Effect of cooling rate on the microstructures of three low carbon alloys with different manganese and molybdenum contents

Low-alloy 16 to 20MND5 steels are used for the production of nuclear reactor components. During manufacturing, austenitization is followed by a quench; different types of microstructures are formed during this step. Characterizing the impact of Mo and Mn and of cooling rate on these microstructures can help understand how mechanical properties will evolve during tempering and ageing. The impact of molybdenum and manganese, as well as the impact of the cooling rate, were studied on microstructures of three model alloys: FeCMo, FeCMn and FeCMoMn. This was done using continuous cooling transformation (CCT) diagrams and electron backscattering diffraction (EBSD) characterizations. FeCMoMn was found to be a good model for 16 to 20MND5 steels, based on its CCT diagram and hardness. The presence of molybdenum or manganese did not modify the misorientation angle/axis pairs of martensite. In bainitic microstructures however, the presence of Mn seemed to favor the presence of block boundaries with a misorientation about 59 degrees [433]. On the prior austenitic grain (PAG) level, the impact of the cooling rate was rather continuous, from martensite to slowly cooled bainite, and the same regardless of the composition, with the presence of block and sub-block boundaries. The microstructure became coarser with decreasing cooling rate, with fewer crystallographic orientations per PAG.