Microstructural evolution in SUS304H steel during long-term creep over 105 h

The microstructural evolution has comprehensively been investigated for type 304H austenitic stainless steel after creep rupture testing for up to about 180,000 h at temperatures between 550 (823) and 750°C (1023 K). Fine M23C6 carbides and σ phase precipitated in the specimens during creep. The concentration of Cr in the M23C6 carbides increased during creep, while that of Fe decreased and they reached substantially equilibrium values at long times over 105 h. No change was found for the concentrations of Cr, Fe and Ni in the σ phase. The Vickers hardness of the specimen gauge portion decreased with time after reaching a maximum at short times less than about 102 h, while it had a tendency of increase again at long times at 600°C. The creep voids formed at the interface between the precipitates of σ phase on grain boundaries and austenite matrix at low stresses and long times.