Relation of small punch creep test properties with microstructure changes for an ASTM A387 Cr–Mo steel

The creep properties of ASTM A387 5Cr-0.5Mo steel, grade 5 and class 2, were evaluated by the Small-Punch Creep test. Small-Punch creep tests were carried out at 550 and 600 °C with loads of 67–133 N under an argon atmosphere. Specimens were tested in the as-received and 600 °C-aged conditions. The analysis of creep curves by Norton's law indicated n exponent values between 8.1 and 12, which suggested that the creep deformation occurred by the transgranular deformation of the ferrite matrix originating a transgranular ductile-fracture mode in tested specimens. The difference of n exponent between the as-received specimens tested at 550 and 600 °C can be related to the different microconstituent and precipitation evolution during testing. In contrast, the isothermal 600 °C-aging of specimens prior to testing at 600 °C promoted shorter time-to-rupture and higher n exponents, in comparison to the as-received specimens tested at the same temperature, because of the coarsening process of M₂₃C₆ carbides.