This paper provides details of an ongoing effort to characterise the microstructure of heavy section low alloy steel forgings used in high integrity applications and correlate these data to the mechanical properties of these materials. Current industry practice is to use chemical etching and visual assessment in acceptance standards to determine nominal averages for microstructure parameters. This work uses electron microscope techniques to provide information on a variety of grain and secondary-phase particle information allowing numerical comparison of key microstructure variables to mechanical properties. For instance, the transition toughness behaviour of low alloys steels is controlled by the combination of the largest grain and particle in the material, i.e. the most potent initiator for cleavage failure. Knowledge of only the average grain size is insufficient to correlate microstructure and transition toughness performance. The programme consists of three main stages: modelling key variables in the manufacturing process to predict microstructure from thermodynamic predictions, developing quantitative microstructure data on archive materials for which mechanical property data are known to allow empirical relationships to be constructed and, a final validation exercise of a complete model by production and assessment of trial forgings.
Rolling and Heat Treatments of Heavy Section Low Alloy Steel Plates