Effects of the variation of the concentration of aqueous polymer solution based on PVP in the quenching of AISI 4140 steel

Abstract

Quenching is a heat treatment whose main objective is to increase the hardness and mechanical strength of the steels by their microstructural transformation. The process is based on increasing the temperature of the material to austenitization, followed by cooling by immersion in a fluid, which needs to be fast enough to avoid diffusion of the carbon, aiming at the formation of a predominantly martensitic microstructure. Aque-ous polymer solutions, which are blends of water with polymer, have been used now for this procedure. The concentration of these solutions is a fundamental factor for the heat exchange capacity of the fluids and, con-sequently, the variation of this factor promotes the development of different mechanical properties in the quenched material. Thus, the present work consisted in the evaluation of the properties of quenched AISI 4140 steel in aqueous polymer solutions based on polyvinylpyrrolidone (PVP) at concentrations of 10, 15, 20 and 25%. For this, the characterization of the steel was made by the microstructural evaluation, microhard-ness and X-ray diffraction. Through these evaluations, it has been demonstrated that the heat withdrawal ca-pacity of the material is being minimized according to the addition of PVP to the solution. As a result, the decrease in the thermal exchange determined a lower martensite formation and a decrease in the microhard- ness profile of the samples, although no phase variation was observed in all the process situations.