Improving strength and ductility of low activation martensitic (LAM) steel by alloying with titanium and tempering

Abstract

In sequel to our previous study (Mater. Sci. Eng. A, 769 (2020) 138,471), we elucidate here the determining role of alloying with Ti and tempering effect on tensile and creep properties in low activation martensitic steel (LAM) and compare with the coarse-grained (CG) LAM steel. When the tempering temperature was decreased from 760 °C to 730 °C, while maintaining time constant at 30 min, grain size and MX carbide density was constant at ~7.0 ± 0.5 μm and (8.20 ± 0.05) × 1024 m−3 in 0.1 Ti LAM steels, compared to ~20.0 ± 0.5 μm and (3.20 ± 0.05) × 1024 m−3 in Ti-free CG LAM steel. However, the thickness of martensitic lath decreased from 450 ± 10 nm to 370 ± 10 nm and the density of M23C6 carbides increased from (4.30 ± 0.05) × 1019 m−3 to (1.10 ± 0.05) × 1020 m−3, which were 320 ± 10 nm and (6.05 ± 0.05) × 1019 m−3 in CG LAM steel. Yield strength (σy) increased with the decrease of tempering temperature. At room temperature, σy of 0.1 Ti LAM steels increased from 660 ± 10 MPa to 700 ± 10 MPa, accompanied by no apparent decrease in elongation (25 ± 1% to 24 ± 1%). In CG LAM steel, σy was 570 MPa and elongation was 23%. Creep life at 600 °C/170 MPa increased from 145 ± 5 h to 190 ± 5 h with the decrease of tempering temperature from 760 °C to 730 °C, while it was 120 h for CG LAM steel. Furthermore, σy increased with the decrease of subgrain size and the increase of M23C6 density. At 600 °C/170 MPa, the coarsening of martensitic lath and the decrease of M23C6 carbide density reduced the strength, leading to crack initiation in the vicinity of large M23C6 carbides and fracture. The mechanisms involving differences in the mechanical behavior are discussed.