Effect of Decarburization Heat Treatment and Chromium Addition on Intergranular Embrittlement of SA-106 Carbon Steel

Abstract

Effect of decarburization heat treatment and chromium addition on intergranular embrittlement of SA-106 carbon steel was investigated by scanning Auger electron spectroscopy analysis on fracture surface. Optical microscopy, scanning electron microscopy, transmission electron microscopy with energy dispersive spectroscopy analyzer and tensile test were performed to clarify the change in microstructure and mechanical properties due to decarburization heat treatment and 0.3–0.4 wt% chromium addition. The experimental results showed that decarburization heat treatment reduced the bulk carbon contents up to 40 mass ppm and caused the low temperature fracture mode to change from cleavage to intergranular under ultra high vacuum. It was also found that the decarburization heat treatment and chromium changed the shape and size of cementite in pearlite and formed the stable Cr-rich precipitates in the grain boundary which had an influence on the phosphorus segregation in the grain boundary. The tensile properties of SA-106 steels also changed due to the decarburization and chromium addition. Considering the direct and indirect effect of residual tensile stress imposed on SA-106 feeder pipes in heavy water reactor during manufacturing and installation, cautions should be taken in the effect of increased phosphorus segregation due to such various causes as tensile stress, decarburization heat treatment and chromium addition on the intergranular embrittlement of SA-106 feeder pipes in heavy water reactors.