Effect of hot rolling process on the mechanical and microstructural property of the 9Cr-1Mo steel

Highlights: → We performed hot rolling to see the mechanical and microstructural effect in modified 9Cr-1Mo steel. → Specimens were hot rolled either at 1050 deg. C or 950 deg. C upon cooling after normalizing. → Continuous annealing right after the hot rolling at 950 deg. C was also performed. → We found hot rolling leads to the precipitation hardening. → Continuous annealing enhanced the hot rolling effect. - Abstract: The effect of hot rolling on the mechanical and microstructural property has been investigated to simulate the effect of hot extrusion during the manufacturing process of the fuel cladding for sodium cooled fast reactors (SFRs). Hot rolling of modified 9Cr-1Mo steel was carried out either at 1050 deg. C or 950 deg. C upon cooling after normalizing. Continuous annealing right after the hot rolling at 950 deg. C for 1 h has been carried out followed by the mechanical testing and microstructural analysis. The results showed that hot rolling without any annealing or tempering treatment leaves residual stress so that it leads to the abrupt increase of material strength that would affect cladding formability. Continuous annealing right after the hot rolling process can alleviate residual stress without decreasing too much of material strength. Hot rolling either at 1050 deg. C or 950 deg. C increases the number density of the remained precipitate which leads to the precipitation hardening. Introduction of continuous annealing results in an increase in the fraction of secondary V-rich MX precipitate that leads to an increase in the stability at high temperature mechanical property.