Evolution of carbide precipitates in 2½Cr–1Mo steels in power industry: morphology and stability, and characterisation techniques

Abstract

Many power generation plants have already been in operation for decades, a few even beyond the design life of 30 to 40 years. To accomplish this goal of extending the service life of these plants, ability to accurately estimate the service history and remaining life of high temperature components is essential to implement maintenance and repair programmes. An indicator of the service age and remaining life of Cr–Mo steel components can be the carbide precipitates that form after the steel component is exposed to prolonged temperature–time conditions. Characterisation of these carbides as a function of these conditions will assist in determining the service history of a boiler component. To learn about the stability and evolution of the carbides, a set of heat treatment was conducted to produce specimens with varied thermal exposures. The carbides were extracted from the steel samples using selective digestion, which proved to be excellent in recovering most of the precipitates for subsequent analysis. Coupled with X-ray diffraction, qualitative and quantitative information regarding the carbides were determined. High resolution image analysis also assisted in the determination of size distributions of the carbide particles. The methodology followed in this work is directly applicable for the determination of service history and indirectly the remaining life of the boiler equipment.

Keywords:

• CARBIDE PRECIPITATION
• CARBIDE STABILITY AND EVOLUTION
• CR–MO STEELS
• SELECTIVE DIGESTION
• X-RAY DIFFRACTION