Characteristic Features of Nanostructured Elements Formation During Pre-Recrystallization Heat Treatment

Abstract

In the work, the features of nanostructured elements formation during pre-recrystallization heat treatment were investigated using the methods of X-ray structural analysis and electron microscopy. It is shown that static deformation by 60…80 % and next short-term holding (1…2 min.) during pre-recrystallization heat treatment allow forming nanostructured elements in technically pure iron, non-ferrous alloys and steels. Using the example of technically pure iron, it was established that the specified treatment provides a reducing in the dislocation density due to the arrangement of the dislocation structure by the formation of networks with small-angle boundaries that are evenly distributed over the grain area. At the same time, the average size of the coherent scattering regions of after pre-recrystallization heat treatment is sharply reduced compared to the deformed state, which indicates the grinding of the substructure to submicroscopic (nanoscale) sizes. The formation features of the sub-grain structure in steels 20, У8 and 20X13 were studied. It is shown that an increase in the amount of carbon and chromium reduces the increase in hardness after pre-recrystallization heat treatment, which is associated with the formation of carbides that accumulate on sub-grain boundaries, block dislocations during plastic deformation and at the initial stage of polygonization. The low thermal stability (several minutes) of the formed crushed substructure was noted, which makes it impossible to use this method of improving properties for large sized parts. It has been proven that the combined deformation (30 % dynamic + 30 % static deformation) ensures the thermal stability of the polygonization substructure for up to 60 minutes. and form from 12 to 62 % of nanoscale elements in technically pure iron, carbon and alloy steels. A holding time of 60 minutes slightly increases the size and decreases the number of nanosized subgrains, which is explained by the increase in misorientation angles due to the beginning of dynamic recrystallization processes, but this is enough to ensure high hardness.