Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-31 Cooperative journals: 《腐蚀科学与防护技术》
Abstract： Multi-layered weld beads were made by shielded metal arc welding process with the domestic nuclear grade welding rod E308L. Then the corrosion performance of the weld beads in high temperature water were investigated by means of weight change measurement as well as SEM, XRD and XPS. The results show that the mass loss of the breads increased faster in the initial stage of corrosion and then slowed down gradually to a stable level while the formed oxide scale composed mainly of oxides of Fe and Cr, of which the grain size grown with time gradually. In general, the weld breads of domestic nuclear grade welding rod E308L shows an acceptable resistance to uniform corrosion in the desired high temperature water.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-31 Cooperative journals: 《腐蚀科学与防护技术》
Abstract： Forged 316LNSS has been selected as the main pipe line material used in the third generation nuclear power plants. The oxide scale formed on the forged 316LNSS after immersion in high temperature pressurized water with 2.3 mg/L of dissolved hydrogen (DH) was studied using scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Results showed that a double-layer oxide scale was grown on 316LNSS. The outer Ferich layer was composed of Fe3O4 and hydroxides (mainly Ni(OH)2 and CrOOH) and the inner layer was composed of Cr-enriched spinel oxides. The mechanism of oxide scale formation was also discussed.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： The reactor coolant piping in the third generation nuclear power plants of AP1000 is manufactured by integrally forging. Therefore, it is of vital importance to investigate the effects of forging and heat treatments on the stress corrosion cracking (SCC) resistance of 316LN stainless steel (316LNSS), which is the candidate material for the reactor coolant piping in AP1000 nuclear power plants. In this work, electron back scattering diffraction (EBSD) and microhardness measurements (HV) were used to characterize the microstructure and residual strain of the as-received 316LNSS, the forged and solution anneal treated 316LNSS and the forged and stress relief treated 316LNSS, respectively. The average grain size of the as- received 316LNSS was the largest, and the forged 316LNSS followed by solution anneal treatment and stress relief treatment showed no obvious differences on grain size. The as-received 316LNSS exhibited the highest residual strain followed by the forged and stress relief treated 316LNSS and then solution anneal treated 316LNSS. Besides, the residual strain in the as- received 316LNSS concentrated on grain boundaries, while the residual strain in the forged and stress relief treated 316LNSS was characterized by a band-like distribution. The U-bend specimens were utilized to investigate the SCC behavior of the 3 kinds of 316LNSS specimens in high temperature caustic solution. After SCC experiments, the crack morphologies of the 3 kinds of 316LNSS specimens were examined by SEM. Then the macro and micro fracture morphologies were examined by OM and SEM, respectively. Grain morphology, residual strain and grain boundary character distribution near the SCC crack tip of the forged and stress relief treated 316LNSS were investigated using EBSD. The results showed that the forged and solution anneal treated 316LNSS exhibited the lowest SCC sensibility, while the as-received the highest, with the most cracks and the highest growth rate. The as-received and the forged and solution anneal treated 316LNSS showed obvious intergranular cracking, while the forged and stress relief treated 316LNSS showed a mixed cracking mode. The larger average grain size and higher residual strain, especially concentrating on the grain boundaries, were considered to be responsible for the highest SCC sensibility of the as-received 316LNSS. Compared with the forged and stress relief treated 316LNSS, the higher content of coincidence site lattice boundary (CSLB) and lower residual strain contributed to the lower SCC sensibility of forged and solution anneal treated 316LNSS. The stress relief treatment failed to eliminate the band-like microstructure effectively, which disadvantaged the SCC resistance.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Plasma arc cold hearth melting (PAM) is an effective technology to produce high purity titanium alloy ingots which are widely used in aeronautic and astronautic industries. To date, the development of PAM in our country is still at initial stage. It is necessary to investigate the melting parameters of PAM and the following thermal mechanical processing of the ingots fabricated by PAM. In this study, the TC1 alloy ingots casted by PAM were cogged at b transus temperature and then rolled by unidirectional rolling and cross rolling in the a+b phase field. The typical widmanstatten structure of cast- ingots turned to transformed b morphology after cogging at b transus temperature in which the a phases forms in smaller colonies of laths. After the unidirectinal rolling in the a+b phase field, the a colonies were distorted and the a laths re-arranged along the rolling direction, while they had weaker directivity after cross rolling. The sheets rolled by both unidirectional and cross rolling showed typical prismatic texture. After annealing treatment below the b transus temperature, the a phases turned to equiaxial morphology. The ambient yield strength of the sheet in transverse direction was significantly higher than in rolling direction, which could be attributed to the strong prismatic texture introduced by hot rolling process.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： The directionally solidificated (DS) Co-based superalloys are widely used in aircraft turbine vanes due to the good stress-rupture parameters and excellent hot corrosion resistance. The cyclic change of temperatures and complex stress state thermal fatigue (TF) cracks happen frequently in vanes during service. However, most of the work are conducted in Ni- based superalloys and there is rare report concerning the TF behavior of DS Cobased superalloys. Furthermore, due to the residual strain accumulated during processing, shot peening, grinding and recrystallization (RX) frequently occur when the DS components are exposed to high temperatures. It is believed that RX may change the microstructure, especially adding more grain boundaries to DS alloys, and result in the reduction of the mechanical properties of DS superalloys. Therefore, in this work, V-notch plate specimens with notch direction perpendicular and parallel to the DS orientation are machined from the DS plate. Local RX grains are prepared (local indented and then heat treated) in the notch areas of some samples. TF test is conducted between 1000 ℃ to room temperature. The effect of DS orientation and RX on TF properties of a DS Co-based superalloy is investigated. The results indicate that the cracks propagate along the interdendritic regions in the samples with notches parallel to the DS direction, which exhibites lower TF properties than samples with notches vertical to the DS direction. TF cracks initiate and propagate along RX boundaries in samples containing RX grains. Precipitation of M23C6 carbides is found along the RX boundaries during TF tests. Due to the oxidation at the tip of crack, M23C6 desquamates and leads to the formation of micro voids, which accelerates the crack propagation and decreases TF properties of tested alloy. In samples with notches parallel to the DS direction, cracks preferentially propagate along the RX grain boundaries.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Casting microporosity defect is one of the important issues for as- cast HK40 alloys preparation, which is of great importance to application performance of the alloy castings. A comprehensive understanding of the mechanism on formation of the casting microporosity defect is still unclear for the alloys. In this work, the casting microporosity defect and influences of boron micro-alloying on the as-cast microstructures and microporosities in HK40 alloys castings were investigated by means of SEM, OM and XRD, etc.. The microstructures in the HK40 alloys with and without boron micro-alloying after quenching at high temperatures were also examined to check the solidification characteristic change attribute to boron addition. The results show that there are two types of casting microporosities in the castings. Type A is mainly caused by the rapid growth of dendrites and thus dendritic bridge connecting which lead to feeding shortages between the bridge dendrites. Type B is, however, resulted by the growth of M7C3 carbides in coarsened dendritic morphology which induce to the feeding channel blockage in adjacent interdendritic regions. Boron micro-alloying decreases the tendency of columnar grain formation and refines the dendrites in HK40 alloys which therefore suppresses the casting microporosity defect of type A. Additionally, boron micro-alloying not only increases the volume fraction of eutectic phases, but also changes the M7C3 carbides in dendritic morphology into the M23C6 carbides in lamellar morphology, which prevents the feeding channal blockage in adjacent interdendritic regions, thus reduces the casting microporosity defect of type B.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Service safety of turbine blades in aircraft enγines are threatened by microstructural and property deγradation instantly caused by overheatinγ durinγ service. Systematic investiγations about microstructural deγradation durinγ overheatinγ exposures and its influence on mechanical properties of turbine blades durinγ service are limitedly reported. In this work, microstructure and mechanical properties of γH4033 alloy, which was sectioned from the shank of a serviced 2nd staγe turbine blade in an aircraft enγine, were studied after overheatinγ at 900~1100 ℃ for 3 min. Microstructural deγradation durinγ overheatinγ exposures as well as its influence on room temperature hardness and stress rupture life at 700 ℃, 430 MPa were analyzed. The results of microstructural characterization indicated that the coarseninγ and dissolution of γ' precipitates were introduced by overheatinγ exposures, and all of the γ' precipitates dissolved at 980 ℃ for 3 min. γradual dissolution of γrain boundary (γB) carbides was observed with the increase of overheatinγ temperature. Complete dissolution of γB carbides at 1100 ℃ resulted in γrain γrowth. The room temperature hardness after overheatinγ exposures decreased γrossly with the dissolution of γ' phase. Due to the dissolution and re-precipitation of γ' phase as well as the dissolution of γB carbides, the stress rupture life under 700 ℃, 430 MPa of γH4033 alloy was initially increased and then decreased siγnificantly.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-18 Cooperative journals: 《材料研究学报》
Abstract： Effect of forging on the average grain size, residue strain and grain orientation distribution of nuclear grade 316LN stainless steel (316LNss) was studied by means of electron back scattering diffraction (EBSD). The morphology and chemical composition of the oxide films formed on the as-received 316LNss as well as the forged and solution annealed 316LNss after immersion in borated and lithiated high temperature pressurized water at 300 oC for 190 h were also investigated. The results show that the average grain size was reduced and the residual strain was eliminated by forging and followed solution annealing. There were no obvious textures in the forged and solution annealed 316LNss. A two-layered oxide film grew on 316LNss after immersion in high temperature pressurized water. The outer layer composes of hydroxides and Fe-enriched spinal oxides and the inner layer composes of Cr-enriched spinal oxides. The forged and solution annealed 316LNss exhibited a lower oxidation rate rather than the as received ones due to the formation of a thinner and more Cr-enriched oxide film. The oxidation mechanism was also discussed.
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