The effect of oil quench severity on the properties of hardened SAE 4140 steel

“This project addresses the need for a correlation between quench severity and physical properties of steels by measuring the bending fatigue performance and fracture toughness of SAE 4140 quenched in oil at three levels of agitation. Thermal anemometry was utilized to measure quench severity. Analytical techniques including Mössbauer spectroscopy, x-ray diffraction, and scanning electron microscopy were used to characterize microstructure and residual stress. Four-point bending fatigue testing showed a correlation between quench severity and fatigue performance, indicating that increasing quench severity lowers fatigue life for this material and specimen. Surface residual stress was found to be tensile in nature, and to increase with increasing quench severity from 320 MPa for the low quench severity to 460 MPa at the high quench severity. Microstructure was not affected by quench severity. Retained austenite measurements were unchanging, as well, with values reported by Mössbauer spectroscopy of 6.5 % to 7.0 % by volume. The addition of measured residual stresses to the four-point bending fatigue results eliminated the trend with quench severity and supported the conclusion that increasing oil quench severity lowers the fatigue life of this specimen by increasing the surface tensile residual stress”--Abstract, page iii