Effects of the phase transformation in alloy structural steels at heating and cooling on their microstructure and impact resistance

The effects of the revealed phase transformation in alloy steels during isothermal soaking either at heating or cooling on theirs microstructures and impact resistance were experimentally investigated. The alloy Steel 35HML in as cast state as well as Steel E36 and Steel 30HGSA both in as hot rolled state were used for the research. Grain size refinement as well as elimination of the typical micro- and meso-structural nonuniformities, formed in the just crystallized and just hot deformed steels, was reached as a result of optimal isothermal soaking during the subsequent cooling. practically full restructuring of the initial steel states is shown for Steel E36 and Steel 35HML meanwhile in Steel 30HGSA visible microstructure changes do not take place under the above mentioned heat treatment conditions. Considerable, not less than 2 fold increase in impact resistance of Steel E36 and Steel 35HML is also reached as a result of optimal heat treatment, contrary to Steel 30HGSA where the resistance is practically constant. Formation of highly homogeneous microstructures but having however increased grain size is reached in Steel E36 and Steel 35HML after the proposed heat treatment, which includes isothermal soaking during austenitizing heating. The effects observed in the investigated steels as a result of the proposed heat treatment were explained in terms of the grain and dislocation subgrain boundary influences on chemical element diffusional redistribution between the structure components and new phase nucleation process. The specific behavior of Steel 30HGSA was attributed to the silicon influence on the above processes