ABSTRACT 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 µm, and 7–8 µm 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 µm and 60 µm 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.

ABSTRACT 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.

ABSTRACT 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°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.

ABSTRACT The entire reformer furnace tube is analyzed and studied through OM, SEM and mechanical property test in this work. The microstructure of centrifugal casting tube and weld joint has deteriorated, with the primary carbides coarsened and growth. The primary carbides at the dendrite boundary have grown from lamellar and skeleton shape to continuous network and chain shape, and the primary carbides M7C3 and Nb(Ti)C transformed into M23C6 and G phase. The high temperature creep strength of reformer furnace tube decreased obviously. Through the investigation of the microstructure and mechanical properties of different zones in the entire reformer furnace tube, the weakest part of the reformer furnace tube is located in the middle and lower zone, and its remnant life has been predicted. On the basis of the design temperature and pressure, the remnant life of this reformer furnace tube is 38000 hours.

ABSTRACT Fatigue crack growth (FCG) studies have been conducted on compact tension specimens of alloy 617M for as-received and aged for 1000, 5000 and 10,000 h at 710°C. Effects of thermal ageing on FCG behaviour of alloy 617M forged has been characterised at 27°C, 650°C, 710°C and 750°C. Results reveal that threshold stress intensity factor ( Δ K th ) decreases as temperature increases upto 710°C and then increases at 750°C for all the ageing conditions except for 1000 h aged. The Δ K th value also decreases with an increase in ageing time till 5000 h, but at 10,000 h a reverse trend is observed. The influence of thermal ageing and temperature reflects major deviations for Paris constant (C) and exponent (m). Additionally, thermal ageing and temperature influences on FCG, the effects of dynamic strain ageing, Cr-rich carbides and γ’-phase precipitations have been discussed and corroborated with fractographic and microstructural changes.

ABSTRACT P92 (9Cr-1.8W–0.5Mo-NbV) martensite steel is widely used in high-temperature pipes of ultra-supercritical boilers in China. During high-temperature operation, microstructural evolution of the heat affected zone (HAZ) across welded joint leads to type IV cracking failure. In this article, microstructural evolution and type IV cracking of P92 steel welded joint in an actual 605℃ supercritical boiler were analysed. The results show the cracking appeared in the fine-grained heat affected zone (FGHAZ). Laves phase and M23C6 phase clustered around the cavity. Their precipitation and coarsening promote the nucleation and growth of cavities. The Z phase was also observed in the heated affected zone. Compared with other zones, FGHAZ has lower dislocation density and more degeneration of tempered martensite lath, the coarsening of Laves phase and M23C6 phase is more severer in FGHAZ. These factors led to the deterioration of the creep properties and type IV cracking.

ABSTRACT The cast coarse-grained polycrystalline Alloy 247 is investigated, which is typically used in the rear turbine blades of land-based gas turbines. High rotational speeds up to 10,000 rpm in combination with high temperatures induce creep deformation. Additionally, gas turbine blades are subjected to high-frequency cyclic stresses. This complex loading state requires the investigation of the interaction of creep damage and high-cycle fatigue (HCF). Therefore, HCF tests with a stress ratio of Rσ = −1 were carried out on as-received and pre-crept specimens. In comparison, an HCF test series with high mean stresses (Rσ = 0.5) was conducted to consider the superposition of creep and fatigue. Since nickel-base alloys exhibit a pronounced elastic anisotropy, the grain orientations were considered in the damage analysis. The results show that creep-induced grain boundary damage is a dominant structural parameter for the HCF behaviour of Alloy 247 in both, sequential and superpositioned creep-fatigue loading.

ABSTRACT The high-temperature mechanical properties of 310S stainless steel were investigated by uniaxial tensile tests and small punch tests at the temperature from 20℃ to 600℃, and the relationship between the mechanical properties (σYS ,σUTS) and the characteristic loads (Fy, Fm) was established. The results revealed that with increasing temperature, the mechanical properties of 310S decrease, and the characteristic loads obtained by Fy_Mao, Fy_EN and Fy_E3205 are more appropriate for determining the yield strength of materials at high temperatures particularly for Fy_E3205. The fracture pattern observed in the SPT specimens indicated a mixed tough-brittle fracture accompanied by the characteristics of cleavage fracture. Additionally, the ultimate tensile strength of the material and the maximum load Fm increased slightly at 400 ℃ compared to 300 ℃ due to the precipitation of a large number of granular carbides and the increase in the grain size.

ABSTRACT The creep resistance of martensitic power plant steels depends strongly on the dispersion of various types of precipitates in the microstructure. Reliable prediction of rupture strength of such alloys thus demands accurate description of the microstructure evolution during service. A material model that calculates the simultaneous precipitation kinetics and hardening in these steels has been reported previously. The creep rupture model reported here is a natural extension of the previous research. This model has been validated against experimental creep rupture data of wide-used 9–12% Cr steels. It allows the effect of variation in alloy conditions, such as composition and heat treatments, on rupture strength to be quantitatively evaluated for a given alloy grade. Results show that such variations can lead to significant differences in the calculated rupture strength, which are believed to be largely responsible for the wide scatter in the experimental data.

ABSTRACT Thermal barrier coatings (TBCs) are susceptible to corrosion by molten calcium-magnesium-aluminium-silicate (CMAS) during high temperature service. The microstructural evolution and thermal cycling lives of double-layer La2Ce2O7 (LC)/yttria partially stabilised zirconia (YSZ) and single-layer YSZ coatings were comparatively studied by a burner rig corrosion test. The results suggest that the thermal cycling life of double-layer LC/YSZ coating is approximately 80% longer than that of single-layer YSZ counterpart. The CMAS penetrated into the whole YSZ coating, which remarkably accelerated the spallation failure of the coating. However, due to the superior CMAS resistance of the LC layer, the failure of the double-layer LC/YSZ coating was mainly caused by the sintering of the LC layer, which resulted in a large in-plane stress and initiation of cracks. These cracks propagated, branched and deflected along the LC/YSZ interface, ultimately leading to the spallation of the LC layer.