COOLING INDUCED SEGREGATION OF IMPURITY ELEMENTS TO GRAIN-BOUNDARIES IN FE-3 WT-PERCENT-NI ALLOYS, 2-1 4 WT-PERCENT-CR-1 WT-PERCENT-MO STEEL AND SUBMERGED-ARC WELD METAL/

Previous theoretical and experimental studies demonstrate that small b ulk concentrations of impurity and solute elements can segregate to gr ain boundaries in ferritic steels during cooling from high temperature s. This results from solute-vacancy pair formation and their subsequen t diffusion to the grain boundary sinks. The grain boundary segregatio n which results from cooling at three different rates from fixed tempe ratures of 1273 and 1323 K respectively, have been measured on ferriti c Fe-3 wt%Ni alloys and 2 1/4 wt%Cr-1 wt%Mo steels containing addition s of either P or Sn. In addition, a C Mn submerged arc weld metal subj ect to a complex thermal cycle has been investigated. The composition of the grain boundaries have been measured on thin foil specimens usin g both conventional and high resolution STEM-EDS X-ray microanalysis t echniques. Segregations of both P and Sn have been observed in the Fe- 3 wt%Ni alloys and 2 1/4 wt%Cr-1 wt%Mo steels and P segregations in th e weld metal. The measured grain boundary segregations of both P and S n are discussed with respect to previous theoretical predictions for t he cooling rates investigated and the interactive role of other elemen ts present.