Investigations On The Structures And Properties Of Fe-Ni-Co Invar-type Low Expansion Alloy With Addition Of Magnesium Or Cerium

Invar-type low expansion alloy, which possess low expansion coefficient in a certain temperature range, is mainly used in standard measure, microwave cavity, shadow mask of color picture tubes, storage and transport containers of liquefied natural gas and liquid hydrogen which require dimensions stability as various temperature. The roles of alloying elements, preparation technology, heat treatment and deformation process, Invar anomalous theories, magnetic property, welding property in this alloy were extensively investigated theoretically and experimentally in the past decades. The effect of magnesium or cerium on the structure and property of invar-type low expansion alloy was rarely mentioned. However, Ni-Mg master alloy or cerium is usually used to deoxidate in steel-making process, and magnesium or cerium would be remained in the ingot. Based on this problem, the influence of remained magnesium or cerium on the as-forged (including solid-solution state) structure, non-metallic inclusions, thermal expansion coefficient, cryogenic structure stability of invar-type low expansion alloy were studied by optical microscope, X-ray diffraction, scanning electron microscope, EDAX, thermal dilatometer, and so on. We expect this work to provide some references for theoretical research and actual production. The main results are as follows.(1) As-forged structures of the alloy is refined due to the addition of magnesium or cerium, obtains a grain size rating of10.0or12.0with the addition of0.021%Mg or0.0072%Ce, respectively. The alloy consists of a single austenitic phase of face-centered cubic which would not be changed with the addition Mg or Ce.(2) The grade of nonmetallic inclusion in the alloy is less than1.0with addition of magnesium. The morphology of sulfide inclusion in the alloy turns from string to ellipse or sphere with addition of trace cerium.(3) The average linear expansion coefficient from20℃to100℃of the alloys decreases with the increase of the Mg or Ce content owing to the significant effect of deoxidation and desulfurization of Mg or Ce. On the one hand, the segregation of the impurity elements on the grain boundary is suppressed; on the other hand, the quantity of inclusions is reduced sharply. The thermal expansion coefficient (-60℃to100℃) of the alloys with0.010%Mg or0.0060%Ce increases with the increase of solution temperature due to the grain coarsening.(4) Phase transformation is observed in most alloys with addition of magnesium or cerium holding at-60℃for two hours, resulting in a significant increase of thermal expansion coefficient (from-60℃to100℃) of the alloys. Martensitic transformation temperature of the alloys is investigated by DSC, thermal dilation method and resonance method respectively. The results indicate that the addition of Mg or Ce would decrease the martensitic transformation temperature of the alloys and increase the cryogenic structure stability. The results obtained by thermal dilation method are similar with the results obtained by the resonance method, which are more accurate than DSC results.