The paper describes a method to produce a nanocrystalline structure in medium-alloyed, commercial steels by use of specially designed heat treatment process. Conventional heat treatment processes of steel consisting on quenching and tempering do not permit reducing the grain size below 300nm of phases formed. It was however shown, that a nanocrystalline structure consisting of grains with thickness below 100nm may be formed in steels with specific chemical composition as a result of isothermal bainitic transformation. To obtain a nanobainitic structure it is necessary to conduct heat treatment process in precisely defined conditions. These are defined on the basis of knowledge of the kinetics and critical points of phase transformation occurring in steel. The general scheme of the heat treatment consisted on austenitizing followed by quenching to the a temperature lying in the lower zone of bainite transformation and isothermal holding at this temperature for a time allowing completion the bainite transformation.
Four kinds of commercial steels have been chosen for investigations: bearing steel, spring steel, hot work tool steel and structural steel. In order to establish the parameters of austempering heat treatment, the processes of phases transformation occurring in selected steels have been investigated by means of dilatometric measurements. The determined kinetics and critical points of phase transformations were used to establish the proper parameters of austempering treatment that would lead to a nanobainitic structure. The structure of steels after austempering treatment was studied by light microscopy and transmission electron microscopy. It was shown, the austempering leaded to the formation of carbide free nanobainitic structure in bearing steel, spring steel and hot work tool steel. This structure was composed of plates of bainitic ferrite with thickness below 100nm embedded in retained austenite matrix. The thickness of retained austenite films separating the plates of bainitic ferrite was below 50nm. The obtained structure can be thus considered as a nano-composite structure. The structural steel after austempering heat treatment exhibited a sub-micrometric structure of lower bainite containing about 11.3% of retained austenite , mostly in form of blocks. This microstructure results from insufficient carbon content 0.35%wt. in this steel. In order to increase the carbon content steel samples were subjected to the vacuum carburizing process. As a result the carbon content was doubled in the surface layer of structural steel. The austempering process leaded in this layer to a nano-composite structure similar to the structure of other investigated steels. The mechanical properties of steels with nano-composite structure were determined by hardness measurements, tensile tests and Charpy impact tests. It was shown, that nano-composite steels exhibited high value of strength parameters combined with relatively high ductility. Moreover it was shown, that the carburized layers in structural steels exhibited higher wear resistance after austempering treatment than after the conventional quenching and tempering process commonly applied in carburized steels.
The technological limitations associated with the production of steels with nanocrystalline structure have been described and the possibilities of eliminating these restrictions have been analysed.
The study was accomplished within the Structural Project “Nanocrystalline structure formation in steels using phase transformation” NANOSTAL (contract no. POIG 01.01.02-14-100/09). The project is co-financed by European Union from the European Regional Development Fund within Operational Programme Innovative Economy, 2007-2013. |