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MICROSTRUCTURAL ASPECTS OF LONG-TERM AGEING OF MARTENSITIC 9% Cr STEEL WITH AND WITHOUT BORON ADDITION

In high-chromium 9–12% Cr steels, the basic precipitates are chromium-rich M23C6 carbides, which play an important role in these steels as a stabilising factor for the martensitic lath microstructure. Previous experiments in the use of modern martensitic 9% Cr steels in the power industry showed that the main mechanisms of degradation of materials are processes of coagulation of M23C6 carbides and the release and increase of the Laves phase [1–3]. The increase in the size of M23C6 carbides and Laves phase precipitates during operation results not only in the limitation of their effect on precipitation strengthening, but also leads to the formation of a continuous network on grain boundaries. Numerous precipitates on grain/lath boundaries not only increase the brittleness of the steel, but also change the cracking mechanism from ductile to brittle. In addition, the diffusion of atoms of substitutional elements (chromium, molybdenum, tungsten) into the released Laves phase and/or M23C6 carbides contributes to the steel’s tendency to recover and polygonise the matrix and reduce resistance to oxidation [2–6]. The introduction of the micro-addition of boron into martensitic steels is kind of an antidote for the tendency of M23C6 and Laves phase precipitates to coagulate. Boron partially substitutes carbon atoms in the M23C6 carbide, forming more stable M23(C, B)6 carboborides. During use, these precipitates tend to coagulate and are characterised by greater dispersion. A similar, positive effect of boron was observed on the retardation of the process of Laves phase precipitation and increase in its stability [2, 5, 7]. The aim of the study was to analyse the influence of longterm ageing on changes in the microstructure of 9% Cr steels – X10CrMoVNbN9-1 (P91) and X13CrMoCoVNbNB9-2-1 (PB2). The study covered 9% Cr martensitic steels – X10CrMoVNb9-1 (P91) and X13CrMoCoVNbNB9-2-1 (PB2) after long-term ageing at the temperature of 620°C and soaking time up to 50,000 hours. The microstructural investigations of the tested steels were carried out using a TITAN 80–300 transmission electron microscope. The analysis of the changes in the microstructure included the identification of precipitates and computer image analysis: the measurement of dislocation density and mean diameter of the precipitates. The tests showed that the analysed steels still retained the lath martensitic microstructure, but also the processes of matrix softening were observed. The microstructure of the examined steels also showed numerous precipitates with various morphologies. The micro-addition of boron in the PB2 steel, compared with the P91 steel, influenced the retardation of the microstructure degradation processes through the limitation of the growth of M23C6 precipitates and Laves phase. This resulted in a slowdown in the processes of recovery and polygonisation of the matrix of the tested steel.