Search Results for ductility

Remaining-life assessment of high temperature components using the small punch (SP) creep testing technique necessitates the evaluation of SP load (F)/uniaxial stress (σ) conversion factor, F/σ, obtained by comparing the SP and uniaxial creep test results. In the present study, the SP creep tests were carried out at 850°C on various Ni-base alloys having different reduction of area in the range of 0.05-0.67 to investigate the influence of creep ductility on the value of F/σ. The F/σ value was determined for each alloy by correlating SP creep rupture data with corresponding uniaxial creep ones. The experimental results revealed that the F/σ value was not well correlated with Vickers hardness, but it increased almost linearly with increasing reduction of area up to around 0.4. This result indicated that the SP creep rupture data could be converted to the uniaxial data if the creep ductility on a given material was available.

Recovery of microstructure and void formation were investigated in creep-ruptured specimens of ASME Gr. T91 steels to understand the cause of loss of creep rupture ductility in the long-term creep condition and its heat-to-heat variation. The specimens studied were two heats (MGA, MGC) of Gr. T91 steels creep-ruptured at 600 °C under the stress conditions of 160-80 MPa. The reduction of area at rupture (RA) was 55% for MGA, but 83% for MGC in the long-term condition (under the creep stress of 80 MPa), while RA was higher than 80 % for the two heats in the short-term conditions (under the creep stresses above 100 MPa). In both heats, equiaxed grains were observed in the vicinity of ruptured surface in the long-term condition, indicating that recovery and recrystallization occurred extensively in the creep condition, while grains were elongated in the short-term conditions. In the uniformly deformed regions with a small area reduction in the long-term crept specimens, recovered and recrystallized grains were observed in the limited region close to high angle grain boundaries in MGA, while they were extended into grain interiors in MGC. In the long-term creep conditions two types of voids were observed: fine ones with a diameter below 1 μm and coarse ones with a diameter from 2 μm up to 50 μm. Fine creep voids were found to grow with necking in MGA while they neither nucleated nor grew with necking in MGC. Coarse creep voids increased in size and in number with necking in both heats and were larger and denser in MGA than in MGC.

The effect of boron nitrides (BN) and aluminum nitrides (AlN) on long-term creep life and rupture ductility has been investigated for martensitic 9 to 12Cr steels at 550 to 700 °C. The BN particles form in 9 to 12Cr steels during normalizing heat treatment at 1050 to 1150 °C, suggesting no change in the amount of BN particles during creep. On the other hand, the AlN particles gradually form during creep at 550 to 700 °C, decreasing the concentration of nitrogen free from the AlN particles. The degradation in creep life takes place more significantly with test duration in the steels containing high Al but not in those containing the BN particles. The rupture ductility is evaluated by using a semi-logarithmic diagram of the RA and total elongation, showing the necking dominant and void swelling dominant regions. The BN and AlN particles are responsible for the degradation in RA at low stresses and long times by accelerating the formation of creep voids at interfaces between the BN and AlN particles and alloy matrix.

Effects of microstructure constituents of α 2 -Ti 3 Al/γ-TiAl lamellae, β-Ti grains and γ grains, with various volume fractions on room-temperature ductility of γ-TiAl based alloys have been studied. The ductility of the alloys containing β phase of about 20% in volume increases to more than 1% as the volume fraction of γ phase increases to 80%. However, γ single phase alloys show very limited ductility of less than 0.2%. Microstructure analysis have revealed that intragranular fracture along γ/γ grain boundary occurred in γ single phase alloy whereas it does not along β/γ interphase in alloys containing β phase. In addition, local strain accumulations along β/γ interphase have been confirmed. The present results, thus, confirmed the significant contribution of β phase, especially the existence of β/γ interphase to enhancement of the room-temperature ductility in multicomponent TiAl alloys.

For the safe operation of high temperature equipment, it is necessary to ensure creep rupture ductility of the components from the viewpoint of notch weakening. In this study, the effect of heat treatment conditions on creep rupture ductility was evaluated and its underlying metallurgical mechanism was investigated with using a forged Ni-based superalloy Udimet520. In order to improve the creep rupture ductility without lowering the creep rupture strength, it is important to increase both intragranular strength and intergranular strength in a balanced manner. For this purpose, it was clarified that 1) secondary γ' phase within grains should be kept fine and dense, 2) grain boundaries should be sufficiently covered by M 23 C 6 carbide by increasing its phase fraction, and 3) tertiary γ' phase within grains should be redissolved before the start of creep. To obtain such a precipitate state, it is essential to appropriately select the cooling rate after solution treatment, stabilizing treatment and aging treatment conditions.

This research compares creep crack growth behavior of two heats of creep strength enhanced ferritic (CSEF) steel, grade 91. These heats represent extremes of creep damage susceptibility, one heat exhibiting low creep ductility and the other high creep ductility. Creep crack growth tests were performed with compact tension specimens and were monitored with direct current potential drop and optical surface measurements. Load line displacement was measured throughout the duration of the tests. Specimens were sectioned, mounted, and analyzed using optical and scanning electron microscopy to assess the presence of oxidation, micro-cracking, creep damage, and void density. Tests were performed over a range of initial stress intensities on the low ductility material to investigate the impact of creep ductility. Metallurgical evidence and test data for each crack growth test was assessed to evaluate crack growth behavior linked to creep crack growth parameter (C*) and stress/creep damage distribution in the vicinity of the crack.

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.

MarBN steels, originally developed by Professor Fujio Abe at NIMS Japan, have undergone significant advancement in the UK through a series of government-funded collaborative projects (IMPACT, IMPEL, INMAP, IMPULSE, and IMPLANT). These initiatives have achieved several major milestones, including operational power plant trials, full-scale extruded pipe production, matching welding consumable development, and most notably, the creation of IBN-1—a new steel demonstrating 30-45% higher creep strength than Grade 92. However, like other creep strength-enhanced ferritic steels, IBN-1 shows reduced creep ductility under the lower stress conditions typical of operational use. Since adequate creep ductility is essential for component damage tolerance and effective in-service monitoring, this study investigates the effects of an alternative normalizing and tempering heat treatment on cast IBN-1. The research presents creep rupture test results showing improved ductility and analyzes the microstructural mechanisms responsible for this enhancement.

Diffusion bonded compact heat exchangers have exceptionally high heat transfer efficiency and might significantly improve the performance and reduce the cost of supercritical carbon-dioxide Brayton cycle power plants using high temperature heat sources, like high temperature nuclear reactors and concentrating solar power plants. While these heat exchangers have an excellent service history for lower temperature applications, considerable uncertainty remains on the performance of diffusion bonded material operating in the creep regime. This paper describes a microstructural modeling framework to explore the plausible mechanisms that may explain the reduced creep ductility and strength of diffusion bonded material, compared to wrought material. The crystal plasticity finite element method (CPFEM) is used to study factors affecting bond strength in polycrystals mimicking diffusion bonded microstructures. Additionally, the phase field method is also employed to simulate the grain growth and recrystallization at the bond line to model the bonding process and CPFEM is used to predict the resulting material performance to connect processing parameters to the expected creep life and ductility of the material, and to study potential means to improve the structural reliability of the material and the resulting components by optimizing the material processing parameters.

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 simple Design by Formula 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. The metallurgical findings from the selected tests are the focus of the Part II paper. The association of performance with notch geometry, weld strength, and other potential contributing factors will be highlighted with a primary objective of informing the reader of the variability, and heat-specific behavior that is observed among this class of alloys widely used in modern thermal fleet components and systems.

Ductility dip cracking (DDC) is known to occur in highly restrained welds and structural overlays made using high chromium (Cr) nickel (Ni) based filler metals in the nuclear power generation industry, resulting in costly repairs and reworks. Previous work explored the role of mechanical energy imposed by the thermo-mechanical cycle of multipass welding on DDC formation in a highly restrained Alloy 52 filler metal weld. It was hypothesized that imposed mechanical energy (IME) in the recrystallization temperature range would induce dynamic recrystallization (DRX), which is known to mitigate DDC formation. It was not shown however that IME in the recrystallization temperature range (IMERT) induced DRX. The objective of the work is to discern if a relationship between IMERT and DRX exists and quantify the amount of DRX observed in a filler metal 52 (FM-52) groove weld. DRX was analyzed and quantified using electron beam scattered diffraction (EBSD) generated inverse poll figures (IPF), grain surface area and grain aspect ratio distribution, grain orientation spread (GOS), kernel average misorientation (KAM), and grain boundary (GB) length density. From the analysis, GOS was determined to be an unsuitable criterion for quantifying DRX in multipass Ni-Cr fusion welds. Based on the observed criteria, higher IMERT regions correlate to smaller grain surface area, larger grain boundary density, and higher grain aspect ratio, which are all symptoms of DRX. High IMERT has a strong correlation with the symptoms DRX, but due to the lack of observable DRX, creating a threshold for DRX grain size, grain aspect ratio, and GB density is not possible. Future work will aim to optimize characterization criteria based on a Ni-Cr weld with large presence of DRX.

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.

Long-term creep strength property of creep strength enhanced ferritic steels was investigated. Stress dependence of minimum creep rate was divided into two regimes with a boundary condition of macroscopic elastic limit which corresponds to 50% of 0.2% offset yield stress (Half Yield). High rupture ductility was observed in the high stress regime above Half Yield, and it was considered to be caused by relatively easy creep deformation throughout grain interior with the assistance of external stress. Grades T23, T/P92 and T/P122 steels represented marked drop in rupture ductility at half yield with decrease in stress. It was considered to be caused by inhomogeneous recovery at the vicinity of prior austenite grain boundary, because creep deformation was concentrated in a tiny recovered area. High creep rupture ductility of Grade P23 steel should be associated with its lower creep strength. It was supposed that recovery of tempered martensitic microstructure of T91 steel was faster than those of the other steels and as a result of that it indicated significant drop in long-term creep rupture strength and relatively high creep rupture ductility. The long-term creep rupture strength at 600°C of Grade 91 steel decreased with increase in nickel content and nickel was considered to be one of the detrimental factors reducing microstructural stability and long-term creep strength. The causes affecting recovery of microstructure should be elucidated in order to obtain a good combination of creep strength and rupture ductility for long-term.

Cold working and bending during boiler manufacturing can induce strain hardening in austenitic stainless steel, potentially compromising creep ductility and leading to premature failures during operation. While design codes like ASME I, PG 19 provide guidelines for maximum strain levels before solution treating is required, industry concerns suggest these limits may be too high, prompting some boiler manufacturers to implement more conservative thresholds. This study examined the creep ductility of four austenitic stainless steels (TP310HCbN, XA704, TX304HB, and Sanicro 25) at prior strain levels of 12% and 15%, with Sanicro 25 demonstrating the highest ductility, followed by TX304HB, XA704, and TP310HCbN. Solution annealing successfully restored creep ductility to exceed 10% elongation in all materials, though this treatment may be necessary at strains of 12% and 15% for all materials except Sanicro 25 to ensure adequate creep ductility. The findings suggest that ASME I PG 19 guidelines for austenitic stainless steels containing Cb, V, and N should be reviewed, as lower strain limits could help reduce strain-induced precipitation hardening failures.

The relationship between the hot workability and the precipitation morphology of γ′ phase in the Alloy U520 was examined with a focus on the presence of γ′-nodule. To change the morphology of γ’ phase, forged bars of the Alloy U520 were solution treated followed by cooling process with the cooling rates of 5~100 K/h. After the heat treatment, both γ’ phases of intragranular particle and nodule along grain boundaries were observed, and the both sizes increased by slowing down the cooling rate. That is, the area fraction of γ’-nodule increased from about 0.1 % in the sample cooled at 100 K/h to about 70 % at 5 K/h. In Gleeble tension test, the slow-cooled samples basically exhibited higher ductility than water-quenched samples below the γ′-solvus temperature. However, the ductility was maximized in the sample cooled at 20 K/h, and excessive decrease of cooling rate resulted in a drop in ductility. EBSD analysis revealed that dynamic recrystallization (DRX) was often occurred in grain interior but suppressed at γ′-nodule area, indicating that presence of γ′-nodule had a negative influence on hot workability at subsolvus temperature.

Tempered martensitic 9-12%Cr steels bearing tungsten, such as P92 and P122 showing higher creep rupture strength than the conventional steel P91, have been developed for thick section components in ultra-supercritical (USC) boilers. However, their creep strength is not sufficient for applying at the steam condition of 650°C/35MPa or above, which is a recent target condition in order to increase plant efficiency. The research and development project in NIMS on advanced high-Cr steels which can be applied at the steam condition of 650°C/35MPa as boiler components with large diameter and thick section has been carried out since 1997. In this project, it has been revealed that the addition of boron more than 0.01 mass% to the 0.08C-9Cr- 3W-3Co-V,Nb-<0.00ЗN steel remarkably improves creep strength. The boron enriched in M 23 C 6 carbides near prior-austenite grain boundaries suppresses coarsening of these carbides during creep deformation, leading to excellent microstructural stability and creep strength. Further improvement of creep strength is achieved by the addition of appropriate amount of nitrogen which enhances precipitation of fine MX. Excess addition of nitrogen to the high-B containing steel reduces creep rupture lives and ductility. The highest creep strength is obtained in the 0.08C-9Cr-3W-3Co-0.2V-0.05Nb-0.0139B-0.0079N (mass%) steel, resulting in excellent creep strength in comparison with that of P92 and P122. This steel shows good creep ductility even in the long term. It is, therefore, concluded that this high-B bearing 9Cr-3W-3Co-V,Nb steel with the addition of nitrogen in the order of 0.008 mass% is the promising candidate which shows superior creep strength without impairing creep ductility for thick section components in the 650°C-USC plant.

The creep strength and ductility of Grade P22 steel (2¼ Cr) was measured at 600°C under standard uniaxial tensile conditions at 150MPa. Test specimens were prepared by solution heat treatment at austenitization temperatures ranging from 900°C - 1200°C followed by normalization at 900°C before continuous air cooling to room temperature. In addition to specimens tested in the solution treated state, creep tests were also performed after tempering. The variable austenitization temperatures gave rise to different prior austenite grain (PAG) sizes, which in turn influenced the crystallographic packet and block boundary misorientation angle distribution. The latter parameters were measured using electron backscattered diffraction which also allowed partial reconstruction of the PAG boundaries. The time to creep failure at 600°C increased as function of PAG size up to approximately 70µm, but significantly decreased when the average prior austenite grain size measured approximately 108 µm. However, the minimum creep rate decreased even up to the largest PAG size with corresponding decrease in creep ductility. The stability of the crystallographic packet and block boundaries influences the high strength-low ductility for the large PAGs in comparison to the dominant effect of PAG boundaries at the smallest grain size where extensive recovery and recrystallization reduces creep strength.

In wrought nickel-base alloys used at elevated temperatures for extended periods of time, it is commonly observed that unwanted phases may nucleate and grow. One such phase is the eta phase, based on Ni 3 Ti, which is a plate-shaped precipitate that nucleates at the grain boundaries and grows at the expense of the strengthening gamma prime phase. In order to study the effects of eta phase on creep performance, Alloy 263 was modified to contain 3 different microstructures: standard (contains gamma prime); aged (contains gamma prime and eta); and modified (contains only eta and no gamma prime). These microstructures were then creep tested in the range of 973-1123 K (700-850°C). An extensive test matrix revealed that the eta-only modified alloy had creep rupture strengths within 10% of the standard alloy even though this alloy had no strengthening gamma prime precipitates. It also exhibited superior creep ductility. A preliminary test matrix on the aged material containing eta and gamma prime prior to the creep tests revealed that the performance of this microstructure was generally between that of the standard alloy (best) and the eta-only alloy (worst). The aged material exhibited far superior creep ductility. These results suggest that the presence of the eta phase may not be deleterious to creep ductility, and in fact, may enhance it.

Haynes 282 alloy is a relatively new Ni-based superalloy that is being considered for advanced ultrasupercritical (A-USC) steam turbine casings for steam temperatures up to 760°C. Weld properties are important for the turbine casing application, so block ingots of Haynes 282 alloy were cast for properties studies. Good, sound welds were produced using Haynes 282 weld-wire and a hot gas-tungsten-arc welding method, and tensile and creep-rupture properties were measured on cross-weld specimens. In the fully heat-treated condition (solution annealed + aged), the tensile properties of the welded specimens compare well with as-cast material. In the fully heat-treated condition the creep-rupture life and ductility at 750°C/250MPa and 800°C/200MPa of the cross-weld specimens are similar to the as-cast base metal, and repeat creep tests show even longer rupture life for the welds. However, without heat-treatment or with only the precipitate age-hardening heat-treatment, the welds have only about half the rupture life and much lower creep ductility than the as-cast base metal. These good properties of weldments are positive results for advancing the use of cast Haynes 282 alloy for the A-USC steam turbine casing application.

Inconel alloy 740, a precipitation-hardenable nickel-chromium-cobalt alloy with niobium addition, has emerged as a leading candidate material for ultra-supercritical (USC) boilers due to its superior stress rupture strength and corrosion resistance at operating temperatures near 760°C. While derived from Nimonic alloy 263, alloy 740's unique chemistry necessitates comprehensive weldability studies to address potential challenges including heat-affected zone liquation cracking, ductility-dip cracking, and post-weld heat treatment cracking. This ongoing investigation examines the alloy's weldability characteristics through material characterization studies comparing its cracking sensitivity to established aerospace alloys like Waspalloy and Inconel alloy 718. The research applies aerospace industry expertise to boiler applications requiring sections up to three inches thick, with gas tungsten arc welding and pulsed gas metal arc welding identified as the most promising processes for producing sound, crack-free welds.