Residual creep life assessment by change of martensitic lath structure in modified 9Cr-1Mo steels

Mod.9Cr-1Mo steel has a martensitic lath structure. Recovery of the lath structure takes place in 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 1053 K, the lath structure is thermally stable at the testing temperatures (848 K - 923 K). However, recovery of lath structure readily takes place during creep, indicating that the recovery is induced by creep deformation. Lath width d increases with creep strain and saturates to a value ds determined by creep stress. The increase of d is faster at a higher stress and temperature. A normalized change in lath width, Δd/Δds, was introduced to explain the variation of lath growth rate with creep stress and temperature. Δd is the change in lath width from the initial value d0, and Δds is the difference between ds, and d0. Δd/Δds is uniquely related to creep strain ε and the relationship is independent of creep stress as well as creep temperature. This Δd/Δds-ε 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 evaluated from the measurement of Δd/Δds based on the Δd/Δds-ε 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. (author)