Effects of Hydrogen and Tempering Temperature on Mechanical Properties and Fracture Morphology of SAE 4135 Steel

Abstract

A high strength low-alloy SAE 4135 Steel was quenched and tempered over a temperature range of 385°C to 635°C. Tensile specimens corresponding to all tempering temperatures were tested in air and in an acid solution (0.1 N H2SO4). Both the yield strength and tensile strength decrease at higher tempering temperatures whereas the ductility increases. The effect of hydrogen on the material ductility is quite considerable. When the tempering temperature rises or the yield strength decreases; the number of crack initiation sites at the surface increases; the aspect of these cracks changes from intergranular to quasi-cleavage and microtearing; the intergranular propagation zones disappear; the size of the slow ductile crack propagation with crack-arrest lines increases, and the shear fracture at 45° becomes predominant.