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Abstract It is well known the effectiveness of ausforming on increasing the dislocation density of fresh martensite. This work demonstrates, using atom probe tomography, that this increase in the dislocation density has a strong influence on the nucleation of nanoprecipitates during tempering. Therefore, ausforming might be a powerful processing route for the two-fold improvement of ferritic/martensitic steels where creep resistance is of paramount importance. Firstly, ausforming refines the martensitic microstructure (high strength and toughness), and secondly, it increases the number density of thermally stable nanoprecipitates (creep resistance). Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Small Punch Creep technique was used as a screening procedure to evaluate the creep properties of different microstructures developed in a thermomechanical simulator. The goal seek was to generate alternative microstructures in a conventional ferritic-martensitic G91 steel grade which boost thermal stability at temperatures as high as 700 °C. The developed microstructures allow studying the effect of the austenitization temperature optimized by thermodynamic calculations and the ausforming on the creep strength and ductility. The improvement in creep strength recorded was attributable to a higher number density of MX precipitates. By contrast, these microstructures showed an important reduction in creep ductility. [ABSTRACT FROM AUTHOR]

The development of thermomechanical treatments (TMT) has a high potential for improving creep-strength in 9Cr-1Mo ferritic/martensitic steel (ASTM T/P91) to operate at temperatures beyond 600 °C. To maximize the number of nanoscale MX precipitates, an ausforming procedure has been used to increase the number of nucleation sites for precipitation inside the martensite lath. Relative to standard heat treatments (consisting of austenitization at about 1040 °C followed by tempering at about 730 °C) this processing concept has enabled achieving a microstructure containing approximately three orders of magnitude higher number density of MX precipitates having a size around four times smaller in ASTM T/P91 steel. On the other hand; this TMT has little effect on the size and number density of M23C6 particles. The optimized microstructure produced by this TMT route proposed is expected to improve the creep strength of this steel. [ABSTRACT FROM AUTHOR]