Non-equilibrium grain boundary segregation of boron in austenitic stainless steel - II. Fine scale segregation behaviour

A combination of TEM, FIM, AP and IAP has been used to study boron grain boundary segregation in austenitic stainless steels of the types 316L (with 40 ppm or with <1ppm boron) and ''Mo-free 316L'' (23 ppm boron). High resolution segregation profiles were determined for cooling rates from 0.29 to 5300C/s for three starting temperatures: 800, 1075 and 12500C. Boron grain boundary segregation was found after all heat treatments. The segregation behaviour was mainly of the non-equilibrium type after cooling from 1075 to 12500C whereas equilibrium segregation dominated after rapid cooling from 8000C. The influence of the relative grain orientation on the amount of non-equilibrium segregation was small for general boundaries. However, no segregation was detected at coherent twin boundaries. The binding energy of boron to austenite grain boundaries was estimated at 0.65 +- 0.4eV for both types of steels. The influence of the composition and boron content of the steels on the segregation behaviour is discussed and the experimental techniques used are presented