International Journal of Engineering and Information Systems (IJEAIS)   Year: 2021 | Volume: 5 | Issue: 2 | Page No.: 17-25 Study of the Influence of Heat Treating Temperatures on the Fracture Properties of the Ultrahigh-strength low-alloy Steel Type 4140 Meshref A. A., Mazen A. A., El-Giushi M. A. and Ashraf T.M. Abstract: The purpose of this work is to study the effect of heat treating temperatures on the fracture strength, fracture strain and fracture surface of the ultrahigh- strength low-alloy steel type 4140. In this work two types of heat treating are used. The first type is stress relieving at temperature 500 0C , 600 0C and 660 0C, the other type is full annealing at temperature 800 0C, 860 0C and 900 0C. Tensile testing is carried out at room temperature on type ZDN 10 t 191 VEB testing machine. The results obtained show that the fracture strength, fracture strain and fracture surface appear to depend on stress-relief and full annealing temperatures. It is found that the fracture strength decreases with increasing the stress-relief temperature. Fracture strain rises with rising stress-relief temperature. Before 860 0C, the fracture strength decreases with raising full annealing temperature. In contrast, the fracture strain increases upon full annealing up to 8600C, and then decrease with increasing full annealing temperature. The appearance of the fracture surface of tested samples is found to change with increasing stress-relief and full annealing temperatures.