Change of steels and alloys structure and properties during long operation at high temperature

Abstract
When defining the stress-strain state (SSS) and evaluating safety factors for turbomashinery components operating at high temperatures under creep conditions, deformation and fracture characteristics obtained during tests of as-heat-treated materials are usually used. At the same time, if the change in material properties during long - term service is taken into account, then the SSS of companents and, consequently, the real safety factors can significantly differ from design values which have been calculated without accounting for these changes. In this report the changes in material structure and deformation characteristics, stress-rupture and creep-rupture properties, and thermal fatigue for a number of materials of different types (perlitic and austenitic steel, nickel-base alloys) are analyzed based on the results of long-standing studies were performed for the materials both in the original state and after long-term aging (to 10000 hour and in some cases up to 50.0000 hour and up) at high temperatures (400-900 °C). The greatest changes in properties were found for 20Ch23N18 austenitic steel and EP99 nickel-base alloy (22%Cr) after long-term aging, resulting in topologically close-packed δ- and µ- phases precipitations. Thus, EP99 alloy plasticity is decreased to about zero at 20 °C, number of cycles to failure is hundred times reduced thermal fatigue tests. The rate of creep for 20Ch23N18 steel 20X23H18 is 150 times increased.