Superior cryogenic toughness of high-Mn austenitic steel by welding thermal cycles: The role of grain boundary evolution

The high-Mn austenitic steel is expected to improve mechanical properties by grain boundary adjustment. The role of grain boundary on cryogenic toughness of high-Mn austenitic steel subjected to various welding thermal cycle was investigated by the means of instrumented Charpy V-notch impact tests and electron backscatter diffraction (EBSD) analysis. The absorbed energy of samples at −196 °C decreased from 203 J to 163 J with the increase of peak temperature from 600 °C to 900 °C. When the peak temperature increased to 1000 °C, 1100 °C and 1300 °C, the absorbed energy of sample increased to 185 J, 204 J and 196 J, respectively. Phase transformation and abnormal grain growth were absent in the samples. The lowest cryogenic toughness in sample with the peak temperature of 900 °C was thus attributed to the highest proportion of the special grain boundary ∑3. Furthermore, during the cryogenic impact test, the crack formation energy was corresponded to the proportion change of the low angle boundary. While, the crack propagation energy was closely related to the proportion change of special grain boundary of ∑3 rather than the high fraction of ∑3 by welding thermal cycle. It is proposed that the increasing low angle boundary of 2–5° and the activation ∑3 special grain boundary are responsible for the superior cryogenic toughness for high-Mn austenitic steel.