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.     

核能用2.25Cr-1Mo钢水蒸气氧化行为

 为了研究核电用2.25Cr-1Mo钢的抗蒸汽氧化性能,在500 ℃、0.1 MPa水蒸气条件下,对2.25Cr-1Mo耐热钢进行了600 h的氧化试验,利用分析天平测定样品氧化增重,获得氧化动力学曲线,通过扫描电镜观察分析了氧化膜的形貌和结构,结合X射线衍射和能谱分析对氧化产物进行物相分析。试验结果表明,2.25Cr-1Mo耐热钢的氧化增重曲线符合立方规律;氧化膜为双层结构,氧化膜内层较为致密的主要物相为(Fe,Cr)₃O₄尖晶石,氧化膜外层疏松多孔主要物相为Fe₃O₄和少量Fe₂O₃;腐蚀速率测定结果表明,该材料具有较好的抗蒸汽氧化性能。     

Fracture toughness behavior of ex-service 2-1/4Cr-1Mo steels from a 22-year-old fossil power plant

Elevated-temperature fracture toughness properties were developed on ex-service 2-l/4Cr-1Mo steel weldments. Fracture toughness was measured on both base and heat-affected zone (HAZ) metals. A composite specimen consisting of base, HAZ, and weld metals was used to develop fracture toughness properties in the HAZ area. It was observed that the J-R curve of the HAZ was significantly lower than that of the base metal. Increasing crack extension increased the difference between theJ-R curves of the base metal and the HAZ. Dimpled fracture was the prime fracture mode in the base metal specimen, and a mixed-mode (ductile and “granular”) fracture was found in the HAZ specimens. Scanning transmission electron microscopy (STEM) examination revealed significant intergranular carbide precipitation and agglomeration within the HAZ. The lower fracture toughness of the HAZ, as compared to the base metal, was attributed to the large accumulation of carbides in the grain boundaries of the HAZ, which weakened the grain boundaries and caused “granular” fracture.     

Creep-rupture behavior of forged, thick section 9Cr-1Mo ferritic steel

Detailed investigations have been performed to examine the creep-rupture behavior of a 1000-mm diameter and 300-mm-thick tube plate forging of 9Cr-1Mo ferritic steel in quenched and tempered (Q + T), simulated postweld heat treatment (SPWHT), and thermally aged (TA) conditions. Creep tests were conducted over a wide stress range (50 to 275 MPa) at 793 and 873 K. The alloy exhibited well-defined primary, steady-state, and extended tertiary creep stages at all test conditions. At 793 K, no significant difference in the creep-rupture properties was noted between Q + T, SPWHT, and TA conditions. On the other hand, SPWHT specimens exhibited lower creep-rupture strength than that of Q + T specimens at 873 K. Applied stress (σ _a ) dependence of rupture life (t _r ) exhibited two-slope behavior. Both the Monkman-Grant (ε _s .t _r = C _MG) and modified Monkman-Grant (ε _s .t _r /ε _f = C _MMG) relationships were found to be valid for 9Cr-1Mo steel, where ε _s is the steady-state creep rate and ε _f is the strain to failure. The two-slope behavior was also reflected as two constants in the Monkman-Grant relationship (MGR) and modified Monkman-Grant relationship (MMGR) in the two stress regimes. Further, two creep damage tolerance factors (λ = 1/C _MMG) of 5 and 10 were also observed in the high and low stress regimes, respectively. The alloy exhibited high creep ductility, which was retained for longer rupture lives at low stresses, and the creep ductility increased with increase in test temperature. The failure mode remained trangranular under all test conditions. The extensive tertiary creep in the alloy has been attributed to microstructural degradation associated with precipitates and dislocation substructure. The creep-rupture strength of the forging was found to be lower than that of thin section bars and tubes.     

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.     

Low cycle fatigue and creep-fatigue interaction behavior of modified 9Cr-1Mo ferritic steel and its weld joint

The aim of the present paper is to study the low cycle fatigue and creep-fatigue interaction behavior of modified 9Cr-1Mo ferritic steel weld joint. Total axial strain controlled continuous cycling tests were conducted between 773 K and 873 K and at strain amplitudes ±0.25%, ±0.4%, ±0.6% and ±1%. Hold tests were also conducted at +0.6% and 823 K and 873 K temperatures to study the creep-fatigue interaction behavior of the weld joint. The alloy exhibited cyclic softening from first cycle onwards irrespective of the loading conditions. Failure location in the weld joint was correlated to the test parameters. Detailed replica study conducted on all the failed specimens revealed that most of the failures occurred in one side of the heat affected zone (HAZ) of the weld joint. Strain localization in the soft zone of the HAZ and subsurface creep cavity formation in this region and their linkage had caused enhanced crack propagation that translated into lower fatigue life of the weld joint at high temperatures. Type IV mode of failure was identified to be operative under tensile hold and high temperatures. The alloy was also found to be compressive dwell sensitive and it was ascertained that the lower life under compression hold compared to tension hold was due to the deleterious effect of oxidation.     

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.     

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.     

The nucleation and growth of voids at carbides in 9 Cr-1 Mo steel

The ductile fracture characteristics of a commercial 9 Cr-1 Mo steel have been studied using tensile specimens of the material, both in the “as heat treated” (normalised and tempered) condition and after ageing. Transmission electron microscopy on foils cut from deformed samples has enabled the nucleation of voids at precipitate (carbide) particles to be monitored as a function of strain and an assessment of the growth characteristics of these voids has been made using this technique. An attempt has been made to account for the progressive nucleation of voids with strain in terms of a carbide size effect and the nucleation and growth data have been combined to allow the effect of ageing on the damage accumulation process associated with ductile fracture to be treated quantitatively.     

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.     

Microstructural zones in the primary solidification structure of weldment of 9Cr-1Mo steel

The present article describes the microstructural heterogeneity in the weldments of 9Cr-1Mo steel, in the as-welded condition. The thermal cycle, which each microscopic region undergoes, depends on its distance from the source of heat. The consequent differences in the type of phase transformations experienced by the various zones are established using detailed analytical transmission electron microscopy studies. Based on these studies, distinct microstructural zones are identified and the temperature profiles of the same are deduced.     

Thermal treatment improving creep properties of nitrogen-added Mod.9Cr-1Mo steels

Generation IV reactors are being developed to produce a reliable energy safely and with an economic benefit, because nuclear energy is being seriously considered to meet the increasing demand for a world-wide energy supply without environmental effects. Ferritic/martensitic steels are attracting attention as candidate materials for the Gen-IV reactors due to their high strength and thermal conductivity, low thermal expansion, and good resistance to corrosion. In recent years, new ferritic/martensitic steels have been developed for ultra supercritical fossil power plants through advanced technologies for steel fabrication. The microstructural stability of these materials for the pressure vessel, cladding and core structure of the VHTR and SFR is very important. Nitrogen is a precipitation hardening element, and the thermal stability of nitrides is superior to that of carbides. So the formation of nitrides may improve the thermal stability of the microstructure and eventually increase the creep rupture strength of high Cr steels. The effect of nitrogen on the creep rupture strength and microstructure evolution of nitrogen-added Mod.9Cr-1Mo steels has been studied. Creep testing was carried out at 873 and 923 K under constant load conditions. The optimum controlled Cr₂X precipitates were developed by special heat treatment, and they were not dissolved after a creep deformation. These fine and stable Cr₂X precipitates contributed to the increase of the creep rupture strength. The prior austenite grain size and martensite lath width were decreased by the resultant stable nitrides.     

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.     

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.     

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.     

Hydrogen attack behavior of the heat affected zone of a 2.25Cr-1Mo steel weldment

The kinetics of hydrogen attack (HA) has been studied in the heat affected zone (HAZ) in a 2.25Cr-1Mo steel weld to determine the relative rates of attack and bubble nucleation in the HAZ, base metal, and weld metal. The HAZ was found to suffer hydrogen attack at nearly twice the rate of the base metal, but not as rapidly as the weld metal. Nucleation of bubbles does not occur during HA of the HAZ of a 2.25Cr-1Mo steel, on exposure to hydrogen pressure of 20.5 MPa or less, but does occur at higher pressures up to 31.5 MPa (4500 psi) at 550 °C, or up to 27.5 MPa (4000 psi) at 580 °C. Such nucleation results in enhancement of the HA rate by a factor of six. The weak dependence of nucleation effects on hydrogen pressure and the saturation of the nucleation effects in a short time suggest some thermally activated nucleation of fresh bubbles. Formerly with The Ohio State University.     

Analysis of the stress-strain curves of a modified 9Cr-1Mo steel by the voce equation

The representation of stress-strain curves by empirical relations can give first-hand information on the deformation behavior of a material. Although the constants in these equations are simple numerals for smooth fitting of the curves, a theoretical basis using dislocation interaction has been out forth bv Berastrom.[1]     

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.