Quantitative microstructural characterization of nuclear grade bainitic steel: Influence of macrosegregations in full-scale forgings

Abstract

Large forgings used for the fabrication of nuclear reactor components often present chemical macrosegregations as a result of the solidification processes in the initial ingot. This paper presents an extensive microstructural characterization of both the macrosegregated area (up to 0.28 wt% of carbon) and a reference non-segregated area (0.18 wt% of carbon) from 16MND5 steel full-scale large forgings after the final heat treatments and tempering. Inside the macrosegregated areas, significant microhardness variations are observed, corresponding to millimetric microsegregations observed by optical microscopy. At a finer scale, the microstructures of the reference non-segregated area and the macrosegregated area are quite similar, as revealed by Scanning Electron Microscope (SEM) and Electron BackScattered Diffraction (EBSD). In both cases, the microstructure is mostly made up of granular bainite and upper bainite. Various types of precipitates were observed by either SEM or Transmission Electron Microscopy (TEM), such as Fe(Cr,Mn)3C, Mo2C, AlN and AlMnSiN precipitates. The main difference between unsegregated and segregated areas is the significantly higher precipitate number density in the segregated area, as a result of the increase in C, Cr, Mn and Mo content, which can explain the lowered fracture toughness in these areas. Thermodynamic calculations confirm that Cr and Mn enrich cementite precipitates after tempering.