EBSD ANALYSIS OF CARBIDES IN X20CrMoV12.1 STEEL TEMPERATURES

摘要

One of the steels that are often used in power plants for the steam pipes is X20CrMoV12.1 due to its excellent combination of high-temperature strength, toughness and creep strength. The microstructure consists of tempered martensite with finely dispersed carbide precipitates along the boundaries of the ex-austenitic grains and the ferritic sub-boundaries. During steel exploitation the working conditions often reach temperatures up to 600 ℃. Therefore, the carbides change their chemical and phase composition as well as their size until equilibrium is reached, which leads to a deterioration of the steel mechanical characteristics. In order to simulate the normal operating conditions in the laboratory and to accelerate the process, some specimens of the same material were annealed under ambient conditions for different times at 800 ℃. The microstructure and the carbide precipitates in the initial state and after annealing were investigated using a field-emission scanning electron microscope Jeol JSM 6500F equipped with an EBSD camera HKL Nordlys Ⅱ using Channel5 software. Graphic processing software and image analysis 3.1 PRO software were used to colour-code areas occupied by carbides. This was the basis for the determination of their volume fraction. During the annealing the tetragonal martensite transformed to the ferrite phase and the carbide precipitates coarsened. The volume fraction of these carbides remained the same, although their number decreased with the annealing time. Some hardness and tensile tests were performed to relate the structure to mechanical properties. Static 100 hours creep tests were also performed and the results will be commented. Based on EBSD the carbide compounds were determined to be M_(23)C_6 and during coarsening their crystal structure remained the same. The carbides nucleate along martensite lath. After specimen annealing of 3 h at 800 ℃ the carbides are in the orientational relationship with the neighbouring martensite and remain in the same orientation after 168 hours of annealing. Using EBSD mapping and spot analysis the carbides' orientational relationships with the neighbouring grains were determined and will be presented and explained.