[en] The microstructural evolution of reactor pressure vessel (RPV) steel and its effect on the mechanical properties during tempering at 650 °C were studied to reveal the time-dependent toughness and temper embrittlement. The results show that the toughening of the material should be attributed to the decomposition of the martensite/austenite constituents and uniform distribution of carbides. When the tempering duration was 5 h, the strength of the investigated steel decreased to strike a balance with the material impact toughness that reached a plateau. As the tempering duration was further increased, the material strength was slightly reduced but the material impact toughness deteriorated drastically. This time-dependent temper embrittlement is different from traditional temper embrittlement, and it can be partly attributed to the softening of the matrix and the broadening of the ferrite laths. Moreover, the dimensions and distribution of the grain carbides are the most important factors of the impact toughness. - Highlights: • The fracture mechanism of reactor pressure vessel (RPV) steels under impact load was investigated. • The Charpy V-notch impact test and the hinge model were employed for the study. • Grain boundary carbides play a key role in the impact toughness and fracture toughness. • The dependence of the deterioration of impact toughness on tempering time was analyzed for the first time.

[en] Zr-Sn-Nb-Fe alloy specimens tempered at 500 and 560 degree C with different time after cold rolling, respectively,were exposed to 70 μg/g Li⁺ LiOH solution at 350 degree C, 16.8 MPa. The microstructures and second phases were examined by transmission electron microscopy (TEM). It is found that the specimen treated by 500 degree C/100 h has the best corrosion resistance among all the specimens. TEM analysis of the specimens tempered at 500 degree C with different time shows that the second phase changes from the seriate strips to the banding particles with the increasing of tempering time, and the fine βNb particles precipitates from the matrix as tempered to 100 h. The transformation process of specimens tempered at 560 degree C is similar to those tempered at 500 degree C except the shorter transition time, and the matrix is recrystallized to the equiaxed grain completely as tempered to 10 h. (authors)

[en] The effect of nickel alloying element on the precipitation of Cu-rich clusters in RPV model steel has been investigated by means of atom probe tomography. 40 kg ingots of RPV model steel were prepared by vacuum induction furnace melting with two different Ni contents (0.79 wt.% and 1.52 wt.%) and with higher Cu content (0.5 wt.%). The specimens with 50 x 40 mm² in 4 mm thickness were heat treated by 880 degree C/20 min water quenching, 660 degree C/10 h tempering and 400 degree C aging for different time from 100-2000 h. The results show that the increase of nickel content in RPV model steels will promote the precipitation of the Cu-rich clusters. The contents of Ni and Mn were detected in the Cu-rich clusters at the early stage. The Ni-rich clusters containing Cu and Mn were detected, and they could act as the nucleation sites for the precipitation of Cu-rich clusters. Mn-rich clusters could also be detected. Mn-rich clusters are not the nucleation sites for the precipitation of Cu-rich clusters, but the segregation of Cu atoms in those Mn-rich clusters containing much higher Ni could be found. Ni-rich clusters will act as the nucleation sites during the precipitation of Cu-rich clusters, therefore the increase of nickel content in RPV steels could promote the precipitation of Cu-rich clusters. This is the essential reason that the presence of nickel in PRV steels increases its sensitivity to neutron irradiation embrittlement. (authors)

[en] The tangential fretting wear of three kinds of zirconium alloys tube mated with 304 stainless steel (SS) plate was investigated. The tests were conducted in an autoclave containing 300 °C pressurized B-Li water for tube-on-plate contact configuration. The worn surfaces were examined with scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and 3D microscopy. The cross-section of wear scar was examined with transmission electron microscope (TEM). The results indicated that the dominant wear mechanism of zirconium alloys in this test condition was delamination and oxidation. The oxide layer on the fretted area consists of outer oxide layer composed of iron oxide and zirconium oxide and inner oxide layer composed of zirconium oxide.

[en] The mechanical properties of lower head of steam generator for the small modular reactor use were estimated to examine the uniformity of material in different parts. The composition analysis showed that the C content varied between 0.20% ∼ 0.21% wt., and S and P contents were less than 0.004% wt. and 0.001 wt. %, respectively. The contents of other alloying elements varied in a very small and reasonable range. Due to large deformation and resultant residue stress arising from integral forging schedule, the hardness slightly varied in 217 ∼ 232 HB with an acceptable fluctuation from the inner and outer surfaces of body ball, four nozzles and their joints. The strength at room (23℃) or elevated temperatures (350℃) was generally and homogeneously distributed in both circumferential and radial directions of the dismembered samples after quench-temper treatment and simulated post-welding heat treatment, whereas the corresponding impact toughness has a relatively larger fluctuation but the minimum adsorbed energy was still much higher than the required value. Uniform mechanical properties can be achieved throughout the integral forged lower head of steam generator via the combination of integral forging and heat treatment processing. (authors)

[en] The effect of size of nanoscale Cu precipitate on the mechanical response of microalloyed steel was investigated computationally and experimentally. A phenomenological constitutive description is adopted to build the computational crystal model. The material is envisaged as a composite; the Cu precipitate is modeled as a monocrystalline core surrounded with a lower yield stress and higher work hardening rate response. Both a quasi-isotropic and crystal plasticity approaches are used to simulate the matrix. The nanoscale precipitate is modeled as ellipsoidal inclusion with different Yound's modulus to matrix. Elastic and plastic anisotropy are incorporated into this simulation. An implicit Lagrangian finite element formulation with von Mises plasticity or rate dependent crystal plasticity is used to study the nonuniform deformation and localized plastic flow. The computational predictions are compared with the experimentally determined mechanical response of HSLA-100 steel with average size of nanoscale precipitates of 2.02±1.89 nm. The tendency of the calculated yield strength attributed to Cu precipitates is in good agreement with experimental result.

[en] Highlights: • Ultra-high strength steel without carbide-forming elements is designed. • Two-stage hardening behavioris discovered. • Strengthening by nanoscale co-precipitation of Cu and β-NiAl precipitates. • The effect of Co addition in precipitation reaction is discussed. A novel ultralow carbon precipitation-hardening Ni-Mn-Cu-Al-Co ferritic steel without adding any strong carbide-forming element was designed to investigate the co-precipitation behaviors of Cu-rich (CRPs) and β-NiAl precipitates by using electron back-scatter diffraction (EBSD) and atom probe tomography (APT). In the light of phase transformation and hardness-temperature curves, it is estimated that the nanoscale clusters and precipitates formed at 450 and 500°C tempering respectively are responsible for the strong precipitation strengthening effect. The number density of the co-precipitated couples is significantly increased by an order of magnitude while the size almost keeps constant. The limited growth of the dispersed small precipitates is in association with the co-precipitation reactions itself and the addition of Co that possibly hinders fast coarsening of the co-precipitate couples. The strength contributions from dislocated martensite, solid solution and precipitation were roughly calculated and compared with the experimental values. It is found the precipitation hardening is dominant in strengthening and the solid solution hardening exhibits a non-negligible effect as well.