Effect of Severe Plastic Deformation on Radiation Hardening of T91 Ferritic-Martensitic Steel

Abstract

Effect of thermomechanical treatment on radiation hardening behavior in T91 ferritic-martensitic steel was evaluated. An applying of severe plastic deformation (SPD) by the “upsetting-extrusion” method and subsequent heat treatment led to a considerable grain refinement, crushing of martensite lamellas, reduction of MX carbides size and their more uniform distribution. Nanoindentation measurements of SPD-modified steel revealed a 1.4-fold increase in the hardness relative to the initial steel. Irradiation response of modified steel was examined after 1.4 MeV Ar+ ion irradiations in the dose range of 10…45 displacements per atom (dpa) at room temperature and 460 °C. Microstructure characterization was performed by means of transmission electron microscopy (TEM). It was found that dislocation loops and nano-sized argon bubbles dominated the damage microstructure after ion irradiation. The effects of SPD-induced transformations as well as nano-bubbles formation are discussed regarding to the hardening phenomenon observed in irradiated steel.

Hardening Behavior of Advanced Structural Alloys Under Ion Irradiation

“…Martensite lamellas with a transverse size of 0.25…0.5 μm are observed inside the former austenite grains. In additions, a significant number of small precipitates of ≤ 50 nm in size, which are usually referred to as MX-type phases is also observed [8].…”

“…After heat treatment at 600 °C for 25 h (hereinafter, sample code T91-MSPD), sufficiently large grains of typical "ferritic" morphology have formed. "Broken" martensite in some zones is transformed into a more "classic" [8].…”

“…Martensite lamellas with a transverse size of 0.25…0.5 μm are observed inside the former austenite grains. In additions, a significant number of small precipitates of ≤ 50 nm in size, which are usually referred to as MX-type phases is also observed[8].The microstructure of T91-MSPD steel after applying of SPD by the "upsetting-extrusion" method comprises a broken martensite type structure and grains with the average size 100 nm. After heat treatment at 600 °C for 25 h, sufficiently large grains of typical "ferritic" morphology have formed (see Fig.5,b), which appear to be the result of recrystallization of fine grains of former austenite or fine-grained areas of "broken" martensite.…”