SB-07-2(070) Assessment of Material Degradation Based on Microstructural Evolution in Type 304H Stainless Steel During Long-Term Creep(Changes in Microstructure 2)

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In order to assess the material degradation, the microstructural evolution and the change in hardness have been investigated for 18Cr-8Ni (type304H) stainless steel after creep rupture tests for up to 180,000 h at 550-750℃. During creep, at first M_<23>C_6 carbides and then σ phase precipitate. The M_<23>C_6 and σ phase along grain boundaries have a close correlation with intergranular creep fracture, which controls the long-term creep rupture strength of this steel. The number density of σ phase decreases or is constant with time after reaching a maximum at a short time less than 10,000 h at 600℃, while the size significantly increases with time for up to 100,000 h or more. At low stress of 61 MPa or less at around 650℃, the hardening during creep can be approximately given by age hardening under no stress except for the final stage just before creep-rupture.
2001-06-03