Creep life assessment of Mod.9Cr-1Mo steel, 2; Quantitative evaluation of microstructural damage in creep-interrupted specimens

Sawada, Kota; Maruyama, Koichi; Komine, Ryuji; Nagae, Yuji 

Mod.9Cr-1Mo steel has a martensitic lath structure. Recovery of the lath structure takes place hl the course of creep. Microstructural degradation due to the recovery results in the acceleration of creep rate and the subsequent failure of a specimen. Change of lath width during creep of the steel was quantitatively investigated to propose a residual life assessment methodology based on the recovery process. Since the steel was tempered at 1053K, the lath structure is thermally stable at the testing temperatures (848K923K). However, recovery of lath structure readily takes place during creep, indicating that the recovery is induced by creep deformation. Lath width increases with creep strain and saturates to a value determined by creep stress. The increase of is faster at a higher stress and temperature. A normalized ehange in lath width, /, was introduced to explain the variation of lath growth rate with creep stress and temperature. is the change in lath width from the initial value , and is the difference between and . / is uniquely related to creep strain and the relationship is independent of creep stress as well as ereep temperature. This /- relationship obtained by an accelerated creep test at a higher temperature or stress is applicable to any creep condition including service conditions of engineering plants. Creep strain can be evaluted from the measurement of / based on the /- relationship. A creep curve under any creep condition can readily be calculated by creep data of the steel. Combining these information one can assess residual life of a structural component made of the steel.