Yanli Wang

This study systematically investigates the effects of thermal aging on the microstructural evolution and mechanical properties of forged Z2CND18.12N2 austenitic stainless steel (ASS) using small punch testing (SPT) and tensile testing. Prolonged thermal exposure at 400 °C for 10 000 h results in no significant changes in grain size or precipitated phase content, while α′‐phase formation is identified within individual shear bands in both aged and nonaged specimens. Fractographic analysis reveals a mixed‐mode failure mechanism involving cleavage fracture initiation at ferrite‐austenite interfaces, followed by microvoid coalescence in stress‐concentrated austenite regions. Contrastingly, mechanical characterization demonstrates that thermal aging enhances strength parameters (yield strength: 338.45–372.32 MPa; ultimate tensile strength: 540.65–588.33 MPa) but severely compromises ductility (elongation: 75.8–42.5%), with ferrite‐phase hardening evidenced by nanoindentation hardness increasing from 5.97 to 8.89 GPa. SPT results further corroborate this degradation trend through reduced total absorbed energy (2.24–2.12 J). The inverse strength‐ductility relationship is attributed to thermally induced ferrite hardening, providing critical insights into ASS degradation mechanisms and essential guidance for ensuring structural integrity in high‐temperature applications.

A multi-analytical approach was used to investigate the effect of thermal aging on the microstructure and mechanical properties of ER308L/Z2CND18.12N2. The results demonstrated that fractures occurred preferentially on the ER308L side. Z2CND18.12N2 exhibited superior fracture toughness compared to ER308L regardless of thermal aging time. The ultimate tensile strength significantly increased from 564.5 MPa in the unaged condition to 592.7 MPa to MPa after thermal aging and the fracture mode changed from ductile fracture into a ductile + quasi-cleavage fracture. The fusion zone (FZ) with the chemical composition gradient was about 40 μm from the Z2CND18.12N2 to ER308L. After thermal aging, spinodal decomposition and G-phase precipitation were observed for the first time in the ferrite phase of the FZ. Moreover, the hardness presented the following trend: FZ > ER308L > Z2CND18.12N2. The hardness of the ferrite phase dramatically increased from 6.13 GPa in an unaged condition to 8.46 GPa in a 10,000 h aged condition.

The Elastic-Perfectly Plastic (EPP) based design method was qualified for ASME Section III, Division 5 Class A construction using Type 304 and 316 stainless steels and Alloy 617 via Code Case N-861 for Strain Limits evaluations. Because the isochronous stress-strain curves used in the EPP Strain Limits analysis were determined from standard creep testing on specimens without any prior cyclic loading history, experimental verification and analysis are required in order to extend the EPP Strain Limits analysis to the remaining three Class A materials, Grade 91, Alloy 800H and 2.25Cr-1Mo. In this study, specialized tests were designed and performed to evaluate the effects of cyclic deformation on the subsequent creep properties of these three Class A materials in the creep regime and the results are compared with standard creep tests. It is shown that reduction factors on the isochronous stress-strain curves are needed for Grade 91 to account for its cyclic softening effect. However, prior cyclic deformation has an insignificant effect on the minimum creep rates of Alloy 800H and 2.25Cr-1Mo, hence reduction factors are not required. The experimental results and analysis in this study provide the technical basis for the extension of Code Case N-861 to these three Class A materials.

Creep-fatigue is the most severe damage mode for high temperate components under cyclic loading. Experimentally, standard creep-fatigue testing is mostly performed at strain ranges of 0.3% and above. There is a major data gap in creep-fatigue failure information for strain ranges at 0.3%, or lower, which are more prototypical for advanced reactor plant operations. To develop design curves for creep-fatigue evaluations, it will require a method for extrapolating the creep-fatigue cyclic lives to lower strain ranges and longer hold times. In this study, methods for creep-fatigue life prediction of Alloy 617 at lower strain ranges and longer hold times at 950°C were assessed. Two types of experimental data were generated to support the development of the creep-fatigue design curves at low strain ranges and long hold times. First, standard strain-controlled creep-fatigue testing using the concept of block-strain range was conducted to generate the information needed for extrapolating the creep-fatigue design curves to lower strain ranges and longer hold times. Second, creep-fatigue experiments with elastic follow-up were performed to failure to verify the proposed design curves. Based on this new testing approach and analysis method, a preliminary creep-fatigue design curve was developed for Alloy 617 at 950°C. This study shows the potential of generating a set of creep-fatigue design curves with different hold times within a reasonable amount of test times and testing effort at the low strain range region.

Hydride artefacts are commonly induced by the TEM sample preparation process in Zirconium alloys as hydrogen-sensitive metals, including electron polishing and focused ion beam (FIB) technology. In the research, we present the application of chemical polishing with a solution of 10HF:45HNO₃:45H₂O to prepare the disk samples for TEM observation in zirconium alloys. The thinning efficiency of chemical polishing is 25 μm per minute. XRD patterns indicate that the chemical polishing actually eliminates the macro- and micro-stress induced by mechanical grinding. TEM observation demonstrates that chemical polishing reduces the amount of hydride artefacts, especially hydrides with large size. It is proposed that induced stress provides driving force for hydride artefact formation. Compared with traditional mechanical grinding, the advantages of chemical polishing are high efficiency, free of induced stress, less induced hydride artefacts and bend contours.

The low cycle fatigue (LCF) behaviors of cast duplex stainless steel (CDSS) thermally aged at different times were investigated under different strain amplitudes. The effects of thermal aging on the LCF lives of CDSS are closely related to the strain amplitude. At a low strain amplitude, the fatigue life of the material increases significantly after thermal aging, while the LCF life decreases with an increasing aging time at a high strain amplitude. After thermal aging at 400 °C for 10,000 h, the fatigue fracture morphologies of CDSS change from fatigue fringes to mixture features including fatigue fringes in austenite and cleavage cracks in ferrite. Severe plastic deformation in ferrite of the aged CDSS under a high strain amplitude causes the cleavage cracking of ferrite. The premature failure of ferrite accelerates the propagation of fatigue crack and shortens the fatigue life at a high strain amplitude.