| The ultra-supercritical technology, with the advantages of high efficiency and low consumption of power, has been widely used in fossil fuel power plants. The Super304H steel, a novel heat-resistant stainless steel with excellent high temperature strength, high temperature oxidation resistance and anti-steam corrosion performance, has become one of the preferred materials for the superheater and reheater tubes in ultra-supercritical power plants. However, the high intergranular corrosion sensitivity (IGCS) of Super304H steel, caused by its high carbon content and more M23C6 precipitation, has become a critical problem for its application. Therefore a full understanding of the precipitation kinetics of M23C6 phase in Super304H steel, which has not been focused by most academic scholars at present, is of great importance for providing scientific guidance for the composition and process optimization to solve the problem of high IGCS of Super304H steel.In this paper, the precipitation kinetics of M23C6 phase in the Super304H steel was systematically studied and compared with that in the TP304H and coarsened Super304H steels. Firstly, the precipitation behavior of the phases in Super304H at high temperature was investigated by means of Optical Microscope (OM), Scanning Electron Microscope (SEM), Energy Dispersive Spectrometer (EDS) and X-Ray Diffraction (XRD). Afterwards, the diffusion kinetics of Cr during vacuum heat treatment in Super304H and TP304H steels, with Cr electroplated on their surface, was detected by EDS quantitative line scanning. Finally, the precipitation kinetics of M23C6 phase in original Super304H, coarsened Super304H and referenced TP304H during high temperature aging, were mainly investigated with quantitative etching method and corresponding Precipitation-Temperature-Time (PTT) curves of the three group samples were established. Further analysis was also made from the aspect of variation in composition and structure of steels.It is found that micro structure of Super304H is made up of matrix γ-phase and a small amount of precipitates, with the grade of grain size ranging from 8 to 10. By contrast, the microstructure of TP304H is made up of only matrix γ-phase without precipitates, the grade of grain size being 5 to 7. The coarsened Super304H with the same grain grade as TP304H was obtained by heating at 1250℃ for 10 minutes with water quenching in order to take into account the effect of grain size.When the aging temperature is between 600℃and 850℃,the amount of precipitates in Super304H steel increases gradually and it decreases apparently when the aging temperature is higher than 850℃,with precipitates rc-dissolved into the matrix.At the same time,the fine precipitates continuously distributed at the grain boundary gradually gather into discontinuous coarsened particles.In addition,M23C6,Nb(C,N)and Cu-rich phases are detected in the Super304H aged at 600℃～900℃ for 500 hours.Using electroplating technology,pure chromium coating with thicknesses of 18mm and 45mm respectively were obtained on the surface of Super304H and TP304H steels. Afterwards,the diffusion behavior of Cr from the pure Cr coating to the matrix after different temperature annealing was investigated by EDS quantitative line scanning.The results show that the diffusion depth and diffusivity of chromium increase gradually with the rising of temperature.At the same temperature,the activation energy for Cr diffusion in the Super304H is smaller than that in TP304H,and the diffusion coefficient of chromium is larger in the Super304H.The diffusion equation of chromium in the Super304H and TP304H steels are D=1.08×10-15exp(-6.08×104/RT) and D=2.29×10-15exp(-7.44×104/RT) respectively.Finally,the PTT curves of M23C6 precipitates were established in original Super304H, coarsened Super304H and referenced TP304H through the Avrami curves,which were fitted by the partial precipitation content data of M23C6 by quantitative etching method.The PTT curves of the three group samples a11 show a typical C shape character,with nose temperature being 800℃～850℃ for original and coarsened Super304H and being 850℃ for TP304H. Furthermore,compared with the TP304H,the begin line of C-shape curve of the Super304H moves to the right,and the end line shifts toi the left.Meanwhile the begin line of coarsened Super304H moves to left and the end line shifts to the right compared with the original Super304H.These phenomena are further analyzed from the differences in composition, structure and atomic diffusion kinetics. |
