Improved Creep Strength Research Articles

Microstructural characterization and creep behavior of dispersion strengthened MRI230D magnesium alloy

The microstructural changes following the dispersion of SiC nanoparticles (SiCnp) and its subsequent significance on the creep response of MRI230D alloy are examined. Nanocomposites (NCs) of MRI230D alloy are fabricated with a dispersion of 1.0SiCnp (NC1SiC) and 2.0SiCnp (NC2SiC) (wt.%) using squeeze-casting. The primary Mg (α-Mg), lamellar C36 (Mg,Al)2Ca, as well as blocky Al8Mn5 phases, are observed in the MRI230D alloy and NCs. Additionally, SiC phase is detected in NCs. The additions of SiCnp refine the grains of the NCs. The fraction of C36 increases with an increase in SiCnp in MRI230D alloy. The alloy and NCs are exposed to impression creep tests under 450–500 MPa stress and 448–498 K temperature. The NCs exhibit better resistance to creep over MRI230D alloy, and NC2SiC is the most creep-resistant. The strengthening from the increased fraction of C36, along with Orowan strengthening from SiCnp dispersion in the α-Mg matrix, give rise to improved creep strength of the NCs than the alloy. The creep in MRI230D alloy and NCs progressed by dislocation-climb with pipe diffusion.

Improved creep strength of 10% Cr steels with low N and high B content via increasing quenching temperature

An increase in quenching temperature from 1050 °C to 1100 °C is found to be an effective approach to improving the creep strength at 650 °C in two Re-containing 10% Cr martensitic steels with low N and high B contents. Creep time at creep tests of 650 °C/120 MPa increases at the expense of the appearance of the steady-state creep stage in both steels. Moreover, the room yield strength and ultimate tensile strength are also increased by 10% with increasing quenching temperature in the steel with 0.8% Cu, whereas tensile tests at 650 °C do not reveal the increment in strength in both steels. Improvement in creep and tensile properties is caused by an increase in particle strengthening due to M23C6 carbides, mainly in both steels. The NbX, VX, and “Cu”-clusters/precipitates make minor contributions to the change in particle strengthening. Refinement of M23C6 carbides in both steels is accompanied by an increase in particle number density along the martensitic laths and an increase in the fraction of fine particles with sizes below 50 nm. Such changes in M23C6 carbide dispersions are caused by an increase in the lath area per unit volume and the enrichment of the solid solution with boron because of the dissolution of W2B particles with increasing quenching temperatures.

Improving creep strength of the fine-grained heat-affected zone of novel 9Cr martensitic heat-resistant steel via modified thermo-mechanical treatment

Improving creep strength of the fine-grained heat-affected zone of novel 9Cr martensitic heat-resistant steel via modified thermo-mechanical treatment

Improving the intermediate- and high-temperature strength of L12-Co3(Al,W) by Ni and Ta additions

The effects of Ni and Ta additions on the mechanical properties in the L12 compound Co3(Al,W), the strengthening phase of Co-based superalloys, have been investigated by compression tests between room temperature and 1000 °C, in order to elucidate the effects of stability of the L12 phase on the mechanical properties. The additions of Ni and Ta, both of which are L12-stabilizers that increase the L12 solvus temperature, increase the yield strength at intermediate and high temperatures. The strength increase is shown to be more significant as the amount of additions of these elements and thereby the stability of the L12 phase increases. Two factors account for the strength increase at intermediate temperatures: The reduction of the onset temperature of yield stress anomaly (YSA-onset) due to the increased complex stacking fault (CSF) energy and the increase in both the base strength and the intensity of the yield stress anomaly associated with an increased anti-phase boundary (APB) energy on (111) planes. The strength increase at high temperatures, on the other hand, arises from the increase in the peak temperature due to the increased L12 solvus temperatures. The increased strength of the L12 phase due to a higher phase stability thus partly accounts for the improved creep strength of Co-based superalloys upon alloying with Ni and Ta.

A new strategy to improve the creep strength of a novel G115 steel by accelerating the precipitation of nano-sized MX and Cu-rich particles

A new thermo-mechanical treatment strategy is proposed to improve the creep resistance of a recently developed G115 steel. The goal of this strategy is to construct a favourable microstructure with superior thermal stability at high temperatures up to 650 °C. The improved creep strength can be ascribed to the reinforced interactions between dislocations and nano-sized MX and Cu-rich precipitates. Creep analysis suggests that these MX and Cu-rich particles exhibit more important contribution to the creep resistance than the M23C6 carbides. Following this strategy, the creep rupture life of the G115 steel can be significantly prolonged by more than two times.

Core Materials for Sodium-Cooled Fast Reactors: Past to Present and Future Prospects

Abstract The Sodium-Cooled Fast Reactor (SFR) is the most promising and technologically evolved among the six nuclear reactor concepts selected by the Generation IV International Forum. Although it is compatible with a closed fuel cycle for sustainable energy production, its competitiveness depends on being able to achieve high fuel burn-up up to 200 GWd/t. This would enable efficient fuel utilization to minimize fuel cycle costs. Hence, development of optimized core materials, especially for fuel cladding tubes that are subjected to extreme conditions of intense fast-spectrum neutron irradiation, high temperatures, and mechanical and chemical fuel–cladding interactions, continues to be a priority worldwide. Resistance to void swelling, irradiation creep, and embrittlement are required to be enhanced to minimize dimensional changes and loss of ductility in core components, which limit achievable burn-up. Current SFR core materials comprise austenitic stainless steels (SS) and ferritic-martensitic (FM) steels. Cold-worked austenitic SS grades 304, 316, 316Ti, niobium-stabilized grades FV548, EI-847, EP-172, and Ti-modified 15Cr-15Ni SS, such as D9, 1.4970, 15/15Ti SS, and ChS-68, have been used for fuel cladding and ducts in SFRs built to date, and they withstand neutron irradiation damage up to 80–100 dpa. Subsequent improvements have been made by optimizing minor elements (titanium, silicon, and phosphorous) and multi-stabilization to develop advanced grades such as Si-modified 15-15Ti, Indian Fast Reactor Advanced Clad (IFAC-1), PNC-316, EK-164, and HT-UPS, which could support burn-up of 150 GWd/t. In the case of FM steels, several commercial grades are found suitable in view of their inherent void swelling resistance to 200 dpa, and are being developed for improved creep strength by oxide dispersion strengthening to realize burn-up of 200–250 GWd/t. This article presents the development of structural materials for SFR fuel pin cladding and ducts and associated enhancement of permissible fuel burn-up. Indian and international experience resulting from extensive in-pile and out-of-pile mechanical testing and fuel–cladding interactions has been covered.

Creep Resistance of S304H Austenitic Steel Processed by High-Pressure Sliding.

Sheets of coarse-grained S304H austenitic steel were processed by high-pressure sliding (HPS) at room temperature and a ultrafine-grained microstructure with a mean grain size of about 0.14 µm was prepared. The microstructure changes and creep behavior of coarse-grained and HPS-processed steel were investigated at 500–700 °C under the application of different loads. It was found that the processing of S304H steel led to a significant improvement in creep strength at 500 °C. However, a further increase in creep temperature to 600 °C and 700 °C led to the deterioration of creep behavior of HPS-processed steel. The microstructure results suggest that the creep behavior of HPS-processed steel is associated with the thermal stability of the SPD-processed microstructure. The recrystallization, grain growth, the coarsening of precipitates led to a reduction in creep strength of the HPS-processed state. It was also observed that in the HPS-processed microstructure the fast formation of σ-phase occurs. The σ-phase was already formed during slight grain coarsening at 600 °C and its formation was enhanced after recrystallization at 700 °C.

Development of energy intensive multifunction cavitation technology and its application to the surface modification of the Ni-based columnar crystal superalloy CM186LC

The present work demonstrates a technique for the hot forging of metal surfaces in water at 1000 °C or higher, termed energy-intensive multifunctional cavitation (EI-MFC). In this process, the energy of cavitation bubbles is maximized, following which these bubbles collide with the metal surface. This technique will be employed to improve the surface structure of CM186LC/DS, a Ni-based columnar crystalline superalloy used to manufacture the rotor blades of jet engines and gas turbines that are exposed to high-temperature oxidizing environments, with the aim of improving creep strength. EI-MFC processing induces compressive residual stress in the metal that prevents the occurrence of surface cracks and also increases surface hardness, improves corrosion resistance, and increases the coefficient of friction. The latter effect can enhance the adhesion of thermal barrier coatings applied to Ni-based superalloys by thermal spraying. The technology demonstrated herein can be applied to present-day jet engine and gas turbine components and also to the production of hydrogen combustion turbines operating at 1700 °C with higher combustion efficiency than the current 1500 °C class gas turbines. In addition, the high processing energy obtained using the EI-MFC technique has the potential to flatten rough surfaces resulting from the stacking pitches of various metals manufactured using three-dimensional printers, and so improve surface strength.

Utilizing local phase transformation strengthening for nickel-base superalloys

Almost 75 years of research has been devoted to producing superalloys capable of higher operating temperatures in jet turbine engines, and there is an ongoing need to increase operating temperature further. Here, a new disk Nickel-base superalloy is designed to take advantage of strengthening atomic-scale dynamic complexions. This local phase transformation strengthening provides the alloy with a three times improvement in creep strength over similar disk superalloys and comparable strength to a single crystal blade alloy at 760 °C. Ultra-high-resolution chemical mapping reveals that the improvement in creep strength is a result of atomic-scale η (D024) and χ (D019) formation along superlattice stacking faults. To understand these results, the energy differences between the L12 and competing D024 and D019 stacking fault structures and their dependence on composition are computed by density functional theory. This study can help guide researchers to further optimize local phase transformation strengthening mechanisms for alloy development.

Fabrication routes for advanced first wall design alternatives

In future nuclear fusion reactors, plasma facing components have to sustain specific neutron damage. While the majority of irradiation data provides a relatively clear picture of the displacement damage, the effect of helium transmutation is not yet explored in detail. Nevertheless, available results from simulation experiments indicate that 9%-chromium steels will reach their operating limit as soon as the growing helium bubbles extent a critical size. At that point, the material would most probably fail due to grain boundary embrittlement. In this contribution, we present a strategy for the mitigation of the before-mentioned problem using the following facts. (1) The neutron dose and related transmutation rate decreases quickly inside the first wall of the breeding blankets, that is, only a plasma-near area is extremely loaded. (2) Nanostructured oxide dispersion strengthened (ODS) steels may have an enormous trapping effect on helium, which would suppress the formation of large helium bubbles for a much longer period. (3) Compared to conventional steels, ODS steels also provide improved irradiation tensile ductility and creep strength. Therefore, a design, based on the fabrication of the plasma facing and highly neutron and heat loaded parts of blankets by an ODS steel, while using EUROFER97 for everything else, would extend the operating time and enable a higher heat flux. Consequently, we (i) developed and produced 14%Cr ferritic ODS steel plates and (ii) optimized and demonstrated a scalable industrial production route. (iii) We fabricated a mock-up with five cooling channels and a plated first wall of ODS steel, using the same production processes as for a real component. (iv) Finally, we performed high heat flux tests in the Helium Loop Karlsruhe, applying a few hundred short and a few 2 h long pulses, in which the operating temperature limit for EUROFER97 (i.e. 550 °C) was finally exceeded by 100 K. (v) Thereafter, microstructure and defect analyses did not reveal critical defects or recognizable damage. Only a heat affected zone in the EUROFER/ODS steel interface could be detected. However, a solution to prohibit the formation of such heat affected zones is given. These research contributions demonstrate that the use of ODS steel is not only feasible and affordable but could make a decisive difference in the future design and performance of breeding blankets.

Tensile Creep Properties of Cr-Si Alloys at 980 °C in Air—Influence of Ge and Mo Addition

The tensile creep behavior of Cr-Si alloys with Cr ≥ 91 at.% was investigated in air at 980 °C with a constant load of 50–100 MPa. Additionally, the influence of substitutional alloying additions of 2 at.% Ge and Mo, leading to ternary alloys was studied. The addition of Ge or Mo results in an improvement in creep strength, with the highest strength achieved with addition of Mo. For longer creep exposure times a strong effect is observed, because of severe nitrogen uptake from the air, depending on alloy composition. Based on the results a novel mechanism for the impact of chromium nitride formation on the creep behavior is proposed.

Development of Energy Intensive Multifunction Cavitation Technology and Its Application to the Surface Modification of the Ni-Based Columnar Crystal Superalloy CM186LC

The present work demonstrates a technique for the hot forging of metal surfaces in water at 1000 °C or higher, termed energy-intensive multifunctional cavitation (EI-MFC). In this process, the energy of cavitation bubbles is maximized, following which these bubbles collide with the metal surface. This technique will be employed to improve the surface structure of CM186LC/DS, a Ni-based columnar crystalline superalloy used to manufacture the rotor blades of jet engines and gas turbines that are exposed to high-temperature oxidizing environments, with the aim of improving creep strength. EI-MFC processing induces compressive residual stress in the metal that prevents the occurrence of surface cracks and also increases surface hardness, improves corrosion resistance, and increases the coefficient of friction. The latter effect can enhance the adhesion of thermal barrier coatings applied to Ni-based superalloys by thermal spraying. The technology demonstrated herein can be applied to present-day jet engine and gas turbine components and also to the production of hydrogen combustion turbines operating at 1700 °C with higher combustion efficiency than the current 1500 °C class gas turbines. In addition, the high processing energy obtained using the EI-MFC technique has the potential to flatten rough surfaces resulting from the stacking pitches of various metals manufactured using three-dimensional printers, and so improve surface strength.

Secondary Recrystallization of Nickel-Base Superalloy CM 247 LC After Processing by Metal Injection Molding

Fabrication of parts by metal injection molding (MIM) results in very fine grain sizes. In the present investigation, heat treatments were employed to gain a larger grain size that is more creep resistant. The alloy under investigation was CM 247 LC. The composition was slightly modified to facilitate grain growth. Secondary recrystallization was observed to occur during post-sintering annealing treatments approximately 50 °C below the sintering temperature. The grain size increased from 25 µm to > 2 mm. The increase of grain size was found to improve creep strength significantly. The samples in the as-sintered condition exhibited a bimodal grain size distribution. The grain size in a thin surface zone after sintering was slightly increased because of a lower C and O content in this zone that promoted normal grain growth. Secondary recrystallization did not occur in the surface zone. This is attributed to a lack of driving force in this area.

On the rejuvenation of crept Ni-Base single crystal superalloys (SX) by hot isostatic pressing (HIP)

In the present work, we study the effect of HIP rejuvenation treatments on the creep behavior and residual life of a pre-crept single crystal Ni-base superalloy of type CMSX-4. The present work combines miniature creep experiments of precisely oriented [001] tensile creep specimens with HIP treatments and quantitative analysis of scanning and transmission electron micrographs. A HIP-rejuvenation treatment after 5.0% creep pre-strain significantly improves creep strength. However, the microstructural damage induced by the creep pre-deformation could not be fully removed. In a series of sequential creep/HIP/creep-experiments, increasing levels of pre-deformation result in increasing levels of creep rates even after identical HIP-rejuvenation treatments. The memory effect, which causes this phenomenon, is related to an accumulation of permanent microstructural damage, which is not associated with rafting or cavitation. The mechanical results obtained in the present work are interpreted based on microstructural results on the γ/γʼ-microstructure (γ-channel widths and γʼ-size), on the pore population (number density of pores, pore size distributions and pore area fractions) and dislocation substructures which have formed during creep. The results are discussed in the light of previous results reported in the literature.

Importance of austenitization temperature and ausforming on creep strength in 9Cr ferritic/martensitic steel

Small Punch Creep technique was used as a screening procedure to evaluate the creep properties of different microstructures developed in a thermomechanical simulator. The goal seek was to generate alternative microstructures in a conventional ferritic-martensitic G91 steel grade which boost thermal stability at temperatures as high as 700 °C. The developed microstructures allow studying the effect of the austenitization temperature optimized by thermodynamic calculations and the ausforming on the creep strength and ductility. The improvement in creep strength recorded was attributable to a higher number density of MX precipitates. By contrast, these microstructures showed an important reduction in creep ductility.

An improvement in creep strength of thermo-mechanical treated modified 9Cr-1Mo steel weld joint

Modified 9Cr-1Mo steel weld joints generally experience the type IV premature failure in the intercritical region (ICR) of HAZ under long term creep exposure at high temperature. Possibility of improving the resistance of this joint to type IV cracking through thermo-mechanical treatment (TMT) of the steel has been explored. Weld joints have been fabricated from the TMT and conventional normalized and tempered (NT) steels using electron beam (EB) welding process. Creep tests have been carried out on NT and TMT steels joint at 923 K (650°C) and 110–100 MPa applied stress. Creep rupture life of the TMT weld joint was significantly higher than the NT steel weld joint. Significant variations of microstructural constituents such as M23C6 precipitate; lath structure and hardness across the joint have been examined in both the joints. The coarser M23C6 precipitate and lath, and subgrain formation in the ICR resulted in the soft zone formation and was predominant in the ICR of NT steel joint. The enhanced MX precipitation through TMT processing and reduction in coarsening of M23C6 precipitate under thermal cycle resulted in improved creep rupture strength of TMT steel weld joint.

Microstructure control to improve creep strength of alumina-forming austenitic heat-resistant steel by pre-strain

At 700°C and 200MPa, the creep rupture time of W-containing alumina-forming austenitic heat-resistant (WAFA) steel increases from 1044 to 3119h by pre-strain. Pre-strain introduces a lot of dislocation into the matrix and most of the dislocations remain stable to the end of creep without recovery and recrystallization. In addition, the dislocations have enabled secondary NbC and Laves phase to be uniformly distributed and refined. However, the dislocations lower grain boundary strength due to the coarsening of precipitates along grain boundaries Therefore, the increase in the creep rupture time in spite of deterioration of grain boundary strength is attributed to the dislocation strengthening and enhanced precipitation hardening within grains.

High-Temperature Mechanical Properties of P91 Weld Metal

Abstract Increasing temperature is one of the key factors for improving the efficiency of steam power plants. Important metallurgical phenomena are activated at such high temperatures and creep resistance becomes a driving criterion for the material selection. Ferritic steels, including 2Cr, 9Cr, and 12Cr steels, are among the best candidates; their continuous development and optimization with the addition of Mo, V, Nb, and W have resulted in a significant improvement in creep strength together with a good weldability. This study investigates the high-temperature mechanical properties of two Grade 91 welded plates and focuses on the creep behavior, proposing a modified expression of the Larson–Miller parameter for the estimation of critical combinations of temperature, stress, and time, which could lead to rupture. The suggested parameter, which is highly sensitive to temperature, is able to outline the criticality of the welding and it is useful for predicting the duration of a creep test with the specimen rupture.

Improved creep strength of alumina-forming austenitic heat-resistant steels through W addition

We investigated the effect of W addition on the creep and precipitation behavior of alumina-forming austenitic (AFA) heat-resistant steels at 700°C and 160MPa. Our results showed that the creep rupture time of the W containing AFA (WAFA) increased more than that of the AFA due to enhanced precipitation hardening of Laves phase and suppression of sigma phase formation. W increased the volume fraction of Laves phase and enhanced precipitation hardening by higher driving force for the precipitation and affinity of the Laves phase. Therefore most W were consumed for the precipitation of the Laves phase, however Mo content contributed to the precipitation of M23C6 as well as precipitation of Laves phase due to the lower affinity of Laves phase. The M23C6 in AFA contained enough Mo to form the sigma phase, which served as an initiation site for cavitation and crack propagation. The sigma phase grew and coarsened at the expense of M23C6 and Mo-rich Laves phase to match the stoichiometric composition of the sigma phase, and finally creep strength of AFA was greatly reduced.

A Study on the Microstructural Characterization of René 142 Deposited Atop René 125 Processed through Scanning Laser Epitaxy

Advancements in the design, optimization and manufacture of turbine engine hot-section components during the past few decades have contributed enormously to the improvement in power-ratings and efficiency levels of gas turbine engines. Nickel-base superalloys are extensively used to produce the hot-section components as this class of alloys offer improved creep strength and higher fatigue resistance compared to other alloys due to the presence of precipitate-strengthening γ' phases i.e. Ni3[Ti, Al, Ta etc.] in the normally face centered cubic (FCC) structure of the solidified nickel. Although this second phase is the main reason for the improvement in properties, it also results in increased processing difficulty as these alloys are prone to crack formation. In this work, we demonstrate powder-bed additive manufacturing of René 142 onto René 125 substrates through scanning laser epitaxy (SLE). René 142 is a high strength, nickel-base directionally solidified (DS) alloy that has high rupture strength, excellent resistance to grain boundary cracking, and superior high-velocity oxidation resistance. Successful deposition of René 142 on René 125 provides an avenue to repair legacy hot-section components by depositing superior quality alloys at the damage locations. The microstructure of the deposited René 142 is observed to follow the polycrystalline or EQ morphology of the underlying René 125 substrate. The SLE processed René 142 exhibits dense and crack-free deposits, and microstructure refinement compared to the underlying cast René 125 substrate. This work is sponsored by the Office of Naval Research through grants N00014-11-1-0670 and N00014-14-1-0658.