Size Effect in Ultra-Miniature Creep Test of 1Cr-0.5Mo Steels

Recently, an ultra-miniature creep (UMC) test by using a small plate specimen has been applied to assessment of the remaining creep rupture life of in-service power plant components in Japan. However, such miniature creep tests have an essential problem of the size effect on creep rupture life. Therefore, in this study, using the UMC specimens with different thickness (0.5 and 1 mm) of 1Cr-0.5Mo steel, the creep test was conducted in an argon gas environment for comparison in creep and rupture properties to the standard round bar specimen (φ6) tested in air. The size effect was discussed from the viewpoint of 3 factors having an influence on the size effect (i.e., number of grains on the cross section of a specimen, oxidation thinning and shape effect). As a result, number of grains had no size effect in the material tested because of small grain size of about 50 µm. Also, a simplified procedure was proposed for evaluating life reduction due to the oxidation thinning. Furthermore, using a shape effect index L0/√A0 (L0: parallel length, A0 : cross section area), the shape effect was found to be observed in the index of 4.5 or less.