal but also in the HAZ of the steels with ≥90 ppm of boron. In addition to the delay in carbide coarsening, the large prior-austenite grain size of the parent metal and the absence of a conventional fine-grained HAZ (FGHAZ) in the weld joints also seem to have a beneficial effect on improving the creep properties of these steels and their weld joints.     

Stress-assisted hydrogen attack cracking in 2.25Cr-1Mo steels at elevated temperatures

Crack growth in 2.25Cr-lMo steels exposed to 3000 psi hydrogen has been investigated in the temperature range 440 °C to 500 °C, using modified wedge-opening loaded specimens to vary stress intensity. Under conditions of temperature and hydrogen pressure, where general hydrogen attack does not occur, the crack propagated by the growth and coalescence of a high density of methane bubbles on grain boundaries, driven by the synergistic influence of internal methane pressure and applied stress. Crack growth rates were measured in base metal, and the heat-affected zones (HAZs) of welds were tempered to different strength levels. The crack growth rate increased with material strength. Above a threshold of about K_l = 20 MPa√m (at 480 °C), the crack growth rate increased rapidly with stress intensity, increasing as roughly K_l ^6.5. Because of better creep resistance, stronger materials can sustain higher levels of stress intensity to drive crack growth and nucleate the high density of voids necessary for crack growth. Stress relaxation by creep reduces the stress intensity, and thus the growth rate, especially in weaker materials. The crack growth rate in the heat-affected zone was found to be substantially faster than in the base metal of the welds. Analysis indicates that K_l rather than C* is the appropriate crack-tip loading parameter in the specimen used here and in a thick-walled pressure vessel. The DC potential drop technique met with limited success in this application due to the spatially discontinuous manner of crack growth and limited crack-tip opening displacement. Formerly Graduate Student, Materials Science and Engineering Department, The Ohio State University