Effect of Heat Treatment Parameters on the Microstructure and Mechanical Properties of Low-alloy Steel (AFNOR 15CDV6)

Abstract

This study investigates the influence of heat treatment parameters on the microstructure evolution and mechanical properties of AFNOR 15CDV6 low-alloy steel. Continuous cooling treatments were carried out starting by austenitization at 960 ˚C and then quenching in water, oil and air. Finally, tempering was performed at different temperatures from 500 to 650 with a step of 25 ˚C. Microstructural investigation was performed using a scanning electron microscope (SEM), equipped with secondary electron (SE) and backscattered electron (BSE) detectors. To identify the crystalline phases in the studied alloy after the different heat treatments, X-ray Diffraction (XRD) was performed. Mechanical characterizations of the heat-treated samples were evaluated through tensile and hardness tests. The results revealed that water quenching resulted in a predominantly martensitic structure with the highest hardness (430 HV) and ultimate tensile strength (1350 MPa), whereas air cooling produced a bainitic–pearlitic structure with improved ductility (15% elongation). Oil quenching provided a balanced combination of strength and toughness. Subsequent tempering treatments promoted carbide precipitation (VC, Mo2C, and Cr7C3 - Cr23C6), leading to secondary hardening behavior. The optimum tempering condition was obtained in the temperature range of 600–625°C, where a favourable combination of strength and ductility was achieved. The results demonstrate that careful control of cooling rate and tempering temperature enables optimization of strength–ductility balance in 15CDV6 steel.