Search

In this paper, a new high-temperature methodology for determining the 'effective' corrosion rates of internal pressure-tested cladding tubes, based on the reduced creep failure times under exposure, is presented. The methodology has been developed to estimate the combined damage accumulation of corrosion and creep and to establish temperature limits for the use of 1515Ti claddings in high-temperature Lead-Bismuth-Eutectic (LBE). The results are compared to static corrosion rates determined from earlier test campaigns in stagnant LBE. All tests are conducted on 24% cold worked 1515Ti cladding tubes that are the SCK CEN reference material for the MYRRHA design efforts. The virgin material creep model that is the basis for the corrosion rate estimation, has been fitted on an extended creep failure database and a life-fraction rule is applied to accommodate for the life reduction caused by the sought 'constant' effective corrosion rate. [ABSTRACT FROM AUTHOR]

Alleima 3R60TM is an AISI 316/316 L type of stainless steel. This alloy shows extra-long creep lives. At 700°C with an applied stress of 45 MPa, the specimen broke first after 240 131 hours, a 140% longer creep life than the predicted. The reasons have been studied using SEM/EDS, EBSD, ECCI and TEM. Two unexpected phenomena have been observed. One is the presence of homogeneously dispersed small Mo2Fe4CrSi(Ni0.5)f063type of precipitates in the matrix. According to the thermodynamic calculation, this phase should not form in this alloy at 700°C. The other is the fine grains formed in the matrix during the creep-testing at 700°C for such an extra-long time. It is believed that both small precipitates and fine grain size contribute to this extra-long creep life of the steel. With the microstructural study and thermodynamic simulation, the mechanisms to form precipitates and fine grains have been discussed. [ABSTRACT FROM AUTHOR]

For heat resistant alloys, long-term structural stability at high temperatures is a critical issue for alloy design and applications. In this paper, the long-term creep behaviours and structural stabilities of six heat resistant high Ni alloys and austenitic stainless steels have been studied. The longest creep rupture life is up to 359 283 hours. High Ni and Cr alloys show a good combination of high creep and oxidation resistances. Precipitation of nano MX particles with a very low growth rate improves long-term creep resistance at high temperatures. Long-term stable multiple nanoprecipitates of MX, Cu-rich, Laves and M23C6 phases can greatly contribute to the creep strength. Low Ni austenitic stainless steels show comparatively low oxidation and creep resistances. It was first found that at 800°C, Cr2N could form in the low Ni steel with a long-term crept by the absorption of nitrogen from the air into the matrix. [ABSTRACT FROM AUTHOR]

The creep response of AISI 316 and AISI 316 L was analysed to provide a coherent picture of the material behaviour, valid for both conventional wrought and additively manufactured steels. Literature evidences were considered. The analysis demonstrated that the presence of a fine distribution of particles, precipitated during creep exposure in both AISI 316 and 316 L(N), should be taken into account. A recent model, expressly developed for particle-strengthened alloys, was successfully used to describe the minimum creep rate dependence on applied stress for AISI 316 and 316 L(N). The same set of constitutive equations, in combination with the composite model, was then used for describing creep data obtained by testing the AISI 316 L produced by additive manufacturing. The main difference with wrought materials was that the microstructure of the additively manufactured alloy was composed by cells (soft zones) and thick walls heavily decorated with ultrafine oxide particles (hard zones). [ABSTRACT FROM AUTHOR]

For modern creep-resistant steels, ductility is primarily controlled by cavitation, since brittle rupture is dominating during long-term service. In recent years, the present authors have formulated fundamental models for nucleation and growth of cavities that have been verified to be able to describe experiments for austenitic stainless steels. These equations, together with models for dislocation creep, are used in this paper to present basic modelling results for the creep ductility of austenitic stainless steels for the first time. New results are also presented for ductile rupture, where the elongation values are mainly governed by plastic instability and necking. The Hart criterion is used to identify the strain where the instability forms. Modelling shows that the instability grows very slowly and that its size is not significant until close to rupture. These facts are used to demonstrate that ductility during ductile rupture can be predicted from necking behaviour. [ABSTRACT FROM AUTHOR]

Microstructural characterisation was performed on the gauge section of two creep rupture samples manufactured from Super 304H; an advanced austenitic stainless steel that is used as superheater and reheater tubes in power plants. The samples had been exposed to uniaxial creep tests (Sample A: 700°C and 75Mpa, Sample B: 650°C and 120Mpa) where both exhibited low creep ductility. Six phases were identified: MX (Nb-rich), sigma, Cu-rich, M23C6, modified Z phase and Fe (BCC). Quantification of Nb-rich and Cu-rich particles revealed ~10% and ~16% more Nb-rich and Cu-rich particles respectively in Sample A versus Sample B. The amount of sigma and the average particle size for the Nb and Cu-rich particles was similar for both samples. There appears to be an association between the sigma phase and creep cavities as shown in 2D microstructural characterisation which could have contributed to the low creep ductility exhibited by these two creep samples. [ABSTRACT FROM AUTHOR]

Creep behaviour of Ti-modified 14Cr–15Ni austenitic stainless steel in solution annealed, 20%, 30%, and 40% cold-worked conditions, was studied at 973 K and 175 MPa stress level in the current investigation. Increase in cold-work level resulted in denser dislocation network resulting in prominent low-angle grain boundaries. Low-angle boundary fraction increased as the amount of cold working increased from 20% to 40% prior to creep loading. The variation in the fraction of low-angle boundaries impacted the creep rupture strength of the steel. During creep exposure of the cold-worked steels, softening occurred due to both recovery and recrystallisation. The extent to which recrystallisation and recovery occurred during creep was dependent on the level of cold work. Subgrain formation and recrystallisation resulted in enhanced recovery and decreased the creep strength of 40% cold-worked steel. The creep rupture life was highest for 30% cold-worked steel. [ABSTRACT FROM AUTHOR]

The UK has unique experience in operating high temperature civil nuclear power systems, known as advanced gas cooled reactors (AGRs). One of the primary challenges for extending the lifetime of the AGR power stations is to understand the interaction that occurs between the AGR CO2 environment and creep-fatigue cracking behaviour. This is one of the life limiting degradation mechanisms for steel components within the reactor pressure vessel. This paper addresses the effect of thermal aging on material internal state that controls both the creep deformation and oxidation behaviour of Type 316H stainless steels when they are exposed at a simulated AGR environment. Experimental results from creep tests are discussed with respect to a multi-scale self-consistent model, while experimental results from oxidation tests are considered with respect to the application of measured short term data to predict the long term oxidation behaviour. Finally, the interaction between oxidation and creep and its impact on high temperature structural integrity of AGR nuclear systems are discussed. [ABSTRACT FROM AUTHOR]

Evaluation of creep-fatigue failure is essential in design and fitness evaluation of high-temperature components in power generation plants. Cyclic deformation may alter the creep properties of the material and taking cyclic effects into account may improve the accuracy of creep-fatigue failure life prediction. To evaluate such a possibility, creep tests were conducted on 316FR and modified 9Cr-1Mo steel specimens subjected to prior cyclic loading; their creep deformation and rupture behaviours were compared with those of as-received materials. It was found that creep rupture life and elongation generally decreased following cyclic loading in both materials. In particular, the rupture elongation of 316FR in long-term creep conditions drastically decreases as a result of being cyclically deformed at a large strain range. Use of creep rupture properties after cyclic deformation, instead of those of as-received material, in strain-based and energy-based life estimation approaches brought about a clear improvement of creep-fatigue life prediction. [ABSTRACT FROM AUTHOR]

This paper investigates the development of intergranular strains and stresses in AISI type 316H austenitic stainless steel during cyclic loading at high temperature. Isothermal cyclic creep tests at 650°C with 2 h displacement controlled creep dwells at maximum strain are conducted with in situ neutron diffraction monitoring at a time of flight facility. The evolution of intergranular strains in five independent { hkl} grain families were successfully measured during consecutive cycles. The grain family with {111} lattice planes normal to the loading direction exhibited linearly reversible behaviour with cyclic load whereas the {200} planes deformed in a non-linear manner forming a hysteresis loop. Intergranular strains during the first dwell remained unchanged with time, but relaxed with time during later dwells. The start of dwell intergranular strains increased from cycle 1 to cycle 2, but markedly less moving from cycle 2 to cycle 3. [ABSTRACT FROM AUTHOR]