Isothermal embrittlement of Fe-8Mn alloys at 450°C

Two Fe-8Mn alloys, one of which is alloy 193, stabilised with 0.17%Ti and 0.18%Al, were austenitised at 900 degrees C, ice brine quenched and their DBTTs determined. In this condition, brittle fractures were predominantly cleavage, and thermodynamic calculations on alloy 193 showed that there were 0.0025 wt-%C and <0.03 ppm N in solid solution. Alloys were tempered for 6 min, 1 h and 10 h at 450 degrees C and their DBTTs again determined; in this case, brittle fractures were mainly intergranular. In alloy 193, DBTT rose from 27 to 125 degrees C in 6 min. Hardness values at 450 degrees C were also monitored and the variation of hardness with time is discussed. It is thought that brittle fracture in alloy 193 is due to segregation of Mn per se to prior austenite grain boundaries, unlike an earlier investigation of a pure Fe-8Mn alloy (K1525), where embrittlement was due to a Mn-N and to a lesser extent a Mn-P interaction at prior austenite grain boundaries. The driving force for Mn segregation to prior austenite grain boundaries is thought to be the initial formation of reverted austenite at such sites.