Embrittlement of ferritic stainless steels

Abstract

The mechanical properties and microstructures of commercial 11 to 29 pct Cr ferritic steels were examined as functions of aging times to 1000 h at 371, 482, and 593°C. Of the properties evaluated, changes in impact transition temperatures were the best measure of embrittlement. Embrittlement at 482°C occurs most rapidly in the 29 pct Cr alloy and somewhat more slowly in the stabilized 26 pct Cr alloy. The stabilized 18 pct Cr alloy embrittles much more slowly while little, if any, embrittlement was detected in a stabilizedll pct Cr alloy. Embrittlement at 482°C was characterized by a rapid change in properties followed by a plateau region and then further property changes. The early property change is attributed to precipitation of interstitial compounds and the later change to classic 475°C embrittlement. The onset of 475°C embrittlement in the two highest Cr alloys was accompanied by clustering of Cr atoms along {100} planes indicative of spinodal decomposition. Concurrent with clustering there was also a change from turbulent slip to a more planar slip along {110} planes. Some embrittlement was observed after longer exposures at 371°C which was attributed to a combination of 475°C embrittlement and the precipitation of interstitial compounds. Two of the alloys also embrittled at 593°C, accompanied by optically observable precipitates. The precipitate in the stabilized 18 pct Cr alloy was identified as Laves (Fe₂Ti) phase. One of the precipitates in the 29 pct Cr alloy was identified as sigma phase.