Effects of Cr and Si on the microstructure and solidification path of austenitic stainless steel

Abstract

The lead-cooled fast reactor (LFR), which features advanced technical maturity and enhanced safety, is an important part of the fourth-generation nuclear power system of China. The superior safety of the LFR results from the choice of a relatively inert coolant, the lead or lead-bismuth eutectic (LBE), which can be rather corrosive to common metallic structural materials. Furthermore, there is basically no cladding material available for the LFR. Austenitic stainless steels feature a combination of excellent corrosion resistance, proper strength, and good workability, and materials such as 316Ti and 15-15Ti, which have been used in the sodium-cooled fast reactor (SFR), are viewed as promising candidate materials for LFR cladding applications. Elements of Cr and Si have been found capable of improving the corrosion resistance of 316Ti and 15-15Ti to LBE. However, as ferrite-forming elements, the influences of Cr and Si on the microstructural stability of 316Ti and 15-15Ti are still unclear. In this work, 316Ti-based materials with various Cr and Si contents were studied through thermodynamic simulation and microstructural characterization. Specifically, the equilibrium phase constitutions of the austenitic stainless steels were investigated by thermodynamic simulation using Thermo-Calc. The solidification microstructures and precipitates of Cr- and Si-bearing austenitic stainless steels were studied by optical microscopy (OM), scanning electronic microscopy (SEM), electronic differential system (EDS), and X-ray diffraction (XRD). The results show that Cr and Si can decrease the solidus and liquidus temperatures of alloys and induce the precipitation of δ-phase. For alloy 18Cr−2.0Si−15Ni, the maximum contents of Cr and Si are determined to be no more than 18.8% and 2.55%, respectively, which hinders δ-phase precipitation. In the ingot of 20Cr−2.0Si, δ-phase is found to be located within dendrites in a skeleton morphology, with a volume fraction of 8.6%, whereas in the ingot of 18Cr−2.5Si, δ-phase precipitates between dendrites, with a volume fraction of 3.4%. Moreover, this work also evaluates two kinds of austenitic stainless steel solidification path criteria.

Keywords

• austenitic stainless steel
• thermodynamic simulation
• solidification microstructure
• delta phase
• silicon