Unique fracture surface in fatigue of shot peened maraging steel

Abstract

In order to investigate the effect of microstructure in the surface layer produced by shot peening on fatigue properties of a maraging steel, rotating bending fatigue tests were carried out using shot-peened specimens and electro-polished ones of a 350 grade 18% Ni maraging steel in the relative humidity of 25% and 85%. The shot-peened specimen tested had a fine-grained structure and a small amount of reversion austenite in the work affected layer. The shot-peened specimens showed a unique fatigue fracture surface with a ring-shaped fracture one along the surface layer in the wide range of fatigue lives. The formation mechanism of the unique fracture was explained from the coalescence of many surface cracks over the entire circumference initiated from dimples formed by shot peening at the specimen surface and the difference in the microstructure between the fine-grained surface layer and the matrix. Fatigue strength in the shot-peened specimen increased due to the grain refinement in addition to hardening and formation of compressive residual stress at the surface layer. The fish eye size, i.e., the internal crack length, increased and the depth of a surface crack inversely decreased with increasing in the fatigue life. The value of stress intensity factor obtained from the fish eye size increased with increasing in the fatigue life and approached to the one of the fatigue fracture toughness of the maraging steel in longer life beyond 107 cycles. This suggested that the fracture mode in the shot peened specimens was changed in the order of a fracture controlled by a propagation of the surface crack, a combined fracture due to propagation and coalescence of a surface crack and an internal one, and an internal fracture induced by an internal crack propagation with decreasing in the stress level.

Keywords

• fatigue
• maraging steel
• shot peening
• grain refinement
• surface crack
• internal crack
• crack initiation
• crack propagation