EFECTO DE LA DEFORMACIÓN EN FRIO SOBRE LAS PROPIEDADES MECÁNICAS DE LOS ACEROS: DIN 32CrMoV12 10, AISI 4340 Y AISI 8620.

Abstract

Automotive and military industries requires rods and tubes of low alloys steel obtained by cold deformation processes by the radial forging technique.These steels must withstand high stresses due to a cyclic heating generated by the impacts, especially in the direction perpendicular to the rolling direction. The radial forging process requires a material of homogeneous structure so that by applying a certain percentage of cold work, they obtain the desired final steel properties such as microstructure, tensile strength, impact strength and ductility. However, it is normal that steels present the phenomenon of banding (alternating longitudinal bands of pearlite and ferrite), because during the solidification, segregation is generated, which is elongated in the longitudinal direction by rolling processes, obtaining steels with anisotropic behavior. In this work we study the effect of three cold deformation rates (17.45, 33.30 and 47.46%) through the radial forging process of three commercial low alloy steels (DIN 32CrMoV12- 10, AISI 4340 and AISI 8620) on their mechanical properties of hardness, tensile strength and impact. A heat treatment was carried out to eliminate the banding. Quenching and tempering were also carried out in order to find the combination of cold deformation microstructure and to obtain the best mechanical properties for the manufacture of highstrength tubes. The results indicate that steels DIN 32CrMoV12-10 and AISI 8620, had problems with banding, which was eliminated with heat treatment at a temperature of 1300 °C for three hours and cooling inside the furnace. However, the grain size grew, so a grain refining heat treatment prior to quenching and tempering was required. According to the norm DIN 50602, DIN steel 32CrMoV12-10 showed the lowest level of inclusions. It was determined that the cold deformation affects the entire mass of material. The initial characterization of steels indicates that the maximum resistance to stress is similar for steels DIN 32CrMoV12-10 and AISI 4340, but the impact strength is two times higher in the steel DIN. Increasing the cold work, the UTS increased and the ductility decreased for the three steels. The impact resistance was increased by 18% steel DIN 32CrMoV12-10 by increasing the deformation. viii The heat treatment and tempering at 600 ° C gave the best relationship between impact strength and yield strength for the steel AISI 4340, improving the mechanical properties of tensile strength, yield strength and impact resistance as well as ductility, with respect to the mechanical properties with which it was purchased. The effect of grain size on the impact test was studied by using the impact test of AISI 8620 steel with heat treatment homogeneity, with and without grain refinement. The best relationship between the properties of impact strength, yield strength and ductility of AISI 8620 steel was obtained with the heat treating and tempering at 450 ° C. Likewise, we identified a significant improvement in mechanical properties with tempering heat treatment at 450 ° C compared to those obtained with the tempering performed at 400 ° C. Steel 32CrMoV12-10 had the highest impact resistance due to the presence of V and Mo that improve.