Search Results for inclusions

This study investigates the behavior of boron nitride (BN) inclusions in high-chromium ferritic heat-resistant steels like P92 and P122. Boron is added to improve creep resistance, but its role is not fully understood. Here, the formation and dissolution of BN inclusions during high-temperature heat treatment were examined. Microscopic analysis revealed coarse BN inclusions (2-5 μm) alongside smaller alumina inclusions. Annealing experiments showed that these BN inclusions only dissolved at temperatures exceeding 1200°C, suggesting they form during casting or forging processes below 1150°C. Chemical analysis identified a critical boron and nitrogen concentration threshold (below 0.001% B and 0.015% N) for BN inclusion formation.

In order to understand the microstructural evolution during service that 9Cr steels experience it is important to be able to quantify key microstructural parameters that define the characteristics of the secondary phases (e.g. precipitated phases and inclusions) and the steel matrix. The average size of M 23 C 6 , Laves phase and MX particles in these materials have been reported in many studies, however comparability between these studies is compromised by variations in technique and different/incomplete reporting of procedure. This paper provides guidelines on what is required to accurately measure these parameters in a reproducible way, taking into account macro-scale chemical heterogeneities and the statistical number of particles required to make meaningful measurements. Although international standards do exist for inclusion analysis, these standards were not developed to measure the number per unit area of hard particles that can act as creep cavity nucleation sites. In this work a standardized approach for measuring inclusions from this perspective is proposed. In addition the associated need to understand the segregation characteristics of the material are described, which in addition to defining the area that needs to be analysed to measure the average number of inclusions per unit area, also allows the maximum number of inclusions per unit area to be determined, a parameter which is more likely to define the damage tolerance of the material.

Creep strength enhanced ferritic (CSEF) steels have shown the potential for creep failure in the weld metal, heat affected zone (HAZ) or fusion line. Details for this behavior have been frequently linked to metallurgical risk factors present in each of these locations which may drive the evolution of damage and subsequent failure. This work is focused on three weld samples fabricated from a commercially sourced Grade 92 steel pipe section. These weld samples were extracted from the same welded section but were reported to exhibit failure in different time frames and failure locations (i.e., HAZ of parent, fusion-line, and weld metal). The only variables that contribute to this observed behavior are the post weld heat treatment (PWHT) cycle and the applied stress (all tests performed at 650 °C). In this work detailed microstructural analysis was undertaken to precisely define the locations of creep damage accumulation and relate them to microstructural features. As part of this an automated inclusion mapping process was developed to quantify the characteristics of the BN particles and other inclusions in the parent material of the samples. It was found that BN particles were only found in the sample that had been subjected to the subcritical PWHT, not those that had received a re-normalizing heat treatment. Such micron sized inclusions are a known potential nucleation site for creep cavities, and this is consistent with the observed failure location in the HAZ of the parent in the sample where these were present. In the absence of BN inclusions, the next most susceptible region to creep cavitation is the weld metal. This has an intrinsically high density of sub-micron sized spherical weld inclusions and this is where most of the creep damage was located, in all the renormalized samples.

Creep brittle behaviour in tempered martensitic, creep strength enhanced ferritic (CSEF) steels is linked to the formation of micro voids. Details of the number of voids formed, and the tendency for reductions in creep strain to fracture are different for the different CSEF steels. However, it appears that the susceptibility for void nucleation is related to the presence of trace elements and hard non-metallic inclusions in the base steel. A key factor in determining whether the inclusions present will nucleate voids is the particle size. Thus, only inclusions of a sufficient size (the critical inclusion size is directly linked to the creep stress) will act directly as nucleation sites. This paper compares results from traditional uniaxial laboratory creep testing with data obtained under multiaxial conditions. The need to understand and quantify how metallurgical and structural factors interact to influence creep damage and cracking is discussed and the significant benefits available through the use of high quality steel making and fabrication procedures are highlighted. Details of component behaviour are considered as part of well-engineered, Damage Tolerant, design methods.

This study aims to elucidate the chemical compositions and microstructural factors that affect longterm creep rupture strength and creep rupture ductility using multiple heats of Gr.92 steel. Evaluating the reduction behavior in long-term creep rupture strength, we propose a relative creep rupture strength value, which is expressed as the logarithmic ratio of the estimated creep strength for each rupture time exceeding 10,000 hours, with 10,000 hours as the reference. Higher initial hardness correlates with greater pronounced strength reduction in the long-term regime. While smaller prior austenite grain sizes lead to greater reductions in creep rupture strength, this effect diminishes above 30 μm. However, no clear correlation was observed between Cr content and creep strength reduction in this study. Brittle creep ruptures with smooth test specimens were observed just below the extensometer ridge in the parallel section of test specimen, indicating notch weakening. Even in heats with excellent creep ductility, the amount of inclusions tended to be higher than in heats with lower creep ductility. Factors other than inclusions also seem to influence long-term creep ductility.

In this work, two unique heats of 9Cr creep strength enhanced ferritic (CSEF) steels extracted from a retired superheat outlet header after 141,000 hours of service were evaluated. These two CSEF steels were a forging manufactured to SA-182 F91 (F91) reducer and a seamless pipe produced to SA-335 P91 (P91) pipe. Their creep deformation and fracture behavior were assessed using a lever arm creep frame integrated with in-situ high-temperature digital image correlation (DIC) system. Critical metallurgical and microstructure factors, including composition, service damage, grain matrix degradation, precipitates, and inclusions were quantitatively characterized to link the performance of the two service aged F91 and P91 CSEF steels. The creep test results show the F91 and P91 steels exhibit a large variation in creep strength and creep ductility. The F91 steel fractured at 572 hours while P91 steel fractured at 1,901 hours when subjected to a test condition of 650 °C and 100 MPa. The nominal creep strains at fracture were 12.5% (F91) and 14.5% (P91), respectively. The high-resolution DIC strain measurements reveal the local creep strain in F91 was about 50% while the local creep strain in P91 was >80%. The characterization results show that the F91 steel possessed pre-existing creep damage from its time in service, a higher fraction of inclusions, and a faster matrix grain coarsening rate. These features contribute to the observed reduction in performance for the F91 steel. The context for these findings, and the importance of metallurgical risk in an integrated life management approach will be emphasized.

Monoblock low-pressure (LP) turbine rotor shaft forgings for nuclear power plants have been produced from up to 600 ton ingots. However, ingots greater than 600 tons are necessary to increase the generator capacity. Segregation, non-metallic inclusions, and micro porosities inevitably increase with the increase in ingot size. Manufacturing such massive ingots with high soundness is quite difficult. Thus, the development of 650 ton ingot production was carried out in 2010. The 650 ton ingot was dissected and investigated to verify its internal quality. The internal quality of the 650 ton ingot was found to be equal to that of 600 ton ingots. Subsequently, in 2011, we produced a 670 ton ingot, the world’s largest, to produce a trial LP rotor shaft forging with a diameter of 3,200 mm. Results show that the internal quality, mechanical properties, and heat stability are the same as LP rotor shaft forgings made from 600 ton ingots.

High-pressure and high-temperature piping in fossil power plants suffer from unexpected and rarely predictable failures. To prevent failures and ensure operational safety, a Quantitative Acoustic Emission (QAE) non-destructive inspection (NDI) method was developed for revealing, identifying, and assessing flaws in equipment operating under strong background noise. This method enables overall piping inspection during normal operation, locating suspected zones with developing low J-integral flaws, identifying flaw types and evaluating danger levels based on J-integral values, and detecting defective components prior to shutdown. Combining continuous and burst acoustic emission as an information tool, the QAE NDI revealed, identified, and assessed significant flaws like creep, micro-cracks, pore/inclusion systems, plastic deformation, and micro-cracking in over 50 operating high-energy piping systems. Findings were independently verified by various NDI techniques, including time of flight diffraction, focused array transducers, magnetic particles, ultrasonic testing, X-ray, replication, and metallurgical investigations.

Theoretical and experimental investigations, including fracture tests, acoustic emission (AE) studies, fractography, micro-sclerometric analyses, and spectral/chemical analyses of specimens, have established the possibility of revealing, recognizing in-service acquired, age-related, and prefabricated flaws based solely on AE data. Results show a linear dependence between AE and mechanical deformation power of steel specimens in original and creep stage 3a-3b conditions, decreasing fracture load and J1c value for aging steel, creep processes at stage 3a-3b having J-integral value below 0.05J1c, possibility of assessing and distinguishing different flaw development stages with ≥87% accuracy, revealing zones of tough and brittle fracture, and recognizing inclusions/pre-fabricated flaws and assessing individual/interacting flaws. Experiments confirmed the absence of the Kaiser effect under repeated loading of flawed specimens and demonstrated using AE for defect revelation. Analysis showed that creep-associated AE is mainly continuous, with repeated loading decreasing burst AE contribution during plastic deformation development.

Creep-resistant austenitic stainless steels are known to be the potential candidate materials for use as super- and re-heater tubes in ultra-super critical (USC) power plants. Among them, ASTM A213/A213M S30432, a novel 18-8 stainless steel (18Cr- 9Ni-3Cu-Nb-N), has attracted considerable attention from electric industry due to its combined lower cost and more excellent performance in contrast to traditional TP347H steel. More than 10 years of service in Japan laid a solid foundation for the steel being selectable USC boiler materials. Steels of S30432 have been recently developed in China during the past few years. This paper presents the evaluation results of S30432 tubes manufactured by four steel plants in China as well as Sumitomo super304H tubes for comparison. A detailed microstructural analysis of the tubes has been performed by using optical and electron microscope, and mechanical properties of the tubes have been evaluated using hardness testing as well as tensile testing up to 700°C. It was found that the impurity elements, nonmetallic inclusions and grain size of the S30432 tubes were well controlled. TEM observation revealed the microstructural changes for a selected batch of S30432 specimens in condition of hot rolled material, as-extruded tube, solution treated tube and 650°C/1000h aged tube. Most attention was paid to the morphology and distribution of precipitates in the microstructure which should be responsible for the enhanced performance of the steel. Although the hardness of all the evaluated tubes was measured to be similar, they showed more or less differences in tensile properties between each other. Creep rupture testing is still in progress, and the steel might exhibit excellent long-term creep rupture strength at 650°C as was predicted from the currently available testing results.

Damage in the grade 91 steel partially transformed zone of weld heat affected zones has historically been associated with many different types of microstructural features. Features described as being responsible for the nucleation of creep damage include particles such as laves phase, coarse M 23 C 6 , inclusions, nitrides, or interactions between creep strong and creep week grains, grain boundaries and potentially other sources. Few studies have attempted to link the observations of damage on scales of increasing detail from macro, to micro, to nano. Similarly, assessments are not made on a statistically relevant basis using 2D or 3D microscopy techniques. In the present paper, 2D assessment using scanning electron microscopy (SEM) and quantification techniques such as energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) are utilized in combination with 3D serial sectioning of large volumes using plasma focused ion beam milling (P-FIB) and simultaneous EDS to evaluate an interrupted cross-weld creep test. Moreover, the sample selected for examination was from a feature cross-weld creep test made using a parent material susceptible to the evolution of creep damage. The test conditions were selected to give creep brittle behaviour and the sample was from a test interrupted at an estimated life fraction of 60%. The findings from these evaluations provide perspective on the features in the microstructure responsible for the nucleation and subsequent growth of the observed damage.

Small punch creep testing (SPCT) is a small-scale, accelerated creep test that allows for the determination of creep data using a limited amount of material. The question, however, remains how the data generated by this technique correlate to more established techniques such as uniaxial testing and ultimately to predictions regarding the remaining service life of a plant component. This empirical study investigated the microstructure-to-property relationship of welded 9-12%Cr steels as measured using SPCT. Virgin P91 (X10CrMoVNb9-1) steel was joined to service exposed X20 (X20CrMoV12-1) steel using two different filler materials (X20 and P91) via fusion welding. Site-specific samples were extracted from the parent plates, heat affected zones and weld metals using electro-discharge machining. Small punch creep testing were performed using a 276 N load at a temperature of 625°C. The untested sample microstructures were quantitatively characterized using a range of electron microscopy techniques to determine the precipitate (M 23 C 6 , MX) spacing, subgrain sizes and dislocation densities for each region of the weldments. Multiple linear regression analysis found that the subgrain size (λsg) played the largest contribution to the SPCT rupture life. The heat affected zones had the lowest SPCT rupture times (49-68 hours), which corresponded to the largest subgrain sizes (1.1-1.3 μm). The P91 parent plate material had the longest SPCT rupture time (349 hours), which corresponded to the lowest subgrain size (0.8 μm). The P91 weld metal sample showed lower initial deflection rates during the SPC testing, however the presence of non-metallic SiO 2 inclusions in this zone contributed to accelerated brittle failure.

As long term laboratory creep data became available the original estimates of the allowable stresses for creep strength enhanced ferritic steels (CSEF) had to be reduced. Thus, even in properly processed steel, the long term performance and creep rupture strength is below that originally predicted from a simple extrapolation of short term data. One of the microstructural degradation mechanisms responsible for the reduction in strength is the development of creep voids. Nucleation, growth and inter linkage of voids also result in a significant loss of creep ductility. Indeed, elongations to rupture of around 5% in 100,000 hours are now considered normal for long term creep tests on many CSEF steels. This relatively brittle behaviour, and the associated creep void development, promotes burst rather than leak type fracture in components. Moreover, the existence of significant densities of voids further complicates in-service assessment of condition and weld repair of these steels. The present paper examines background on the nucleation and development of creep voids in 9 to 12%Cr martensitic steels and discusses factors affecting brittle behavior.

In this study, 25Cr2Ni2Mo1V filler metal was deposited to weld low pressure steam turbine shafts, which are operated in fossil power plants. A comparison experiment was conducted on the weld metals (WMs) before and after varied various aging duration from 200 hours up to 5000 hours at 350 ℃. Microstructure was characterized by means of scanning electron microscopy (SEM) and electron back-scattered diffraction (EBSD) techniques. In addition, mechanical properties of corresponding specimens were evaluated, e.g. Vickers microhardness, Charpy V impact toughness and tensile strength. It is shown that the tensile strength remained stable while impact energy value decreased with increasing aging duration. Based on the experiment above, it was concluded that the variation of mechanical properties can be attributed to the redissolution of carbides and reduction of bainite lath substructure.

The current research adopts a novel approach by integrating correlative microscopy and machine learning in order to study creep cavitation in an ex-service 9%Cr 1%Mo Grade 91 ferritic steel. This method allows for a detailed investigation of the early stages of the creep life, enabling identification of features most prone to damage such as precipitates and the ferritic crystal structure. The microscopy techniques encompass Scanning Electron Microscopy (SEM) imaging and Electron Back-scattered Diffraction (EBSD) imaging, providing insights into the two-dimensional distribution of cavitation. A methodology for acquiring and analysing serial sectioning data employing a Plasma Focused Ion Beam (PFIB) microscope is outlined, complemented by 3D reconstruction of backscattered electron (BSE) images. Subsequently, cavity and precipitate segmentation was performed with the use of the image recognition software, DragonFly and the results were combined with the 3D reconstruction of the material microstructure, elucidating the decoration of grain boundaries with precipitation, as well as the high correlation of precipitates and grain boundaries with the initiation of creep cavitation. Comparison between the 2D and 3D results is discussed.

In this paper, the microstructural evolution of P92 steel were studied in the viewpoint of degradation mechanism based on the creep rupture experiment results obtained at elevated temperature by means of macroscopic, metallographic, electronic microscope, energy spectrum, XRD and TEM examination. The results show that the decrease of mechanical properties of P92 steel is mainly due to the change of microstructure and the transformation of carbides, and there is definite relationship between microstructure evolution, mechanical properties and life loss of P92 steel. The results are beneficial to the further study of mechanism of high temperature creep rupture strength and microstructural evolution of heat-resistant steel. It also has important instructive significance to quantitative identification of scientific selection of materials.

Higher steam temperature in steam power plants increases their thermal efficiency. Thus there is a strong demand for new materials with better creep and corrosion resistance at higher temperatures, while retaining the thermal flexibility of martensitic steels. Z-phase strengthened 12% Cr steels have been developed to meet the 923 K (650°C) challenge in these power plants. Ta, Nb, or V forms Z-phase together with Cr and N. A new trial steel was produced based on combining Ta and Nb to form Z-phase. It was shown that Z-phase was formed with a composition corresponding to Cr1+x(Nb,Ta)1-xN. The Nb/Ta ratio in Z-phase precipitates was higher than that in MX precipitates. Z-phase precipitates based on Ta and Nb were coarser than precipitates in a similar trial steel based on Ta alone.

Grade 91 steel has been widely utilized in power plants over the last 20 years. Its specification worldwide has dramatically increased since the acceptance of Code Case 1943 for this material in 1983. Recent evaluation of a combination of ex-service Grade 91 steel components and virgin material has provided a unique opportunity to independently assess the performance of a combination of base metal and weldments. This approach has been grounded in the fundamental objective of linking metallurgical risk factors in Grade 91 steel to the cross-weld creep performance. Establishing critical risk factors in 9Cr steels is regarded as a key consideration in the integration of a meaningful life management strategy for these complex steels. The potential metallurgical risk factors in Grade 91 steel have been fundamentally divided into factors which affect strength, ductility or both. In this study, two heats of ex-service Grade 91 steel which exhibit dramatic differences in strength and ductility have been evaluated in the ex-service condition and re-heat treated to establish a relevant set of strength:ductility variables. This set of variables includes [strength:ductility]: low:low, medium:low, low:high and medium:high. The influence of these strength:ductility variables were investigated for feature type cross-weld creep tests to better evaluate the influence of the initial base material condition on cross-weld creep performance.

The time-dependent behavior of 9Cr creep strength enhanced ferritic (CSEF) steels has long fixated on the creep life recorded in uniaxial constant load creep tests. This focus is a consequence of the need to develop stress allowable values for use in the design by formulae approach of rules for new construction. The use of these simple rules is justified in part by the assumption that the alloys used will invariably demonstrate high creep ductility. There appears to be little awareness regarding the implication(s) that creep ductility has on structural performance when mechanical or metallurgical notches (e.g., welds) are present in the component design or fabricated component. This reduced awareness regarding the role of ductility is largely because low alloy CrMo steels used for very many years typically were creep ductile. This paper focuses on the structural response from selected tests that have been commissioned or executed by EPRI over the last decade. The results of these tests demonstrate unambiguously the importance that creep ductility has on long-term, time-dependent behavior. This is the second part of a two-part paper; Part I reviewed the selected tests and discussed them from a mechanical perspective. The association of performance with specific microstructural features is briefly reviewed in this paper and the remaining gaps are highlighted for consideration among the international community.

Additive manufacturing is being considered for pressure boundary applications for power plant service by ASME Boiler and Pressure Vessel Code and regulators. Both existing and new plants could benefit from the reduced lead times, design flexibility, and part consolidation possible with additive manufacturing. Various ASME code committees are working towards rules and guidance for use of additive manufacturing. To further the industry's understanding, this research program was undertaken to evaluate the properties of wire arc additive manufactured 316L stainless steel. This study included microstructural characterization, chemical composition testing, mechanical testing, and nondestructive evaluation of multiple large (1600-pound (700 kg)) 316LSi stainless steel valve bodies produced using the gas metal arc directed energy deposition process followed by solution annealing. The results showed the tensile behavior over a range of temperatures was comparable to wrought material. No variation in tensile behavior was observed with change in tensile sample orientation relative to the build direction. Room temperature Charpy V-notch absorbed energy toughness was comparable to wrought material. Large grain sizes were observed in the metallographic samples, indicating that lowering the solution anneal temperature may be worthwhile. The results of surface and volumetric examination were acceptable when compared to forged material acceptance criteria. Together these results suggest that GMA-DED can produce acceptable materials properties comparable to forged materials requirements.