The heat treatment processes, which lead to the formation of homogeneous nanostructure in low and medium-alloy steels were designed within the Project “Production of nanocrystalline steels using phase transformations” which is implemented at the Warsaw University of Technology, Faculty of Materials Science and Engineering. This technology is based on a carefully designed heat treatment which allows to produce nanostructure in the finished elements made of steels that have specially selected chemical composition. The main step of that treatment is isothermal annealing within the range of temperatures in which bainite is formed. Isothermal hold temperatures have been selected to be above Ms temperature to avoid martensite transformation which causes brittleness. Heat-treatment parameters were selected on the basis of dilatometric tests and transmission electron microscopy. The microstructures obtained as a result of austempering consisted of a nano-size carbide-free bainite and retained austenite which provide high ductility. High strength, ductility and good service parameters of nano-size carbide-free bainitic steels make themcompetitive in comparison with highly alloyed martensitic steels. Within the Project another technology of heat treatment which allows to obtain nano-sized carbide-free bainite or low-temperature bainite in carburized surface layer was also developed. Such microstructure in carburized surface layers have better wear resistance in comparison to microstructure obtained by hardening and low-temperature tempering. Moreover, isothermal holding within the range of temperatures in which bainite is formed results in reduction of residual stress level and distortion in comparison to quenching and tempering.  Because of long-time isothermal holding which sometimes last several days, the endeavor to adapt austempering to  industrial production was carried out. As a result a new technology of heat treatment which allows obtain in short time microstructure composed of nano-size bainite, nano-size martensite and retained austenite was developed. This technology allows to obtain optimum mechanical properties such as high tensile strength while maintaining high ductility. Our research shows that nanocrystalline steel besides of high mechanical parameters is characterized by great performance and technological parameters.

1. PRESENTATION: Nanocrystalline structures in steels produced by heat treatment processes, Zip archive data, at least v2.0 to extract, 3MB

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1. Production of nanocrystalline structure in steels by heat treatment process
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