Influence of thermo-mechanical treatments on recrystallization behaviour in the MYRRHA cladding material DIN1.4970

DIN 1.4970 austenitic steel is a candidate cladding material for MYRRHA at SCK•CEN. In this research the static recrystallization behaviour of 24% and 46% cold-worked tubes of this steel was investigated, at different heating temperatures and times. The volume fraction of recrystallized grains in function of time was measured for annealing temperatures 800 °C , 850 °C, 900 °C, 950 °C and 1000 °C. The 46% cold-worked samples recrystallized faster than the 24% cold-worked samples. At higher temperatures recrystallization kinetics are significantly increased. The hardness of all the samples in the axial and transversal direction was measured and linked with the recrystallization behaviour. The hardness of 46% cold-worked samples was higher due to the higher cold-work level. The presence of recrystallization could reliably be detected with hardness measurements, but a direct correlation with the volume fraction of recrystallization proved difficult to establish. The evolution of the grain size and the number of annealing twins were also investigated. More annealing twins were found in the 46% cold-worked material as compared to the 24% cold-worked. In both materials, there were less annealing twins for higher heat treatment temperatures. Texture development was also investigated. The texture of non-recrystallized 24% samples differed from that observed in 46% cold-worked samples. Pole figures of the non-recrystallized 46% cold-worked samples showed some fibre texture. There were also differences in recrystallization texture of the recrystallized 24% and 46% cold-worked samples.