THERMODYNAMIC CALCULATION AND ANALYSIS ON EQUILIBRIUM PHASE IN LOW ALLOY STEEL FOR REACTOR PRESSURE VESSEL

In this paper, the thermodynamic equilibrium phase of the 508-4N steel is introduced by using the Thermo-calc Software. The effect of the content of alloy elements, such as carbon, Cr, Mo, on the precipitated phases of the 508-4N steel under different temperatures (300°C, 343°C, and 450°C) was studied. The result shows that the main precipitated phases are M23C6 and M7C3 in thermodynamic equilibrium condition. As for M23C6, it’s quantity reaches a maximum value and then remains stable as the increase of the carbon, Cr and Mo, but the temperature shows a less influence on that. The M7C3 increases linearly, and then linearly decreases after reaching the maximum value, with the increase of carbon, Cr and Mo. However, the temperature shows a different characteristic. The whole curve, of the relationship between Cr content and M7C3 phase precipitation, shifts to the right when temperature rises. While the opposite behavior can be observed for Mo content and M7C3 phase precipitation. The calculation results can be used to provide a theoretical guidance estimating the influence of precipitated phases on the properties in long-term service conditions.