Sanicro 25 is a heat-resistant steel with high performances used in the advanced ultra-super critical power plants. This paper provides a discussion on the creep-fatigue interaction behaviours of this steel with different strain ranges and dwell times at 700 degrees C. The influences of dwell time on the cyclic deformation behaviours and life time have been evaluated. Increase of dwell time will generally decrease number of cycles but increase the time to failure. The influence of dwell time at a large strain on the life cycle is small. Dwell time can significantly impact both the initiation and propagation of cracks during a creep-fatigue interaction process, resulting in more intergranular cracking. The influences of dwell time on the cyclic plastic deformation, precipitation behaviour, recovery phenomena, and crack initiation and propagation have also been discussed. Fracture investigations show that dwell time results in more intergranular cracking.
This study investigates the premature failure of 10 GH4033 superalloy studs in a catalytic cracking unit double-acting slide valve after 2 years at 680 degrees C. Failure investigation included fractography, mechanical testing, metallography, and EDS. Fractures initiated at threads, exhibiting brittle intergranular cracking. Copper powder infiltration was found in the thread surface layer, accompanied by austenite coarsening and grain boundary distortion in the subsurface. Copper enriched at grain boundaries reacted with segregated Cr and Ni to form a brittle Cu-Cr-Ni phase, weakening intergranular bonding and causing stress-induced crack propagation. The root cause was the first-time use of a copper-based anti-seize lubricant during maintenance, facilitated by porous machined surfaces. Prevention requires strict control of machining quality, selection of a suitable anti-seize lubricant, and application of graphite or MoS2 during assembly.
Irreversible viscous behaviour is often assumed to occur only after yielding, yet experiments show measurable irreversibility within the nominally elastic regime. In this work, we experimentally demonstrate multi-timescale viscoelastic relaxation in 316 SS between 500 and 700 and show that this behaviour cannot be represented by conventional unified viscoplastic models alone. A thermodynamically based viscoelastic-viscoplastic model is calibrated using stress relaxation, cyclic, and anisothermal waveforms, achieving R-2 > 0.87 and RMSE < 28 MPa for all isothermal conditions while maintaining reasonable accuracy under anisothermal cycling (R-2 = 0.72, RMSE = 43.41 MPa). The model captures both recoverable and permanent time-dependent deformation across temperatures, providing improved predictive capability for components subjected to dwell periods and flexible power plant cycling. These results highlight the need to incorporate viscoelasticity into high temperature constitutive models for austenitic steels.
Creep deformation is a dominant failure mode in metallic materials during service, governing structural stability, reliability and design life. However, conventional creep testing is time-consuming and resource-intensive, limiting the availability of high-fidelity creep data for efficient prediction. To address this issue, this study aims to develop a material-level digital twin system capable of generating high-fidelity creep strain data under limited experimental conditions. The proposed system is demonstrated using a 7-series aluminium alloy and employs a physics-guided gated recurrent unit model that integrates real-time sensor data with dynamic updating for creep strain prediction. Results show that incorporating physical constraints improves prediction accuracy by approximately 12% compared with the GRU model. Comparison with continuum damage mechanics-based creep models further demonstrated the competitive predictive capability of the proposed approach. Furthermore, a standalone application was developed to enable real-time monitoring and prediction of the creep process, facilitating efficient experimental implementation.
A decrease in number density of MX carbonitrides due to Z phase precipitation is the most serious factor for the degradation in creep strength of Gr.92 steel at long times. The degradation in RA of Gr.92 steel at long times is caused by the boron nitride particles and by very short normalising time. The rupture ductility is further reduced in steam. The reduction of boundary hardening due to poor M23C6 carbides along PAGBs in HAZ is the most important for the significant decrease in time to rupture of HAZ in welded joints of Gr.92 steel. The formation of protective Cr2O3-rich scale is achieved on the surface of 9Cr-3WVNb steel by the pre-oxidation treatment in argon gas containing small amount of oxygen. The application of pre-oxidation treatment in argon gas to Gr.92 steel is a future challenge.
High-chromium martensitic steels in power plants must endure decades of creep at 500-650 degrees C under moderate stress. Direct testing is impractical, making extrapolation essential - yet small modelling errors can lead to overdesign or premature maintenance. While Dyson's multiplicative ordinary differential equation (ODE) model is a benchmark for creep damage modelling, its strain-driven cavitation lacks explicit calibration with a time-temperature-stress (TTS) framework. This study introduces a TTS-calibrated constitutive framework with three specific refinements: (1) micro-to-macro cavitation damage mapping via ${{\boldsymbol{{\omega }}}_{\boldsymbol{{n}}}}\left({{\boldsymbol{{\sigma }}},{\boldsymbol{{ T}}}} \right) = {{\boldsymbol{{U}}}<^>{'}}\left({{\boldsymbol{{\sigma }}},{\boldsymbol{{ T}}}} \right){{\boldsymbol{{t}}}<^>{\boldsymbol{{m}}}}$omega n sigma,T=U 'sigma,Ttm ${\boldsymbol{{a}}}{\bf{nd}}\,{\boldsymbol{{ }}}{{\boldsymbol{{D}}}_{\boldsymbol{{n}}}}\left({{\boldsymbol{{\sigma }}},{\boldsymbol{{ T}}}} \right) = {{\boldsymbol{{U}}}<^>{''}}\left({{\boldsymbol{{\sigma }}},{\boldsymbol{{ T}}}} \right){{\boldsymbol{{t}}}<^>{\boldsymbol{{m}}}}$and Dn sigma,T=U ''sigma,Ttm, grounded in cavitation kinetics; (2) a modified Sinh law for minimum creep rate: epsilon min = Asinh(C ${{\boldsymbol{{\sigma }}}<^>{\boldsymbol{{q}}}}$sigma q); and (3) a stress-dependent amplification term for tertiary creep: 1 & frasl;(1-Dn)<^>P(sigma). P91 steel was chosen for its industrial relevance and synchrotron cavitation data at specific temperature. The refined model aligns well with experimental creep curves and cavitation data and enhances lifetime prediction, offering improved physical traceability and extrapolation fidelity.
This study aims to investigate the effects of static sodium exposure with carbon concentration of < 10 ppm, and its impact on the microstructure and mechanical properties of 316LN stainless steel for the durations of 3000 to 9000 h at 873 K. The specimen exposed to sodium for 3000 h and 9000 h showed a ferrite layer with a depth of 6-7 m, and 7-8 mu m respectively. XRD and EDS analysis confirmed the presence of a ferrite layer in sodium-exposed specimens. The carburised layer depth estimated by EPMA was 30 mu m and 60 mu m for 3000 h and 9000 h sodium exposed specimens respectively. Nano-hardness revealed an increase in surface hardness of sodium exposed specimens as compared to thermal aged and as-received specimens. Tensile test results showed an increase in yield and ultimate tensile strength while reduction in ductility as compared to as-received steel. This is attributed to the carburisation phenomenon.
Creep deformation and damage critically affect the safety of high-temperature components in aerospace, nuclear power, and transportation. A rigorous description of deformation and damage under complex creep loading from an energy perspective still requires further development. This study develops a thermodynamically consistent constitutive framework combining kinematic hardening and damage evolution. Long- and short-range back stresses describe history effects and anelasticity, while damage is modelled through creep free energy degradation and stress-dependent driving forces, capturing the transition from hardening to softening. The framework reproduces key features such as variable-load creep, cyclic torsional creep, and the shift from decelerating to accelerating strain rates. Results reveal that early creep is governed by back stress hardening, whereas long-term softening and rupture arise from progressive damage. This unified, thermodynamically based approach provides a practical tool for predicting creep behaviour and lifetime, offering guidance for the design and assessment of high-temperature structural materials.
This study investigated the creep-fatigue interaction (CFI) behaviours and failure mechanisms of 13Cr10Mo1W1VNbN steel, a newly developed material for use in China's independently designed heavy-duty gas turbines. The results indicate that tensile dwell, particularly long-term dwell of 1800 s, significantly reduces failure life, while compressive dwell has a comparatively minor influence on cyclic performance. Typical features such as creep voids, oxidation, and microcrack branching, confirm the combined effects of creep and oxidation-induced degradation as primary contributors to life reduction under long-term tensile dwell. However, there is no evident features of creep damage observed on the fracture surfaces under compressive dwell conditions. Notably, compressive stress-induced crack closure, along with microstructural changes such as low and high angle boundaries, was found to effectively suppress crack propagation. This study offers theoretical support for evaluating the service life of high-temperature turbine components and guiding the design of damage-resistant alloys for advanced gas turbine applications.
For the requirements of special gradient service environments, it is critical to fabricate high-temperature structural components with micron-scale gradient microstructures. The double-frustum specimens with different top diameters were designed to achieve a gradient distribution of microstructure and properties along the edge to the centre by thermal compression. The hot compression of double-frustum specimens at 1050 degrees C, 1 s(-1), directly leads to a gradient strain distribution. The thermal deformation samples show a low degree of recrystallisation in all regions of the samples under the effect of dynamic recrystallisation. Additional holding for 30 s after deformation and sufficient static recrystallisation increase recrystallisation degrees in all regions of the Phi 10 sample. The samples showed an almost linear gradient distribution of grain size and hardness along the radial direction from the centre to the edges. The average grain size ranges from 22.82 to 10.49 mu m, and the hardness ranges from 260.1 to 352.4 HV.
In the present paper, creep fracture characteristics of diffusion bonded joint under lower bonding temperature are investigated. The results indicate that although the bonding temperature is 50 degrees C lower than that of the conventional bonding method, the incorporation of nickel interlayer significantly enhances the joint strength, with creep strength increased by at least 100%. Neglecting the effect of creep primary strain on life assessment may result in a considerable overestimation of structural service life, a modified creep damage constitutive model that incorporates all three creep stages is crucial for improving the life prediction accuracy. Creep crack growth behaviours of the joint achieved through the proposed method demonstrate a substantial enhancement compared to that of conventional bonded joint. Under the equivalent C* values, the crack growth rate is reduced by at least one order of magnitude, thereby contributing to a significant enhancement in long-term operational reliability of high-temperature equipment.
High-performance components made from high-temperature alloys often operate under prolonged exposure to elevated temperatures and multiaxial stresses, where creep fracture is a dominant failure mechanism. Accurate prediction of long-term creep life is therefore crucial for ensuring structural reliability. In this study, we propose a data-driven framework for rapid multiaxial creep life prediction, combining the multiaxial stress rupture criterion (MSRC) with the skeletal point concept. A recurrent neural network (RNN) is trained on finite element data from 1278 notched specimens to predict skeletal point stress, while a fully connected neural network (FCNN) is used to estimate rupture time based on material properties and stress states. Testing results show that the proposed method maintains prediction errors within a two-fold tolerance range, demonstrating high accuracy, generalisation, and computational efficiency. This approach offers a practical tool for evaluating the long-term creep behaviour of high-temperature components under complex multiaxial loading.
Reliable long-term creep rupture life prediction of high-temperature materials demands a deep understanding of rupture-controlling mechanisms. Conventional analytical models for creep rupture extrapolation rely heavily on experimental data and adjustable parameters, potentially neglecting the critical failure mechanisms. This study employs fundamental creep models for HR3C(25Cr20NiNbN) austenitic steels, incorporating ductile and brittle creep mechanisms with well-defined physical parameters and no adjustable parameters. The ductile creep models account for dislocation hardening, precipitation hardening, solid solution hardening, and stacking faults, while the brittle creep models in addition consider creep cavitation along sliding grain boundaries. Key physical parameters are derived as follows: precipitate evolution is simulated using thermodynamic computations and validated against experiments, while high-temperature elastic properties and atomic-size misfit are determined through first-principles calculations, with lattice vibrations incorporated via the quasi-harmonic Debye model. By combining first-principles and thermodynamic calculations, the mechanism-based fundamental models successfully predict the creep rupture strength of HR3C quantitatively.
An investigation into the mechanisms influencing the interface between an alloy used to manufacture gun barrels and a chromium layer used to reduce erosion rates. The coated substrates were subjected to a range of heat treatments. SEM and EDS were used to analyse the development of the interfacial region between substrate and coating. Nano-indentation was used to assess the mechanical integrity of the interface. The results indicate that heat treatment promotes interdiffusion of elements between the two materials, thereby enhancing the mechanical properties at the interface. The fracture mechanism evolves with temperature: prior to diffusion layer formation, cracks preferentially initiate at the coating/substrate interface; after formation, they transfer to the diffusion layer/coating interface, exhibiting distinct two-stage failure behaviour.
Binary solid solution alloys that show class A alloy (CAA) behaviour with low stress exponents, absence of substructure and reduced normal or inverted primary creep are analysed. These characteristics are referred to as the class A state (CAS). Basic dislocation climb based models are used to compute properties of Al-Mg and Al-Zn alloys. It is demonstrated that a single climb based model can accurately represent the creep strain rate versus stress. It is no longer necessary to consider a transition from climb to glide and back to climb again. The absence of substructure in CAS is allotted to the recovery of screw dislocations by cross-slip. An expression for the rate of this type of recovery is derived. The presence of inverted primary creep is believed to be due to a significant initial dislocation density. This is consistent with simulated primary creep curves.
The flow behaviour of Modified 9Cr-1Mo (Grade 91) steel base and weld has been evaluated at room temperature (RT), 350 degrees C, 450 degrees C, and 550 degrees C. Tensile test specimens were extracted from the Grade 91 base material and from weldments produced using the automated Gas Tungsten Arc Welding (GTAW) process, followed by a post-weld heat treatment (PWHT) at 760 degrees C for 3 hours. The experimental true stress - plastic strain curves obtained from these specimens were fitted to several constitutive models, including the Holloman, Ludwik, Ludwigson, Swift, Voce and Ramberg-Osgood equations. The Voce model most accurately represents the flow behaviour across the different temperatures for both the base and weld metal. Furthermore, the tensile properties predicted using the Voce equation viz. yield strength, ultimate tensile strength and uniform plastic strain have been found to be in good agreement with the experimental results. SEM images show a higher void size dependence on temperature in weld metals.
