Formation of a Gradient-Layered Structure during Thermal Deformation Treatment of Reinforced Steel

Abstract

In this study, we consider the mechanism and kinetics of structure formation in the hardened zone during thermal deformation treatment of reinforced steel. Depending on the cooling rate and temperature conditions of austenite decomposition, pearlite and martensitic transformations are shown to occur with the formation of a gradient-layered structure, leading to structure modification of the surface layer of steel at a constant chemical composition, structure, and properties of the central layers of the workpiece. A high cooling rate due to a large temperature gradient near the surface is the reason for the formation of a finely dispersed layered structure. A diffusion-free martensitic transformation develops in the surface zone, leading to the formation of acicular martensite. In the underlying layers, the decomposition of austenite proceeds by diffusion and is accompanied by the formation of a lamellar ferrite–carbide mixture of varying dispersion degrees. An increase in the cooling rate leads to a strong refinement of the structure (grain point according to GOST 5639–82 is 11 in the surface layer and 8 in the core) characterized by an increase in the dispersion degree of the ferrite–carbide mixture, which causes an increase in strength and reduction of the plastic characteristics of steel. It is noted that the formation of a gradient-layered structure in the surface layer of strain-hardened reinforced steel allows excluding a sharp transition from the martensite structure to troosto-martensitic and mixed pearlite structures. This increases the contact-fatigue strength of reinforced steel and its crack resistance.