Effects of yttrium and zirconium additions on inclusions and mechanical properties of a reduced activation ferritic/martensitic steel

Abstract

The effects of two alloying elements, i.e., yttrium (Y) and zirconium (Zr), on the inclusion, microstructure, tensile property and impact toughness of reduced activation ferritic/martensitic (RAFM) steel are analyzed. The size and number of particles were determined by optical microscopy, and the particle types were examined by scanning electron microscopy. The size of ~ 20% and ~ 85% of the inclusions lied in the range of 0.5–1.0 μm and 0.5–3.0 μm, respectively. In Y- and Y–Zr-containing specimens, the density of the fine inclusions, with a size less than 0.5 μm, was found to be 1.06 × 1017 and 9.82 × 1016 m−3, respectively. All specimens were normalized at 1323 K for 30 min and tempered at 923 and 1023 K for 90 min, resulting in the formation of tempered martensite with M23C6 carbides and MX precipitates. Zr-containing RAFM steel tempered at 923 K formed M23C6 carbides and nano-sized carbides with Zr, conferring superior strength balance and impact toughness. The yield strength of alloy reached 695 MPa, and a low ductile–brittle transition temperature of 238 K was maintained.