Phase Transformation Behaviors And Welding Of The Modified High Cr Ferritic Heat-resistant Steel

High Cr ferritic heat-resistant steels are widely used in the keycomponents of the super/ultra-supercritical advanced power plants. Withthe increasingly prominent energy and environmental issues, the study onelevating the heat-resistant temperature of high Cr ferriticheat-resistant steel becomes imperative. In this paper, for the modifiedhigh Cr ferritic heat-resistant steel developed by our group, its phasetransformation behaviors during quenching and austenization process, theweldability were systematically analyzed, respectively. Furthermore, themicrostructure evolutions and precipitation processes during isothermalaging were also studied. The following conclusions could be drawn basedon the experimental observation and theoretical analysis.(1) Phase transformation: firstly, the microstructure ofaustenitization at the intercritical temperature was composed ofaustenite and ferrite dual-phase structures. At full austenizationtemperature, it was found that the dissolution of M23C6precipitates waswithin the range of950～1000℃. The delta ferrite gradually starts toform after1050℃. The experimental data were fitted by the phasetransformation model. The results indicate that: The activation energyfor growth of austenite grain is58.7kJ/mol; both the activation energyof growth and the interface velocity of martensite plates decreases withthe increase of austenization temperature. Secondly, only the martensitictransformation was found during quenching at various temperatures; thedelta ferrite and a small amount of residual austenite is also found inthe final microstructure. With decreasing quenching temperature, it isfound that both the starting and finishing temperatures of martensitictransformation increase, as the same as the size and amount of theprecipitates particles, whereas the delta ferrite content decreased. Whenquenching at700℃, retained austenite between martensite laths disappearcompletely. The results of calculation based on spread model indicatethat the aspect ratio of martensitic lath increases with the decrease ofquenching temperature, and the nucleation rate decreases.(2) Welding: first, during welding heat cycles, only the delta ferrite phase exists at the peak temperature,1320℃, which can not completelytransform to austenite. Amount of the residual delta ferrite is left atroom temperature. It is also found that the Boron-rich M23C6phaseprecipitates in the delta ferrite. Based on the grain boundaries sitesaturated nucleation theory, assuming nitrogen diffusion-controlledaustenite transformation, the results of model demonstrates that nitrogenis the main alloy elements to control the phase transformation duringwelding. Second, based on analyzed the welded joints, it is found thatthe location of M23C6precipitated affectes the recovery andrecrystallization of microstructures. More than70%M23C6phase isprecipitated at the grain boundaries of austenite and the lath boundaries.Annealing time is the significant factor for the size of martensite lathduring post weld heat treatment.(3) During isothermal aging, with time prolonging, recovery occursin the martensite lath, which leads to the decrease of dislocationdensities, the formation of equiaxed grains, and the change ofprecipitates shape at grain boundaries. The alloying elements around M23C6due to diffusion and the formation of dislocation network cells promotethe formation of Laves precipitates. The Mo-rich Laves phase forms in thewelded joints. For the high alloying elements contents and kinds of themodified high Cr ferritic heat resistant steel, it is easier to form Lavesphase which is prior located around M23C6precipitates and rapidly growsup.