The magnetism and microstructure of Cr25Ni35Nb and Cr35Ni45Nb alloy tubes after 5 years of service were investigated in this paper. The saturation magnetization of the Cr25Ni35Nb alloy tube in the thickness direction is more than 20 emu/g, and the tube becomes ferromagnetic. The inner and outer walls of Cr35Ni45Nb alloy tubes also become ferromagnetic. But the saturation magnetization of the Cr35Ni45Nb alloy tubes approaches to zero in the center zone. The primary carbides M₇C₃ and NbC are changed into M₂₃C₆ and G phase at the outer region of the furnace tube. However, the M₂₃C₆-type carbides were replaced by carbon-rich carbides M₇C₃ at the carburization zone. Cr-depleted zones are formed at the inner and outer walls of the furnace tubes owing to oxidation. Carburization and oxidation reduce the Cr content of the matrix. Accordingly, the saturation magnetization is very high at the carburization zone and Cr-depleted zone. The magnetism of Cr25Ni35Nb and Cr35Ni45Nb alloy tubes has a high correlation with the Cr content of the matrix. Carburization and oxidation are the main reasons that make the paramagnetic ethylene pyrolysis furnace tube change to ferromagnetic.

The microstructural characterization and mechanical behavior of Cr25Ni35NbM/15CrMo and Cr25Ni35NbM/SUS321 dissimilar weld joints were studied in this paper. The microstructure, room temperature and high temperature (1173 K) tensile strength of dissimilar weld joints were analyzed through optical microscopy (OM), scanning electron microscopy (SEM) and electronic universal tensile testing machine. The microstructure of HAZ in 15CrMo steel of Cr25Ni35NbM/15CrMo dissimilar weld joint transformed from ferrite-pearlite into ferrite-martensite. A large volume fraction of alpha phase was found to have precipitated in the HAZ of SUS321 austenitic stainless steel for Cr25Ni35NbM/SUS321 dissimilar weld joint. At room temperature, the tensile strength and yield strength of these two type dissimilar weld joints is less than Cr25Ni35NbM alloy similar weld joint. The high temperature tensile strength of these two type dissimilar weld joints is less than Cr25Ni35NbM alloy similar weld joint. Both at room and high temperature, the fracture locations of two types dissimilar weld joints are the HAZ of the base metal 15CrMo and SUS321 stainless steel, respectively. It indicates that the weak part of the Cr25Ni35NbM alloy dissimilar weld joints is the low-performance base metals 15CrMo and SUS321 stainless steel.

In order to investigate microstructure and creep rupture strength of a weld joint after aging, Cr35Ni45Nb alloy tubes were welded by the GTAW process of two-type filler metals. The tensile test, hardness and creep test were conducted to analyse the mechanical properties of the weld joint and base metal. The results show that the creep rupture time of the base metal at 1373 K is longer than a new weld joint, while the tensile properties of the base metal at room temperature are similar to a new weld joint. With aging, the microstructure of the weld metal and the base metal tended to be homogeneous, the creep rupture time of the weld joint improved and was similar to that of the base metal.

Iron-based superconductors (FeSCs), particularly the air-stable (Li,Fe)OHFeSe, have drawn considerable attention due to their complex crystalline structures and intriguing superconducting properties. This study focuses on the synthesis of (Li,Fe)OHFeSe, using hydrothermal ion exchange technology with a nearly vacancy-free (PY) _x FeSe precursor. We successfully synthesized samples in both iron-poor and iron-rich reaction systems, with the iron-rich samples exhibiting superconductivity at a transition temperature (T _c) of 40 K, while iron-poor samples did not. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) results showed an obvious difference in Se content between the original solutions of iron-poor and iron-rich systems after the hydrothermal reaction. We propose that in iron-rich system, Fe²⁺ ions migration into the (Li,Fe)OH layer from the reaction solution directly maintains the stability of the FeSe layer in the precursor, facilitating superconductivity. Conversely, in iron-poor system, Fe²⁺ ions migrate into the (Li,Fe)OH layer partly from the reaction solution and partly from the FeSe layer, creating Fe vacancies in the FeSe layer, which inhibit the occurrence of superconductivity. Our findings provide valuable insights into the synthesis of intercalated FeSe-based superconducting materials and the realization of superconductivity in FeSe based superconducting materials.