Textural development, martensite lath formation and mechanical properties variation of a super strength AISI4340 steel due to austenitization and tempering temperature changes

Abstract

In this investigation, the textural development, as well as martensite lath formation on the microstructural changes and mechanical properties variation of a super strength AISI 4340 was studied under different austenitization and tempering conditions. To study the influence of tempering temperature, austenitization temperature of 900 °C was followed by tempering in the range of 200–425 °C on the sheet samples. As well, to investigate the effect of austenitization temperature, austenitization temperature of 850–1200 °C was followed by rapid quenching and tempering at 200 °C. Microstructural investigations were conducted using optical microscopy (OM), field emission scanning electron microscopy (FE-SEM) that was equipped with the electron backscattered (EBSD) as well as energy dispersive spectroscopy (EDS) detectors, X-Ray diffraction (XRD), and transmission electron microscopy (TEM). In addition, mechanical properties were evaluated using tensile testing, macro-hardness testing, and Charpy impact testing. Results showed that increasing austenitization temperature up to 950 °C increased ductility which was followed by a sharp drop as the austenitization temperature increased thereafter. Also, it was found that increasing the austenitization temperature from 850 to 1200 °C decreased yield strength (Y.S), and ultimate tensile strength (U.T.S) for about 24%, 26%, respectively. It reduced impact energy by about 35%. Such reductions were related to the increased mean diameter of the prior austenite grain size from 2 μm for the former temperature to 110 μm in the latest temperature. EBSD analysis showed that the steel texture was mostly randomized at high austenitization temperature. (100) pole figure analysis showed that Cube ({100} 〈001〉) and Copper ({112} 〈111〉) components were developed during the optimized heat treatment of 900 °C followed by tempering at 200 °C. PF analysis also indicated that the texture intensity was weakened at lower austenitization temperatures. The presence of the Copper component at the very low austenitization temperature could explain the low ductility of this scenario. Orientation distribution function (ODF) analysis showed that the Cube component was dominant, and the Copper component severely weakened, during the optimum austenitization temperature, which would explain the optimum mechanical properties combinations, particularly ductility. Finally, the mentioned optimized route provided Y.S of 1720 MPa, U.T.S of 1900 MPa, El% of 11%, and Impact energy of 44 J, which confirmed the development of super-strength steel.