Approach to Describe the Cooling Behavior at Geometric Notches of Rotationally Symmetric Machine Elements by Means of Thermal Form Factors

Abstract Heat treatments are generally used to adjust the required material properties at certain positions in the component, particularly at geometric notches where high stress concentrations are present due to the geometric form factor. During the hardening of steel components in evaporating liquid media, the quenching process can lead to significant variations in local material properties due to inhomogeneous cooling behavior. The geometry of the component – size, shape (e. g., geometrical notches), and surface roughness – directly impacts the cooling behavior. Geometric notches, in particular, change the surface area to volume ratio, which affects local cooling rates and must be assessed for design considerations. It is crucial to compare local stress at the notch base to relevant material standards to ensure component safety under specified loads. Following the definition of the geometric form factors, a thermal form factor model is developed in the present work, which describes the cooling in the notch base. The thermal form factor relates the t 8/5 -times in the notch base to the t 8/5 -times at the unnotched surface, giving insight into locally expected material properties. Experimental investigations on austenitic steel, unnotched and notched cylindrical samples are presented which show the influence of the notch during quenching in oil.