Effects of yttrium on microstructure and properties of reduced activation ferritic-martensitic steel

Abstract

This study investigates the inclusions, microstructure, tensile properties, and impact toughness of reduced activation ferritic/martensitic (RAFM) steels with different Y contents (0.006, 0.034 and 0.071 wt-%). Owing to the pinning of the grain boundary to the Y inclusions (which refines the original austenite grain size) and an existing Fe–Cr–Ta–Y–S–O phase, the tensile strength at both room and high temperatures increased with increasing Y content (below 0.071%). Increasing the Y content to 0.034 wt-% decreased the ductile-to-brittle transition temperature (DBTT). However, when the Y content reached 0.071 wt-%, the DBTT increased as the Y precipitated into blocky yttrium-rich inclusions. The microstructure, tensile properties, and impact toughness of the RAFM steel were optimised at a Y content of approximately 0.034 wt-%.

Effects of Yttrium on Microstructure Stability and Tensile Properties of China Low Activation Martensitic Steel

“…Grains coarsened during the 3000 h of aging but were refined through the hindrance of grain boundaries by Y-containing inclusions, as previously confirmed [19]. The lower coarsening rate of the CLAM steel with 0.071 wt.% Y (11.6 → 13.1 µm) as compared to that of the reference base steel (0Y alloy) (14.7 → 16.9 µm) was attributed to the pinning of the Y inclusions [19]. Surface energy is the driving force of grain boundary migration [20].…”

“…The microstructures of the CLAM alloys after long-term aging are presented in Figure 2. The original heat-treated matrix [19] remained unchanged after aging at 550 °C for 3000 h. Martensitic lath was present in all samples. Table 2 shows the APGZ of the aged samples measured through the linear intercept method.…”

“…The solubility of RE elements would increase when additional RE elements are added, while the carbide nucleation would intensify. The presence of residual Y-rich inclusions in the 71Y alloy would destroy the continuity of the matrix and degrade the mechanical property of the alloy [19,24], as shown in Figure 3d. Several Laves phases appeared in the Y-added CLAM steels and the number of Laves phases in the 0Y alloy increased (Figure 3e-h) when the exposure time was extended to 3000 h. The solubility of RE elements would increase when additional RE elements are added, while the carbide nucleation would intensify.…”

“…Several Laves phases appeared in the Y-added CLAM steels and the number of Laves phases in the 0Y alloy increased (Figure 3e-h) when the exposure time was extended to 3000 h. The solubility of RE elements would increase when additional RE elements are added, while the carbide nucleation would intensify. The presence of residual Y-rich inclusions in the 71Y alloy would destroy the continuity of the matrix and degrade the mechanical property of the alloy [19,24], as shown in Figure 3d. Several Laves phases appeared in the Y-added CLAM steels and the number of Laves phases in the 0Y alloy increased (Figure 3e-h) when the exposure time was extended to 3000 h. The details of the microstructures of the CLAM steels were revealed through TEM (Figure 4).…”

“…Shi and Han [18] reported that the tensile strength of RAFM steel improved when dislocation movement was effectively hindered by micron-sized Y 2 O 3 . The effect of trace (<0.1%) Y addition on CLAM steel was evaluated previously [19]. The thermal aging property shown by nuclear power steels during exposure to operating temperatures is important for ensuring the safe operation of nuclear power plants.…”