Papers by Author: Stanislaw Zajac

Abstract: This paper provides a detailed description of complex bainitic microstructures obtained during the recent development of low carbon linepipe steels with strengths in the range of X100 to X120. New experimental techniques based on a high resolution FEG-SEM and EBSD have been used to characterise and quantify the mixture of ultrafine bainitic ferrite and nanosize second phases in these steels. It was found that the occurrence of incomplete transformation generates new, previously unexplored bainitic microstructures with a wealth of microstructural features that is beyond classification based on conventional concepts. Clear differences in distributions of boundary misorientations and effective grain size were noted between upper, lower and granular bainites. Based on these results a new classification scheme and definition of bainite is proposed.

Abstract: A brief summary is given of the desired effects of precipitation of microalloy nitrides and carbides in austenite and ferrite prior, during and after g−a transformation. Precipitation of microalloy compounds of Nb(C,N), TiC and V(C,N) is discussed in relation to several grain refinement and precipitation strengthening mechanisms. An improved understanding of the thermodynamics and kinetics of precipitation and phase transformations is presented using the approach based on the chemical driving force. Nucleation of intragranular polygonal ferrite on VN particles that grow in austenite and the formation of acicular ferrite inside the austenitic grains in Vsteels is described. The role of carbon in increasing the driving force for nucleation is elucidated and the benefits of using microalloy carbo-nitrides for precipitation strengthening of bainitic steels are reviewed. An expanded view on the role of microalloy carbo-nitrides in grain refinement and precipitation strengthening is presented.