| Because of the energy deficiency and the security of conventional nuclear reactor, the molten salt breeder reactor which has the inherent safety becomes one of the most promising generation IV nuclear energy systems, and has attracted more and more attentions in recent years around the world. GH3535 corrosion resistant superalloy, which developed in America and used as an important structural material of the core vessel and loop pipe in MSBR(Molten Salt Breeder Reactor). But it still has lots of problems, because the study about this alloy was stopped due to some political reasons in 1960s.A new corrosion resistant superalloy (GH3535) was developed based on Hastelloy N in this paper. The influence of heat treatment and alloying elements on microstructure and properties of the alloy has been investigated. The micro structural stability during long term thermal exposure for 10000 h, oxidation and FLiNaK molten salt corrosion resistance of the alloy had also been studied. The results can be outlined as follows:(1) The influence of heat treatment on microstructure and stress rupture properties has been investigated. The grain size increased with the increase of solution temperature and the distribution of caribdes changed from grain boundary to intragranular. The primary M6C dissolved noticeably at 1220℃ and its volume fraction had a great decrease. The M12C carbides which had a coherent orientation relationship with matrix precipitated along grain boundaries aged at 900℃/2 h. The secondary carbides also precipitated during creep test, the amount and size was less than that of aged samples when tested at 650℃/320 MPa; however, the amount and size was nearly the same as that of aged samples when tested at 700℃/240 MPa. When tested at 650℃/320 MPa the stress rupture lives decreased with the increase of solution treatment temperature, while at 700℃/240 MPa the stress rupture lives increased slightly at first and then decreased for samples solution at 1220℃. The elongation all shows a descendent trend for both testing conditions. The stress rupture life and elongation for aged samples all show a noticeable improvement at 650℃/320 MPa, but there was no noticeable improvement at 700℃/240 MPa. The reasons were analyzed based on the grain size, test conditions, and the initial and secondary carbides.(2) The influence of carbon on microstructure and mechanical properties had been studied. The results showed that, the amount of carbides and stress rupture life improved with the increase of the carbon content. And the elongation and charpy impact property decreased with the increase of the carbon content. The carbide can impede the grain growth effectively when exposured to high temperature. The best carbon content is 0.04 wt.% -0.06 wt.%.(3) The influence of Si on microstructure and stress rupture properties of GH3535 superalloy during long term thermal exposure at 700℃ was studied in this dissertation. The results showed that, Si had influence on the type of secondary carbides which precipitated along grain boundaries during long term thermal exposure. The type of secondary carbide was M12C for the alloy with silicon and M6C for the alloy without silicon. There is coherent orientation relationship at the interface of M12C carbide and matrix, but there was no orientation relationship between M6C carbide and matrix. The amount of secondary carbides increased with the growth of aging time, and reach the peak at near 1000 h. The primary and secondary carbides coexisted stability during long term thermal exposure. The stress rupture life and elongation increased along with the increase of aging time, and reached the peak at near 1000 h, the properties decreased later for the alloy without Si, but there were no noticeable decrease of the properties for the alloy with Si. The secondary M12C carbide was beneficial to the stress rupture properties. It can inhibit the propagation of the crack and improve the stress rupture life and elongation. But the M6C carbide was prone to become the crack source. The proper (0.45 wt.%) addition of Si is good for stress rupture properties of GH3535 during long term thermal exposure.(4) The isothermal and cyclic oxidation behavior of the alloy at 700℃ and 900℃had been investigated. The results showed that the formation of spinel oxidization products with the participation of Mn and Si which diffused through the oxidation film was beneficial to the oxidation propertiey during isothermal oxidation at 700℃ and 900℃, the oxidation rate decreased gradually, the continuous, dense and protecting O2O3 oxidation film formed after 500 h, and there was no peeling of oxidization film. But it became worse during cyclic oxidization, the peeling of oxidization film occurred during cyclic oxidization originated from the difference of the expansion coefficient between the oxidation film and the matrix, and Mo element could participate in oxidization continuously, but the oxidation of Mo cannot form protective film. the peeling of oxidization film was severed. The accelerated oxidization phenomenon even occured at 900℃.(5) The corrosion mechanism of GH3535 and the influence of Al2O3 oxidation film on corrosion resistant of the alloy in FLiNaK at 700℃ had been studied. The results showed that the representation of corrosion was the depletion of Mo and Cr; the Al2O3 oxidation film dissolved in FLiNaK, it not only cannot protect the alloy but also increase the corrosion rate of the alloy.The investigation done in this dissertation can be used as theoretical guidance and reference for the optimization and engineering application of the alloy. |
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