Part II Materials Science and Metallurgy Example of a Literature Survey Power Plant Steels: Remanent Life Assessment

The steels used in the power generation industry are given a severe tempering heat–treatment before entering service. This gives them a stable microstructure which is close to equilibrium. They nevertheless undergo many changes over long periods of time. This article is a review of some of the methods which exploit the changes in order to estimate the life that remains in alloys which are only partly exhausted. INTRODUCTION Many of the safety–critical components in power plant are made of steels developed to resist deformation when used in the range 480–565 ◦C and 15–90 MPa. They are expected to serve reliably for a period of about 30 years, giving a maximum tolerable creep strain rate of about 3 × 10−11 s−1 (approximately 2% elongation over the 30 years). The design stress must be set to be small enough to prevent creep rupture over the intended life of the plant. The steels are able to survive for such long periods because the operating temperature is only about half of the absolute melting temperature, making the migration of atoms very slow indeed. Creep therefore depends on the ability of dislocations to overcome obstacles with the help of thermal energy. The obstacles are mainly carbide particles which are dispersed throughout the microstructure. Suppose that the microstructure and the operating conditions do not change during service. The accuracy with which component life might then be predicted would depend only on the quality of the experimental data. The so–called safety factors common in design could then be greatly reduced with obvious benefits. Of course, this never happens in practice; the steels are always heterogeneous and the service conditions vary over a range of scales and locations. The design life is therefore set conservatively to account for the fact that measured creep data follow a Gaussian distribution with a significant width. In spite of this,