Shot Peening or Ceramic Thermal Spraying for Controlling the Delayed Fracture

Abstract

Authors studied how we could control the delayed fracture of three point bent spring steel using shot peening or thermal spraying of oxide ceramic. We first introduce properties of the shot peened spring steel and then explore the effect of hydrogen-supplement method （hydrogen charge） on the susceptibility of delayed fracture. Next we discuss threshold strains by Conventional Immersion Test （called as CIT method） of bent specimens. Further, we propose an alternate evaluation method using acoustic emission （AE）. Here we determine the threshold strain by measuring AEs during Step-wise Strain Increasing （SSI-AE method） of hydrogen pre-charged specimens. We introduce two cases where the shot peening can or can’t control the delayed fracture, depending on both the hydrogen charge method and the strength level of the steel. We also discuss problems and advantages of the CIT and SSI-AE methods. We studied a method for controlling the diffusion rate of hydrogen into the steel using thermal spraying of conductive titania, and demonstrated how this method was effective for improving the threshold strain for the system where the shot peening could not be an effective countermeasure against the delayed fracture.