Views:1 Author:Ronsco Publish Time: 2021-04-28 Origin:Ronsco
Hastelloy N is a nickel-based superalloy used in molten salt reactor , which has excellent corrosion resistance, anti-neutron irradiation properties and good high-temperature strength.
However, the outlet temperature of the reactor reached 750°C, which exceeded the allowable temperature of Hastelloy N alloy 704°C, ie, the alloy could not be stably serviced in molten salt environment at 750°C for a long time. Therefore, there is an urgent need to optimize and improve Hastelloy N alloy to meet the requirements of higher temperature molten salt reactors.
This article takes Hastelloy N as the research object and designs the content of trace element Mn.By means of optical microscopy (OM), scanning electron microscopy (SEM+EDS+EBSD), universal stretching machine, X-ray diffractometer(XRD)，electron probe (EPMA) and other experimental analysis methods to study the effects of different Mn content on the microstructure, mechanical properties, and isothermal oxidation performance on Hastelloy N alloys.
The following research results can be obtained:
（1）Mn element refines the grain of Hastelloy N alloy and precipitates more carbides. The carbides gradually condense into a massive, long chain shape and accumulate at the grain boundary.
（2）At room temperature, the tensile strength of 0.5Mn alloys is poor. When the Mn content exceeds 1wt%, the tensile strength is improved, and there are dimples and step-like grain structures on the fractures. The fracture mode is a mixed fracture consisting of cleavage fracture and ductile fracture. At 850 °C high temperature, Mn has no obvious effect on the tensile properties of the alloy. The fracture mode is a mixed fracture composed of cleaved fractures and ductile fractures.
（3）With the increase of Mn content, the oxidation resistance of the alloy is enhanced. At 700 °C, 1wt% Mn content alloy has the best oxidation resistance, and the oxidation rate is 25.9% lower than that of 0Mn alloy. At 850 °C, the 0.75wt% Mn content alloy has the best oxidation resistance and the oxidation rate is 52.1% lower than the 0Mn alloy.
（4）The oxide film has a layered structure. After oxidation at 700°C/200h, all alloy oxide films are two layers. The outer layers are NiO, Fe2O3 and other oxides, and the inner layers are composed of Cr2O3, MoO2 and NiMn2O4 oxides. The surface of the alloy did not show any obvious spallingand the NiO layer was completely dense. The oxide layer of the alloy gradually becomes thinner as the Mn content increases. After oxidation at 850° C/100 h, Oxide film of 0～0.2wt% Mn content alloy is divided into three layers, the outer layer is mainly NiO, the intermediate layer is NiO, NiMn2O4 and other composite oxides, and the inner layer is oxides such as Cr2O3 and MoO2. When the Mn content is 0.5～1wt%, the oxide film is divided into two layers, the outer layer is NiO and a small amount of NiFe2O4, NiMn2O4, the inner layer is Cr2O3, MoO2 and other oxides.Internal oxidation of the alloy gradually weakens as the Mn content increases.
（5）The addition of Mn promotes the formation of a NiMn2O4 spinel protective layer between NiO and the matrix, effectively impedes the intrusion of external O and the outward diffusion of alloying elements, so that the oxidation resistance of the alloy can be enhanced.