The effect of low temperature transformation time on microstructural & textural evolution, mechanical properties and fracture behavior of a low alloy, medium carbon, super strength AISI 4340 steel

In this study, the influence of low temperature transformation time on the microstructural & textural changes, mechanical properties and fracture behavior of a two-phase AISI 4340 steel was investigated. The process was employed to achieve balanced strength, substantial ductility and superior toughness characteristics. For this purpose, samples with the dimension of 120*100*4 mm were first austenitized at 900 degrees C for 40 min followed by a rapid quenching in a salt bath that was held at 330 degrees C. They were held at this temperature from 0.5 up to 30 min. Optical microscopy (OM), scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM) equipped with electron backscattered diffraction (EBSD) detector, and transmission electron microscopy (TEM) techniques were employed to assess microstructural/textural evolution due to the application of the mentioned process. Tensile testing, Charpy impact testing and hardness testing were employed to evaluate mechanical properties variation resulting from the alteration of the austempering condition. Results showed that elongation percentage and impact energy were continuously increased with increasing the percentage of lower bainite as austempering time increased uninterruptedly. At the same time, yield strength was reduced from 1600 MPa in a sample with the maximum amount of martensite, austempered for a very short period, to 1300 MPa in a sample with 50% lower bainite. The yield strength was mildly increased up to 1350 MPa in a sample with 90% lower bainite that was austempered for 30 min. Texture analysis indicated that a randomized texture has prevailed as austempering time increased. {001} pole figure analysis showed that the Copper ({112}< 111 >) texture was the most important component present in the early stage of austempering which was substantially weakened at longer austempering time (i.e. 30 min). Fractography of the tensile/impact toughness test samples showed that the dimples of the fracture surfaces were increased with increasing the lower bainite percentage. The percentage of the dimples were 85% and 75% for the tensile/impact toughness samples with 90% lower bainite, respectively. TEM micrographs of the optimum condition, heat treated at 30 min, showed a combination of the mentioned phases. The optimum condition of tensile testing and impact energy for such microstructures were 1350 MPa for the yield strength, 1470 MPa for the ultimate tensile strength, 13.25% for the elongation percentage, and 70 J for the impact energy.