Creep Performance Research Articles

Experimental investigation on durability of prefabricated rollable asphalt: Creep and fatigue performance evaluation

Rollpave, a thin prefabricated rollable asphalt layer produced in a factory and rolled on a reel for laying on the existing surface, has attracted attention in the pavement industry due to its rapid treatment capabilities. However, the long-term durability of Rollpave needs further evaluation before considering widespread use. This study aims to assess the durability of Rollpave by evaluating the fatigue and creep performance of its modified bitumen and asphalt mixture. The results highlight that the creep stiffness of modified bitumen at −12°C is half of the corresponding value of virgin bitumen, and lowering the temperature to −18°C increases its creep stiffness to 78.44 MPa. At the same time, it is still 20 MPa less than virgin bitumen. Besides, it is showed the fatigue life of modified bitumen is 23 times greater than the corresponding value of virgin bitumen at 20°C and strain levels of 5 %, and modified bitumen has better stress relaxation ability leading to damage endurance. Additionally, it is observed that the flexural stiffness of the Rollpave mixture drops by only 23 % after 600,000 cycles under 400 µɛ repeated loading. Furthermore, it is indicated that although the creep performance of Rollpave is sensitive to stress and temperature, it has a noticeable recovery ability and rutting resistance, with the average value of non-recoverable creep compliance and percent recovery at stress level of 3.2 kPa at 64°C being 0.213 kPa−1 and 54 %. The asphalt dynamic creep test results of the Rollpave mixture also indicated nearly 20,000 µɛ of cumulative permanent strain at 50°C in the 9000th loading cycle.

Influence of powder particle size distribution on the creep performance of Ti-48Al-2Cr-2Nb alloy fabricated via electron beam-powder bed fusion

The effects of particle size distributions (PSDs) on the microstructure and creep behavior of Ti-48Al-2Cr-2 Nb fabricated via electron beam-powder bed fusion (EB-PBF) were investigated. Three PSDs of 45–150 (PSD-A), 38–150 (PSD-B), and 38–180 µm (PSD-C) were used as powder feedstock. Widening the PSD range decreased the relative density of fabricated parts and promoted the formation of banding microstructure. Moreover, samples fabricated with wider PSDs had a higher volume of lack-of-fusion (LoF) defects; however, the content of gas pores decreased significantly in PSD-C, which was related to the presence of coarse powder particles. The volume of the LoF defects in PSD-C was approximately 2.8 times of the same defects in PSD-A, which influenced the creep performance (at 750℃) and failure mechanism of Ti-48Al-2Cr-2 Nb. A longer steady-state creep period at different stress levels was identified for the samples fabricated with the narrowest PSD (45–150 µm). Widening the PSD span also caused weakness in the creep properties and data scattering at higher stress levels of 300 and 350 MPa. The LoF defects and coarse grains accelerated the crack initiation and propagation in Ti-48Al-2Cr-2 Nb during creep. This study represents the first report on the impact of PSD on the creep properties of TiAl alloys fabricated via EB-PBF, which offers new insights and enhances understanding of the powder-process-microstructure-property relationship.

A novel nickel-based superalloy with excellent high temperature performance designed for laser additive manufacturing

The nickel-based superalloys prepared by additive manufacturing (AM) commonly exhibit inferior mechanical properties at high temperatures, particularly in terms of ductility and creep resistance, compared to the corresponding conventional manufactured (CM)alloys, and this significantly hinders the widespread application of AM nickel-based superalloys in the aerospace industry. To address this problem, a novel nickel-based superalloy for additive manufacturing, Hastelloy X-AM (HX-AM), using the yttrium (Y) alloying strategy has been developed in this work. The results show that the Y element is present in the matrix as Ni5Y phase and yttrium oxide nanoparticles after laser additive manufacturing, exerting a significant inhibitory effect on the precipitation and coarsening of detrimental second phases at grain boundaries (GBs) during subsequent heat treatment or service processes. The HX-AM alloy exhibits an exceptional matching relationship between strength (ultimate tensile strength = 315 ± 3 MPa) and ductility (total elongation = 56 ± 1.7 %) at 900 °C. What's more, the creep performance of HX-AM alloy is significantly improved simultaneously compared with CM Ni-based alloys at 900 °C, overcoming the longstanding challenge of inferior creep performance in AM nickel-based superalloys at high temperature. The fatigue lifetime of HX-AM alloy reaches 295.6 h at 900 °C/40 MPa, showing over fifteenfold enhancement compared to that of CM Hastelloy X alloy.

Effect of solid solution heat treatment duration on microstructure evolution and the high temperature and low stress creep properties of a low-cost fourth-generation single crystal superalloy

Low-cost Ni-based single crystal superalloy with high performance attracted much attention to the researchers in recent years. A Ru-free low-cost fourth-generation single crystal superalloy was designed in this study, which was rarely reported previously. This study mainly focused on the effect of solid solution heat treatment duration on the microstructure evolution and high temperature and low stress creep properties on the experimental superalloy. By the metallographic method, it was revealed that the dendritic segregation was gradually eliminated with the increasing solid solution heat treatment duration. Segregation of Al and Ta were well homogenized as well as W. Segregation of Re could still be observed after 32 h solid solution heat treatment. Quantity and fraction of pores increased, which has a significant influence on the creep properties. Compositional variation between dendritic and inter-dendritic region was the main factor that caused the qualitative difference in the size and volume fraction of the γ' phase. Gap of the γ' size and volume fraction between dendritic cores and inter-dendritic region after solid solution heat treatment became minor. Homogeneous dislocation networks formed during the creep test and the ruptured dislocation networks of the specimen subjected to 32 h solid solution heat treatment were found. In the tertiary stage of the creep test, the primary creep deformation mechanism is the super-dislocations with Burgers vector of a 〈010〉 shearing the rafted γ' phases. The specimen subjected to 16 h solid solution heat treatment has the optimal creep performance, followed by those subjected to 32 h and 8 h, respectively.

Evaluation of Bending Creep Performance of Laminated Veneer Lumber (LVL) Formwork for the Design of Timber Concrete Composite (TCC) Structures

Evaluation of Bending Creep Performance of Laminated Veneer Lumber (LVL) Formwork for the Design of Timber Concrete Composite (TCC) Structures

Microstructure evolution and assessment of creep behaviour of SiC nanoparticles dispersed squeeze-cast Mg-5.0Al-2.0Ca-0.3Mn alloy

The microstructural modification and creep behaviour of Mg-5.0Al-2.0Ca-0.3Mn (AXM520) alloy with SiC nanoparticles (SiCnp) dispersion have been investigated. The concentrations of the SiCnp are varied from 0.5 to 3.0 (wt.%), and the squeeze-cast nanocomposites (NCs) are abbreviated as NC0·5SiC, NC1·0SiC, NC2·0SiC and NC3·0SiC. The microstructures of the AXM520 alloy and NCs consist of a primary solid solution (α-Mg), a eutectic of α-Mg and (Mg,Al)2Ca (C36), and an Al8Mn5 phase. Additionally, the SiC phase is also present in the NCs. The continuous network of the C36 phase in the NCs is fragmented and becomes discontinuous with the increase in the fraction of the SiCnp. All the NCs revealed improved creep performance compared to the AXM520 alloy. The creep rate of the NCs decreases with the increase in the SiCnp content. The NC2·0SiC exhibited an improvement in creep resistance by 73.2 % compared to the alloy. However, the creep resistance deteriorated since the amount of the nanoparticles was further increased in the NC3·0SiC, leading to agglomeration. The stress exponents vary from 5.0 to 6.7, and activation energies vary from 89.8 to 101.8 kJ/mol, implying that the creep in the materials is controlled by the climb of dislocation assisted by the pipe diffusion. The pile-ups of dislocations took place around the C36 phase and near the SiCnp. The additional strengthening owing to the presence of the SiCnp in the NCs was responsible for their improved creep performance compared to the AXM520 alloy.

Temperature-dependent creep damage mechanism and prediction model of fiber-reinforced phenolic resin composites

Carbon fiber-reinforced phenolic resin (CF/PF) and glass fiber-reinforced phenolic resin (GF/PF) composites are crucial for thermal insulation and load-bearing capabilities in the aerospace industry. Understanding their creep behavior at various temperatures is essential. This study investigates the impact of temperature on the bending creep properties of CF/PF and GF/PF composites, focusing on deformation response, damage mechanisms, and prediction models. Results reveal that the decrease in creep properties at elevated temperatures is mainly attributed to the increased viscoplastic strain. At room temperature, CF/PF composite demonstrates exceptional creep properties. CF/PF composite exhibits heightened sensitivity to elevated temperatures, undergoing creep rupture at 210 °C and 240 °C with failure strains of 0.48 % and 0.49 %, respectively. High-temperature damage mechanisms have been analyzed in depth using a combination of thermal stress finite element simulation and microscopic characterization. Thermal stress is directly related to the difference in thermal expansion between the fibers and the resin. The influence of fiber distribution and geometry results in the interface being subjected to both normal and shear stresses. The accumulation of interfacial damage increases the plastic deformation and degrades creep performance. Additionally, an advanced temperature-dependent Burgers model has been developed and effectively predicted the creep curves under different temperatures and loads. This model demonstrated great potential as a general predictive model.

Synchronously enhancing the creep performance along two different loading directions in hot-rolled Mg-2wt%Sn alloy plate

To save rare-earth (RE) resource and develop creep-resistant Mg alloys in a low-cost way, RE-free Mg alloys are strongly recommended recently. However, serious creep anisotropy and poor creep resistance are still the main problems of safely using RE-free Mg alloys. In the present research, one binary Mg-2wt%Sn hot-rolled plate was employed for studying the compressive creep performance at 393 K. It was found that different activities of basal slip, twinning and pyramidal slip weakened the creep performance along the rolling direction and caused the inevitable creep anisotropy. To weaken the creep anisotropy between the rolling direction and normal direction, multi-directional forging (MDF) with three passes was performed at 393 K. Through inducing high-density twins before creep loading, detwinning was frequently activated during creep loading instead of inducing new twins. As a result, the creep anisotropy was successfully weakened in the MDF processed alloy. Moreover, the creep performance was synchronously enhanced along both loading directions rather than deteriorating the creep performance along the normal direction. It is thus expected that this RE-free way of preparing creep-resistant Mg alloys can be widely accepted in the relative application fields.

Enhancing Porous Asphalt Performance by Adding Waste Mask Fiber: Experimental Analysis of Stiffness, Fatigue, and Creep at Optimum Asphalt Content

Purpose: This study aims to evaluate the impact of incorporating waste mask fibers into porous asphalt mixtures, focusing on stiffness, fatigue, and creep performance at the optimum asphalt content (OAC). Theoretical Reference: The study is based on sustainable construction and materials engineering theories, emphasizing the recycling of waste materials to enhance the performance of construction materials and mitigate environmental impacts. Method: Laboratory experiments were conducted by adding waste mask fibers at varying levels (0%, 1%, 2%, and 3% by weight of asphalt) to the porous asphalt mixtures. Test specimens were designed and evaluated for stability, flow, Marshall Quotient, Voids in Mineral Aggregate (VIM), indirect tensile strength (ITS), stiffness (ITSM), fatigue, and creep performance. Results and Conclusion: The addition of 1% waste mask fibers at the OAC is recommended as it meets the criteria for stability, flow, Marshall Quotient, and VIM. The Cantabro Test showed enhanced resistance, while the ITS Test indicated increased indirect tensile strength. The ITSM Test revealed heightened stiffness of the mixture. Although fatigue testing demonstrated that the porous asphalt mixture could bear loads until collapse at a certain stress level, the creep test confirmed the mixture's resistance to permanent deformation due to traffic loads. Implications of research: The findings suggest that incorporating waste mask fibers into asphalt mixtures can improve mechanical properties and durability, offering a sustainable solution for mask waste disposal. This approach can be adopted by policymakers and engineers to enhance road construction practices while addressing environmental concerns. Originality/value: This study provides novel insights into the recycling of waste mask fibers in asphalt mixtures, demonstrating their potential to improve performance and sustainability in road construction. The research offers a pioneering approach to addressing the environmental challenges.

Egg White and Eggshell Mortar Reinforcing a Masonry Stone Bridge: Experiments on Mortar and 3D Full-Scale Bridge Discrete Simulations

In this study, experimental and numerical investigations were conducted to examine the time-dependent creep and earthquake performance of the historical Plaka stone bridge, which was constructed in 1866 in Arta, Greece. During the original construction of the bridge in 1866, Khorasan mortar with an egg white additive was used between the stone elements. Furthermore, when the bridge underwent restoration in 2015, Khorasan mortar with an eggshell additive was employed between the stone elements. Consequently, two distinct 3D finite-difference models were developed for this study. In the first bridge model, egg white was used in the Khorasan mortar, replacing water at various proportions of 0%, 25%, 50%, 75%, and 100%. In contrast, for the second model, eggshell was incorporated into the Khorasan mixture at percentages of 25%, 50%, 75%, and 100%, relative to the lime amount. Subsequently, the mortars were subjected to curing periods of 1 day, 7 days, and 28 days, and their mechanical properties were determined through unconfined compression strength experiments. Taking into account the determined strengths of the mortars, the kn and ks stiffness values of the interface elements between the stone elements and Khorasan mortar were calculated. In the 3D model, each stone element was individually represented, resulting in a total of 1,849,274 stone elements being utilized. Non-reflecting boundary conditions were applied to the edge boundaries of the bridge model, and the Burger creep and Mohr–Coulomb material models was employed for time-dependent creep and seismic analyses, respectively. Subsequently, time-dependent creep analyses were conducted on the bridge, and seismic events that occurred in the region where the bridge was located were simulated to assess their impact. Based on the results of the time-dependent creep and seismic analyses, we observed that the use of 50% eggshell-mixed Khorasan mortar between the stone elements had a positive influence on the earthquake and creep behaviors of both restored and yet-to-be-restored historical bridges.

Large feature cross-weld creep test performance in the novel creep strength enhanced ferritic steel Thor® 115 including consideration of the post-weld heat treatment temperature

No present manuscripts have assessed the cross-weld creep performance of Tenaris high oxidation resistance (Thor®) 115. Furthermore, few studies (if any) have compared the performance of martensitic creep strength enhanced ferritic (CSEF) steels in the as-welded or post-weld heat treatment (PWHT) temperature, and for feature test geometries. As CSEF steels are widely specified in modern thermal fleet applications, it is critically important to assess weldment performance and consider the influence of PWHT for new construction or repair applications. In this study, it is shown that the life and failure location in Thor® 115 is not a function of PWHT temperature and that failures in this steel are identical to those for the mainstay CSEF steel Grades 91 and 92.

Short term creep properties of WPC at different temperatures—An experimental and numerical investigation at different stress levels

AbstractUsing a two‐step molding process, wood plastic composite (WPC) were prepared under temperatures ranging from 160 to 180°C. Uniaxial compression tests of WPC were conducted at temperatures of −20°C, 0°C, 20°C, and 40°C, determining stress levels of 30%, 45%, and 60% based on the test results. Subsequently, 24‐hour compression creep tests were performed for WPC under 12 different conditions. The Findley power law model was employed to fit the experimental results, indicating that creep deformation increased with both stress level and environmental temperature. Notably, specimens at 0°C failed after sustaining a load for 13.6 hours at 60% stress, while those at 20°C failed after only 0.72 hours under the same conditions. Overall, the Findley model effectively predicted the creep behavior of WPC under all temperature conditions and stress levels.Highlights The influence of temperature variation on the strength of WPC. Short term creep of WPC under different stress levels. Creep of WPC at different temperatures. Prediction of creep performance under different operating conditions.

Low-temperature creep performance of additive manufactured Ti–6Al–4V

Abstract Additive manufacturing enables the fabrication of versatile and cost-effective metallic-alloy components from a digital data model. This study explores the prospects of selective laser melting (SLM), an additive manufacturing technique, for fabricating Ti6Al4V alloy components from Ti6Al4V alloy powders. Selective laser melting parameters, such as laser power, scanning speed, powder thickness, hatching space, and scanning strategy, are carefully selected through a series of experiments. The metallurgical characteristics (microstructure, grain orientation, and phase composition), microhardness, and creep performance of the as-fabricated specimens are tested and analyzed. The kinetics of phase transformation and rupture mechanism are determined using advanced instrumental characterization tools, such as field emission scanning electron microscope, energy dispersive X-ray spectroscope, X-ray diffractometer, and transmission electron microscope.

Atomic-scale investigation of interactions between Cu solute and defects in Zr alloys

The effects of dilute Cu solute on dislocations and grain boundaries in Zr alloys have been investigated at the atomic scale by atom probe tomography measurements, molecular dynamics, and molecular statics simulations. We demonstrate that Cu can segregate at dislocations and form nanoclusters in Zr alloys to retard the dislocation motion at high-stress creep. The solid solution strengthening of Cu is stronger than Sn and Nb at the same concentration and 673 K. Moreover, Cu is inclined to segregate at grain boundaries in Zr alloys, which is expected to reduce the Coble creep rate. As such, minor Cu alloying is an effective strategy to tailor the properties of dislocations and grain boundaries for enhancing the creep performance of Zr alloys.

Elevated-temperature creep properties and deformation mechanisms of a non-equiatomic FeMnCoCrAl high-entropy alloy

This study investigates the high-temperature creep behavior of a novel (Fe50Mn30Co10Cr10)91Al9 non-equiatomic high-entropy alloy (HEA) spanning the temperature range of 873–973 K and stress conditions from 50 to 90 MPa. Microstructural examinations and theoretical analyses are conducted to elucidate the performance parameter and deformation mechanisms of the HEA under sustained high-temperature conditions, providing an theoretical basis for application prospects. The results indicated that the cold-rolled and annealed alloy exhibited excellent creep resistance under conditions of 923 K/50 MPa and 873 K/50 MPa. The creep mechanism of the HEA at 923 K was identified as being primarily controlled by dislocation climb, as determined by stress exponent fitting calculations. Differing from common equiatomic HEAs, creep in these cases is controlled by solute atoms and mobile dislocations mechanisms. Microstructural characterization validated identified creep trends and deformation mechanisms, revealing diverse micro-mechanisms, such as dislocation jogs and new phase precipitation under various deformation conditions. Additionally, the 'climb-dominant mechanism model' was used to discuss the self-consistency of experimental parameters, providing in-depth insights into the contribution of elemental parameter variations to dislocation climb, as well as the reasoning behind creep performance concerning stress and temperature dependencies.

Investigating the Dynamic Creep and the Tensile Performance of Zeolitic Tuff-modified Warm Asphalt Mixtures

Aims This research investigates the effect of adding natural Jordanian Zeolitic tuffs to the SUPERPAVE asphalt mixture at the dynamic creep and the indirect tensile performance. Background Rutting and fatigue are considered the most common types of distress that affect pavements all over the world. Many factors can affect these properties, such as different aggregate properties, asphalt grades, and properties, as well as the type and amount of the mineral filler. Mineral fillers play an important role in enhancing the asphalt mixtures' performance by filling the voids in the mixture besides improving the cohesion of the binder as it has a small weight in addition to a very large surface area. Generally, the powder of limestone is the most used type of filler. However, studies showed that many materials could also be used as mineral fillers successfully. The focus of this study is using natural Jordanian Zeolitic tuff as a mineral filler. Jordanian zeolitic tuff mainly consists of Phillipsite, chabazite, and faujasite, which are considered the most abundant Zeolitic tuff minerals. The pyroclastic material is widely distributed in the Badia region of northeast Jordan. Zeolitic tuffs are located in Jabal Aritayn (30km northeast of Azraq), Tlul AlShahba (20 km east of Al Safawi), Tal Al-Rimah (35 km northeast of Al Mafraq), and other small deposits in central and south Jordan. Objectives The aim of this research was to study the impact of Zeolitic tuffs on overall Superpave asphalt mixture performance. Methods The specimens were prepared using the optimum asphalt content obtained by the SUPERPAVE method. Dynamic Testing System DTS-16 was used to apply both dynamic creep and indirect tensile tests that give a good idea about the rutting and fatigue behavior of asphalt mixtures. Both tests were conducted at 25 °C. Results The Zeolitic tuffs modified asphalt mixtures have lower accumulated strains and higher creep stiffness compared to the control mixture. Subsequently, the indirect tensile test results showed that the modified asphalt mixtures have a higher resilient modulus than the control asphalt mix. Generally, using Zeolitic tuffs as a modifier of asphalt mixtures enhanced the rutting and fatigue resistance. The results showed that asphalt mixtures fortified with 25% Zeolitic tuffs emerged as the best contenders for rutting resistance. Also, mixtures enriched with 50% Zeolitic tuffs stood out in fatigue resistance performance. Conclusion The best dynamic creep performance was by adding 25% Zeolitic tuffs by the mineral filler mass, while the best resilient modulus was by adding 50% Zeolitic tuffs by the mineral filler mass. Other The overarching goals encompass understanding the dynamic creep behavior of Zeolitic tuff-infused asphalt blends, deciphering its indirect tensile performance, pinpointing the ideal replacement ratio for mineral fillers, and drawing a performance comparison between WMA and conventional Hot Mix Asphalt (HMA) compositions.

Study on failure mechanism of suspension lug elbow in an ethylene cracking pyrolyzer

The cracking of a suspension lug elbow caused a shutdown in a certain petrochemical enterprise. In this study, the authors conducted physical and chemical analysis to investigate the cracking mechanism. Macroscopic observations indicated no signs of corrosion or thinning on the inner and outer surfaces of the head, and the cracks were distributed in the body of the head and the ear. Chemical composition analysis revealed that the main chemical components of the failed elbow met the standard requirements. Metallographic observations showed significant creep damage in the substrate material, and after long-term high-temperature service, both the room temperature and high-temperature mechanical properties of the failed elbow significantly deteriorated. The failure sample exhibit a fracture elongation of 0 %, and the high-temperature creep performance of the head material was only 6.868 h, both lower than the standard requirement of a fracture elongation of 4 % and 120 h of high-temperature creep performance. Fracture surface observation revealed obvious creep voids and fatigue striations, indicating severe creep damage and the influence of alternating loads on the head. Field observations indicated that the head experienced significant shaking inside the furnace. Therefore, simulation analysis suggested that the maximum stress coincided with the crack location, ranging from 121 MPa to 131 MPa, far exceeding the tensile strength at the operating temperature when the elbow was subjected to alternating loads generated by left–right and back-forth swinging. In conclusion, the failure of the suspension lug elbow was caused by creep-fatigue cracking. This study reveals that, even when the internal medium is in single gaseous phase, elbows are still susceptible to cracking under the combined effects of high temperature vibration and creep interaction. To avoid future failures, strengthening the monitoring of the cracking furnace operation, controlling the load appropriately, and preventing severe vibration and shaking of the furnace tube are recommended.

Homogenization effects by Co additions to high temperature and low stress creep performance of a 4th-generation Ni-based single crystal superalloys

The important factors limiting the fourth-generation Ni-based single-crystal superalloys (Ni-SXs) are the increases in cost and density. A cheaper and lighter potential alternative element of Ru, Co's comprehensive effects were studied. The effects of Co and inhomogeneity between dendrite core (DC) and interdendritic region (ID) on creep performance at 1100 °C and 137 MPa in a 4th generation Ni-SXs were systematically investigated. The results show that the Ni-SXs with 9 wt% Co has the longest creep lifetime. Co additions alleviate the overall element segregations between DC and ID, especially Re, Cr, and W. And reverse partitioning behavior of Co also changes the partitioning ratio of elements between γ and γ′ phases, resulting in the inhibition of TCP phases precipitation for most creep time to avoid excessive consumption of solid solution strengthening elements and local stress concentration. The decrease of the lattice constant of γ reduces lattice misfit between γ and γ′ with the addition of Co, leading to the enlargement of dislocation network spacing and less resistance for movement or shearing of dislocation. The decrease of γ′ solvus temperature with Co addition promotes the dissolution of γ′ at high working temperature, resulting in severe topological inversion of γ/γ′ microstructures at ID. In addition, the main creep cracks of all Ni-SXs appear at ID, so the inhomogeneity between DC and ID is concerned.

Role of Austenite Stability in Elevated Temperature Mechanical Properties of Gas Metal Arc-Directed Energy Deposition Austenitic Stainless Steels

A gas metal-directed energy deposition process was used to fabricate builds using two commercial weld fillers used in power generation applications, 16-8-2 and 316H. Microstructure stability and mechanical properties were investigated through room-temperature and elevated temperature tensile testing and creep testing at 650°C, 750°C, and 825°C. 16-8-2 exhibited reduced austenite stability which resulted in athermal martensite formation after aging at 650°C for 1000 h and strain-induced martensite formation during room-temperature tensile testing. 316H exhibited relatively higher austenite stability due to increased alloying content, resulting in no athermal martensite or strain-induced martensite. Due to lower austenite stability, ferrite formed during creep at 650°C in 16-8-2, which resulted in reduced creep life and lower creep ductility compared to 316H. At 750°C and 825°C, when ferrite is no longer thermodynamically stable, 16-8-2 exhibited longer creep life and similar creep ductility as 316H. The formation of ferrite in 16-8-2 appears to have a greater impact on creep performance than the formation of embrittling topologically close-packed phases like the σ phase, as 316H exhibited superior creep performance while predicted to form 14 vol.% σ phase at 650°C.

Enhanced disperse Ta(C, N) precipitation in reduced activation ferritic/martensitic steels by optimized two-step austenitizing

High-density MX precipitates block the movement of dislocations during creep at high temperature and increase sink strength to point defects under irradiation. Therefore, the precipitation control of MX phase is crucial for the advancement of reduced activation ferritic/martensitic (RAFM) steels. In this paper, an optimized two-step austenitizing strategy was proposed to enhance Ta(C, N) precipitation in RAFM steels, and the temperature effects of the first and second austenitizing on the Ta(C, N) precipitation enhancement were studied. During the first austenitizing, Ta(C, N) particles with a small radius (∼12 nm) and a high number density (∼8.5 × 1019 m−3) precipitated by heterogeneous nucleation on the M23C6 phase boundaries retained by the comparatively low temperature (900 °C). Then in the second austenitizing, enhanced Ta(C, N) precipitates kept good size and thermal stability below 1050 °C. Compared to RAFM steels treated by conventional normalizing and tempering, the strength and creep performance were significantly improved by the pinning effect of the enhanced dispersed Ta(C, N) particles. Furthermore, the optimized two-step austenitizing strategy was validated in the other two RAFM steels, showing broad application prospects in RAFM and heat-resistant steels.