Microstructures and Mechanical Properties of Alumina and/or Yttrium Oxides Strengthened 9Cr Steel Fabricated by Vacuum Casting

Abstract

The oxides with Al2O3‐containing and/or Y‐containing are introduced into 9Cr ferritic/martensitic steel fabricated by vacuum casting to modify its microstructures and improve its mechanical properties. The results show that the micron nonmetallic particles are refined, and the number of submicron particles is increased after the introduction of Al2O3‐containing oxides, while the number of submicron particles is decreased when both Al2O3‐and Y‐containing oxides are introduced. The average grain size, the width of martensite, and the M23C6 phases in the tempered steel are optimized when the Al2O3‐containing oxides are introduced. However, with the further introduction of Y‐containing oxides, these tempered microstructures do not change much, except for the precipitation of the δ‐Fe phase. Due to the grain refinement and second‐phase particle strengthening after the introduction of Al2O3‐containing particles, the tensile strength is improved not only at room temperature but also at 550 °C, but the impact energy drops significantly, which is caused by a large number of TiN particles promoted by Al2O3 particles through heterogeneous nucleation. At the same time, the further introduction of Y‐containing oxides reduces the tensile strength and increases the elongation at room temperature, which is attributed to the precipitation of soft δ‐Fe promoted by the addition of Y.