Degradation behavior of a 12 % Cr tempered martensite ferritic steel after 20 years of service

Abstract

The 12 % Cr tempered martensite ferritic steel, X20CrMoV12-1, is an important heat exchanger material for boiler tubes used in fossil-fired power plants over the past three decades. Understanding the degradation behavior of these steels after long-term service is critical, as it directly determines the safe operation and life extension of power plants. However, most investigations have focused on the properties and microstructure based on the laboratory-accelerated test, while the long-term serviced materials are rarely reported. Here, an X20CrMoV12-1 boiler tube that has been serviced as a superheater for 20 years was investigated in terms of hardness, tensile strength and creep rupture tests with corresponding microstructural analysis, aiming to understand its degradation behavior. We found the stability of microhardness and a decrease in room and high temperature tensile strength by 13 % and 17 %, respectively. Importantly, the creep rupture life of the serviced tube is reduced by ∼80 % at 600 °C, attributing to the significant decrease of dislocation density and unstable features of the coarse Laves phases. The microstructural investigation results showed no significant change in the dominant strengthening precipitates of M₂₃C₆, MX and tempered lath structures. However, microstructures of M₂₃C₆, Laves phase and substructures became unstable upon further creep at 600 °C, appearing as spheroidization, dissolution and rapid coarsening, respectively. These results suggested the overheating of the materials should be avoided during the afterward operation and life extension.