Division II works on how to evaluate and improve the service performance, durability, and safety of materials or components for different types of power plants. The high-temperature and high-pressure aqueous environment is an important increasing factor in the failure of structural materials in thermal power plants. Irrespective of the material used, degradation occurs to a greater or lesser extent with serving time in aggressive environment. Accurate and reliable data of the properties and residual life of materials over their service time is required for plant safety.

Research Areas

1. Service behaviors and life evaluation of materials and components in different types power plant

2. Environmentally-assisted cracking (SCC and CF) in high temperature pressurized water

3. Fretting wear and fretting corrosion of engineering materials

4. Creep behavior and mechanism of heat-resistant steels

5. Grain boundary engineering and control for metallic materials

6. Computational modelling of environmentally-assisted cracking

7. Online monitoring techniques of material failures in extremely complex environment

Research Highlights

1. Corrosion fatigue: crack growth

The corrosion fatigue crack growth (CFCG) behavior of 316LN SS was studied in the high-temperature pressurized water with and without dissolved hydrogen (DH) using DCPD real-time crack measurement. The corrosion fatigue crack tip morphologies and fracture features were subsequently analyzed by using SEM and BSE.

2. Corrosion fatigue: fatigue life

The pressure boundary components of the power plants are subjected to alternating loads from various transient operating conditions. Typical fatigue failure behaviors such as corrosion fatigue, thermal fatigue, and high-cycle fatigue were studied for components such as heat transfer tubes, main pipelines, pressure vessel and bolts, and corresponding damage mechanism models and life prediction models were established.

Figure: (a) Fatigue test data of different Alloy 690TT specimen types in different environments. (b)comparison of corrosion fatigue life predicted by ANL model and actual SG tube test life.

3. Fretting wear: tube

Comparative study on fretting wear mechanism of alloy 690TT tube in high temperature pressurized water under different motions: impact, sliding, and impact-sliding were carried out. The running trajectory characteristics, coefficient of friction and wear volume, induced by fretting wear under different motions were analyzed.

4. Fretting wear: titanium alloys

The fretting wear behavior and wear models of TA15 alloys at high temperatures were studied. The fretting running characteristics, coefficient of friction and wear volume were analyzed. The wear models at different environments were established.

5. Stress corrosion cracking: grain boundary engineering

Intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) are the main failure mechanisms of austenitic stainless steels in subcritical water environment. For austenitic alloy materials such as austenitic stainless steel (316L, 304) and nickel-based alloys (690, 600, 625) used in subcritical water, the grain boundary engineering (GBE) treatment was developed and used to control the grain boundary characteristic distribution of these materials, and a high fraction (>75%) of low-∑ coincidence site lattice (CSL) boundaries, most of which are twin boundaries (∑3), were formed. The low-∑ CSL boundaries can significantly improve the resistance to intergranular corrosion and IGSCC.

Figure: (a) The intergranular cracks mapped using EBSD and OM in a 1/2T CT specimen of 316 stainless steel after SCC test in simulated PWR primary water, and the normalized cracking susceptibility of each types of grain boundaries. (b) The slow strain rate tensile test (SSRT) of a GBE-treated and a conventional 304 stainless steel specimens in simulated PWR primary water shows that the GBE-treated 304 stainless steel has lower susceptibility to IGSCC.

6. Stress corrosion cracking: tube

The effects of micro-defects (such as scratches, impact dents, wear, etc.) on stress corrosion cracking and corrosion fatigue of heat transfer tubes were systematically studied. The mechanism of micro-defects on oxidation and crack initiation was revealed, and the corresponding life evaluation model was established. The SCC behavior induced by creep of Alloy 690TT in high temperature water was investigated

Figure: (a) SEM Morphology of PbSCC in scratched Alloy 690TT specimen, (b) crack tip HAADF image, (c-g) EDS mapping results of Fig. (b), (h) results of EDS points S1-S4 in Fig. (b), (i) HRTEM image of the crack tip area in Fig. (a), (j) schematic diagram of the influence of creep on the SCC process.

7. Stress corrosion cracking: small punch test

The stress corrosion cracking (SCC) behavior of 316LN stainless steels weld joint in high-temperature and high-pressure (HTHP) water were investigated by the newly developed HTHP water small punch test (SP-SCC). The results indicated that SCC behavior of weld joint was strongly influenced by the inhomogeneous microstructure. Intergranular fracture dominated the SCC process of heat affected zone (HAZ) and partial hardening was believed to be the main reason for the increased sensitivity of HAZ to SCC. All samples exhibited cleavage fracture characteristics, indicating that film-induced cleavage dominated the SCC process of welded joints in HTHP water.

Figure: (a) The cross-sectional morphology of weld joint with the illustration of SP sampling locations, and the load-displacement curves of the SP-SCC in HTHP water. (b-e) SEM images showing the typical surface deformation morphologies after SP-SCC test in HTHP water.

8. Thermal aging

The investigation on the microstructure evolution and mechanical degradation and oxidation behavior of stainless steel weld overlay cladding, weld joint and low alloy steels during thermal aging was conducted. One of the results shows that the precipitates along the dislocation and within ferrite matrix contained the face-centered cubic structural G-phase and silicide phase. Silicide phase, supposed to be (Fe, Mn)3Si phase, preferred to form with G-phase along the dislocations and within ferrite matrix. Silicide phase showed a cube-on-cube relationship with ferrite phase and G-phase. Heterogeneous distribution of elements within Ni-Mn-Si clusters was responsible for silicide phase formation during thermal aging process, and the corresponding precipitation behavior discrepancy at different forming sites (dislocations, ferrite matrix and phase boundary) was discussed.

Figure: HRTEM images of the precipitates formed in the aged ferrite phase along (a) [001], (b) [011], (c) [111] directions, and (d) schematic illustration of FFT images. Schematic diagrams of precipitation process (a) along the dislocations and within the ferrite matrix, and (b) along the phase boundary in the ferrite phase during thermal aging.

9. Creep: air and water vapor

The creep tests in air and water vapor environment at high temperature were carried out to study creep life and the effect of water vapor on the creep property, creep deformation and creep fracture mechanisms of a series of heat-resistant steels. The results indicated that the water vapor environment did not affect the creep deformation mechanism but affected the creep behaviors and life, which was determined by the stress level. Different creep damage forms induced by the interaction of different applied stresses and creep environments affected creep fracture process and creep life. Oxidation in water vapor accelerated the connection of existing creep cavities on lath boundaries by promoting crack growth, thereby accelerating creep damage, which led to a significant reduction in creep life.

Figure: SEM images of surface near the fracture region of specimens that crept in air and in the water vapor under applied stresses of 220 MPa, 200 MPa, 180 MPa and 155 MPa.

10. Computational modelling: environmentally-assisted cracking

The atomic scale polycrystalline model of 316LN stainless steel was established. The classical molecular dynamics software LAMMPS was used to simulate the tensile properties and uniaxial ratchetting fatigue behavior of 316LN stainless steel, and normal-inverse Hall-Petch relationship at nanoscale, the mechanical properties response such as ratchet strain, fatigue plastic work density, and the evolution of microstructure such as grain boundary orientation and dislocation structure were analyzed during cyclic loading were analyzed.

Figure: The ratchetting strain-stress curves, ratchetting strains and dislocation structure of 316LN during corrosion fatigue in subcritical water were investigated by joint study of experiments and molecular dynamics simulation.

• Professor Yonghao Lu

• Associate Professor Tingguang Liu

• Associate Professor Long Xin