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.     

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.     

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.     

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).     

Tempering of 2.25 Pct Cr-1 Pct Mo Low Carbon Steels

The effect of carbon level on the tempering behavior at 700°C of 2.25 pct Cr-1 pct Mo steels having typical weld metal compositions has been investigated using analytical electron microscopy and X-ray diffraction techniques. The morphology, crystallography and chemistry, of each of the various types of carbides observed, has been established. It has been shown that each carbide type can be readily identified in terms of the relative heights of the EPMA spectra peaks for iron, chromium, molybdenum, and silicon. A decrease in the carbon level of the steel increases the rate at which the carbide precipitation reactions proceed, and also influences the final product. Of the carbides detected, M₂₃C₆ and M₇C₃ were found to be chromium-based, and their compositions were independent of both the carbon level of the steel and the tempering time. The molybdenum-based carbides, M₂C and M₆C, however, showed an increase in their molybdenum contents as the tempering time was increased. The rate of this increase became greater as the carbon content of the steel was lowered.     

Creep Rupture Strength and Microstructure of Low C-10Cr-2Mo Heat-Resisting Steels with V and Nb

The new ferritic heat-resisting steels of 0.05C-10Cr-2Mo-0.10V-0.05Nb (Cb) composition with high creep rupture strength and good ductility have already been reported. The optimum amounts of V and Nb that can be added to the 0.05C-10Cr-2Mo steels and their effects on the creep rupture strength and microstructure of the steels have been studied in this experiment. The optimum amounts of V and Nb are about 0.10 pct V and 0.05 pct Nb at 600 °C for 10,000 h, but shift to 0.18 pct V and 0.05 pct Nb at 650 °C. Nb-bearing steels are preferred to other grades on the short-time side, because NbC precipitation during initial tempering stages delays recovery of martensite. On the long-time side, however, V-bearing steels have higher creep rupture strength. By adding V to the steels, electron microscopic examination reveals a stable microstructure, retardation during creep of the softening of tempered martensite, fine and uniform distribution of precipitates, and promotion of the precipitation of Fe₂Mo.     

Effect of Boron Addition and Initial Heat-Treatment Temperature on Microstructure and Mechanical Properties of Modified 9Cr-1Mo Steels Under Different Heat-Treatment Conditions

The effect of initial heat treatment on microstructure and mechanical properties of boron-free and boron-containing modified 9Cr-1Mo steel (P91 and P91B, respectively) has been studied under different heat-treatment conditions. The prior austenite grains evolved in P91 steel, having different prior austenite grain sizes, were found to be similar in size after heat treatment in the range of 1073 K to 1448 K (800 °C to 1175 °C) for 5 minutes. The microstructural evolution in P91B steel having different prior austenite grain sizes appeared to be uniform when subjected to different heat-treatment temperatures with the prior austenite grain size being similar to that of initial grain size. Lath martensite was observed in P91B steel after all heat treatments. On the other hand, lath martensite was observed in P91 steel only when subjected to high-temperature heat treatment, whereas subgrain/substructure as well as coarse precipitates were observed after a lower temperature heat treatment. Large differences in the hardness/strength values between different microstructures corresponding to coarse-grained heat-affected zone (CGHAZ) and intercritical HAZ (ICHAZ) of P91 steel weldment were due to the distinct difference in these microstructures. The difference in hardness/strength values between the CGHAZ and ICHAZ was found to be insignificant in P91B steel under similar heat-treatment conditions.     

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.     

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.

     

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 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.     

Creep Strain Modeling of 9 to 12 Pct Cr Steels Based on Microstructure Evolution

Creep deformation is simulated for 9 pct Cr steels by using the Norton equation with the addition of back stresses from dislocations and precipitates. The composite model is used to represent the heterogeneous dislocation structure found in 9 to 12 pct Cr steels. Dislocation evolution is modeled by taking capturing and annihilation of free dislocations into account. Recovery of immobile dislocations is derived from the ability of dislocation climb. In spite of the fact that the initial dislocation density is high and is reduced during creep, primary creep is successfully modeled for a P92 steel. Subgrain growth is evaluated using a model by Sandström (1977). The long time subgrain size corresponds well to a frequently used empirical relation, with subgrain size inversely proportional to the applied stress.     

Characterization of Ferrite in Tempered Martensite of Modified 9Cr-1Mo Steel Using the Electron Backscattered Diffraction Technique

Ferrite was identified and characterized in tempered martensitic modified 9Cr-1Mo steel using the electron backscattered diffraction (EBSD) technique. Microstructural examination of the as-received modified 9Cr-1Mo steel revealed the presence of polycrystalline grains without lath morphology having low hardness within a predominantly tempered lath martensitic matrix. These grains were identified as the ferrite phase, and subsequent EBSD data analysis confirmed that the image quality (IQ) index of these grains is higher and boundary line length per unit area is lower than those of martensitic matrix. Therefore, it is proposed that characterization of ferrite phase in martensitic matrix can be carried out using microstructural parameters such as IQ index and boundary line length per unit area obtained from EBSD data analysis.     

The Creep Rupture Properties of 9% Chromium Steels and the Influence of Oxidation on Strength

Abstract

Creep rupture data for the 9% chromium steels Fe9CrlMoVNb (P91), Fe9CrlMolWVNb (E911) and Fe9Cr Mo2WVNb (P92) have been evaluated using the secondary creep rate as well as the stress rupture life and compared with literature data for Fe9CrlMo (P9) and 12CrlMoV. Extrapolation procedures have been carried out in order to predict the long-terms stress rupture strengths of the 9% Cr Steels. The factors affecting the reliability of the extrapolations are discussed. The 600°C/100 000 h stress rupture strength of P92 was slightly higher than that of E911 based on data of up to 30 000h duration. The effect of oxidation on rupture life was assessed; for components of wall thickness below about 6 mm, the loss of load-bearing cross-section due to oxidation should be taken into account for service life prediction.     

Creep-fatigue crack behaviour of a mod. 9Cr-1Mo steel structure with weldments

An assessment of a creep-fatigue crack initiation and growth for a Mod. 9Cr-1Mo steel structure with weldments has been carried out based on an extended French RCC-MR A16 procedure. The A16 defect assessment guide provides assessment procedures for a creep-fatigue crack initiation and growth for an austenitic stainless steel, but no guidelines are available yet for a Mod. 9Cr-1Mo steel. A creep-fatigue structural test was carried out for a Mod. 9Cr-1Mo specimen with two hours of a hold time at 873 K and various primary nominal stresses. σ _d approach was employed to evaluate a creep-fatigue crack initiation, and an assessment of a creep-fatigue crack growth at a defect has also been carried out. The evaluation results were compared with the observed images from the structural test.     

Effect of Application of Short and Long Holds on Fatigue Life of Modified 9Cr-1Mo Steel Weld Joint

Modified 9Cr-1Mo steel is a heat-treatable steel and hence the microstructure is temperature sensitive. During welding, the weld joint (WJ) is exposed to various temperatures resulting in a complex heterogeneous microstructure across the weld joint, such as the weld metal, heat-affected zone (HAZ) (consisting of coarse-grained HAZ, fine-grained HAZ, and intercritical HAZ), and the unaffected base metal of varying mechanical properties. The overall creep–fatigue interaction (CFI) response of the WJ is hence due to a complex interplay between various factors such as surface oxides and stress relaxation (SR) occurring in each microstructural zone. It has been demonstrated that SR occurring during application of hold in a CFI cycle is an important parameter that controls fatigue life. Creep–fatigue damage in a cavitation-resistant material such as modified 9Cr-1Mo steel base metal is accommodated in the form of microstructural degradation. However, due to the complex heterogeneous microstructure across the weld joint, SR will be different in different microstructural zones. Hence, the damage is accommodated in the form of preferential coarsening of the substructure, cavity formation around the coarsened carbides, and new surface formation such as cracks in the soft heat-affected zone.     

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.     

The effect of ageing on the ductility of 9Cr-1 Mo steel

A study has been made of the effects of ageing at 550°C on the ductility and fracture mechanisms in a 9 Cr-1 Mo steel. Although the fracture mode remains ductile, ageing at times up to 5000 h causes a progressive reduction in ductility. Voids are nucleated at both coarsely distributed inclusions and at much more finely distributed precipitates, and the principal effect of ageing is to enhance void formation at precipitates. This enhancement is due to a combination of phosphorus segregation to carbide precipitate-matrix interfaces and, after ageing for 5000 h, to the precipitation of Fe₂Mo (Laves phase). An important aspect of the observations is that void nucleation at precipitates occurs continuously with increasing strain until failure occurs and the results are consistent with a failure criterion based on the achievement of a critical void spacing.     

Ductile-brittle transition temperatures and dynamic fracture toughness of 9Cr-1Mo steel

The ductile-brittle transition temperature (DBTT) of 9Cr-1Mo steel was characterized by an RT _NDT-based K _IR curve approach and a reference temperature (T ₀)-based master curve (MC) approach. The MC was developed at a dynamic loading condition (loading rate of 5.12 m/s), using precracked Charpy V-notch (PCVN) specimens, and the reference temperature was termed T ₀ ^dy . The RT _NDT and T ₀ ^dy were determined to be −25 °C and −52 °C, respectively. The T ₀ ^dy was also estimated from instrumented CVN tests, using a modified Schindler procedure to evaluate K _Jd ; the result shows close agreement with that obtained from the PCVN tests. The ASME K _IR -curve approach proves to be too conservative compared to the obtained trend of the fracture toughness with temperature. The cleavage fracture stress, σ* _f , estimated from the critical length, l*, shows good agreement with that estimated from the load-temperature diagram (2400 to 2450 MPa), which was constructed from the CVN test results. The crack initiation mechanism has been identified as decohesion of the particle-matrix interface, rather than as the fracture of the particles.