Hot Deformation Behavior Of As-cast 12%Cr Ultra-super-critical Rotor Steel

For the purpose of improving the thermal efficiency of coal-fired power plants, reducing the CO2 emission as well as fulfilling the requirement of protecting the environment and saving energy sources, many developed countries are vigorously developing ultra-super-critical(USC) generator sets with high capacity and high efficiency. Recently 12%Cr martensitic stainless steel has been widely used as ultra–super–critical generator sets due to its excellent properties. However, as many kinds of alloying elements exist in the steel, the hot workability is generally low. There are usually many problems, such as coarse grains, mixed grain structures, and surface cracking in the large rotor. This research tracked back to the casting stage(the initial stage in manufacturing the rotor) in order to solve these problems. Ingots with different initial grain structures were obtained by the medium-frequency induction furnace. Various casting parameters were taken into consideration, such as the preheating temperature, the superheating temperature and the pouring temperature. Then the influence of processing parameters(temperature and strain rate) and initial as-cast grain structures(growth direction-columnar grains, radial direction-columnar grains, and equiaxed grains) on the deformation behavior of 12%Cr steel were investigated through compression test by using Gleeble-3800 thermal-mechanical simulator. Meanwhile, microstructure evolution was studied by using optical microscopy(OM), scanning electron microscope(SEM), and electron back scattering diffraction(EBSD). Based on the dynamic material model(DMM), the processing mapof 12% Cr steel was established. Also the dynamic recrystallization grain size model was built to systemically study the effect of initial microstructure and process conditions on the hot deformation behavior and microstructure evolution rule of 12% Cr steel. The main results are as follows:1) The hot deformation activation energy of the as-cast 12%Cr steel is 477.73 k J/mol, and the hot deformation equation is established with linear regression method.2) The dynamic recrystallization grain size model was built. It reveals the relationship between the dynamic recrystallization(DRX) grain size and the deformation conditions(temperature, strain rate). Both the DRX grain size and the fraction of DRX are increased while the temperature increasing and the strain rate decreasing.3) The processing map was built up based on DMM model. The safe processing window was suggested with a temperature range of 1050–1200 °C and a strain rate range of 0.001–1s-1. At a high strain rate region(10 s-1), flow instability was easy to occur, where the microstructure was in the form of the elongated–deformation bands in the columnar grains, necklace structure and cracks(cavities) in delta ferrite phase. Therefore, deformation in this region should be avoided in the hot working process.4) The initial as-cast grain structure, the content and the shape of the second phase(delta ferrite) have significant influence on hot deformation behavior and microstructure evolution of 12% Cr steel. The smaller the initial grain size is, the higher the volume fraction of dynamic recrystallization will be, then the finer the DRX grain size is. The more content of the delta ferrite it has, the much more recrystallization nucleation location points it has, thus accelerating the DRX rate. The microstructure is uniform and fine.