Effects of Si on the stress rupture life and microstructure of a novel austenitic stainless steel

A novel Si-bearing austenitic stainless steel was designed as the cladding material for lead-cooled fast reactors with an operating temperature range of 450 °C ∼ 550 °C, and the effects of Si (1.0 wt.% - 2.0 wt.%) on the creep life and fractured microstructure of material were studied at 550 °C and 650 °C. Results show that the addition of Si increased the stress rupture life of steel at 550 °C whereas brought in an opposite effect at 650 °C, which was closely related to the precipitation behavior of σ phase at these two temperatures during creep. At 550 °C the precipitation of σ phase was quite slow, but it was substantially accelerated at 650 °C. Since the experimental materials used in this work were in a cold worked state, so deformation twins existed and they were discovered to be able to promote the nucleation and growth of σ phase on the grain boundaries. With the increase of Si content at 550 °C, σ phase became larger in quantity and denser in distribution on the grain boundary, which improved the anti-cracking ability of grain boundary and enhanced the creep life of material. However, at 650 °C, σ particles apparently coarsened and congregated on the grain boundary under thermal activation, leading to an evident drop in the stress rupture property of steel.