Influence of microstructure on yield stress and cleavage fracture stress at −196°C of SA 508 Class 2 pressure vessel steel

Tests have been performed to examine the yield stress and the cleavage fracture stress at −196°C of SA 508 Class 2 pressure vessel steel in a range of heat treatment conditions. The heat treatments either varied the austenitization temperature before isothermal transformation at 450°C or varied the tempering treatment after austenitizing at 900°C and transforming at 450°C. The resultant microstructures have been characterized by metallographic and fractographic examination. Austenitization at temperatures below about 1000°C produced a polygonal ferritic structure, whereas an upper bainite was produced following austenitization above this temperature. Increasing the austenitizing temperature caused first the ferrite grain size and then the bainitic packet size to increase. However, the lath width of the bainitic microstructures decreased, which is consistent with an increase in hardenability accompanying the increased prior austenite grain size. Tempering caused the precipitation of rodlike carbide particles which then spheroidized slowly with further tempering. Both yield stress and clea vage fracture stress showed the same general dependence on microstructure. They increased during the initial stages of tempering and only began to decrease significantly when carbide particle spheroidization was well advanced, i.e. after more than 5 h at 650°C. Neither the yield stress nor the cleavage fracture stress decreased systematically with increasing ferrite grain size or bainitic packet size in as-transformed material. The lowest values occurred after austenitization at 1050°C, which was the lowest austenitizing temperature to give a fully bainitic microstructure. A general correlation existed between cleavage fracture stress and yield stress. The form of this correlation could be predicted from the model proposed by Cottrell for cleavage fracture in which dislocations react to generate a crack nucleus. It is concluded that cleavage fracture of Mn-Mo-Ni steels proceeds by a micromechanism which involves dislocation interaction.