TEM replica analysis of particle phases in a tempered martensite ferritic Cr steel after long term creep

Abstract

Tempered martensite ferritic steels (TMFSs) have been and are being used for critical components in high temperature plant operating in the 600 °C range. They are exposed to creep conditions for long time periods, exceeding 100,000 h. In the present study we investigate a 12% Cr TMFS, after creep-testing at 550 °C at 120 MPa for 139,000 h. We had previously investigated this material in the TEM using thin foils. We now use an extraction replica technique to analyze four particle families: M23C6, MX, Laves-phase and Z-phase, considering statistically relevant numbers of particles (between 120 and 720). We show how EELS mapping can help in identifying Z-phase particles and use Cr-V-maps to differentiate between the four particle families. The chemical evolution of particles is investigated. The experimental results are discussed in the light of previous thin foil data and with respect to predictions from computational thermodynamics. The strength and weakness of thin foil and replica procedures are compared. Improvements for thermodynamic databases are suggested.