Effects of finishing processes on the fatigue life improvements of electro-machined surfaces of tool steel

• Published in: International journal of advanced manufacturing technology Vol. 52; no. 5-8; pp. 583 - 595
• Main Authors: Ghanem, Farhat, Fredj, Nabil Ben, Sidhom, Habib, Braham, Chedly
• Format: Journal Article
• Language: English
• Published: London Springer-Verlag 01.02.2011; Springer Nature B.V; Springer Verlag
• Subjects: Brushing; CAE) and Design; Compressive properties; Computer-Aided Engineering (CAD; Control stability; Crack propagation; Cyclic loads; Engineering; Engineering Sciences; Fatigue cracks; Fatigue life; Fatigue tests; Free surfaces; High cycle fatigue; Industrial and Production Engineering; Martensitic stainless steels; Materials; Mechanical Engineering; Mechanical properties; Mechanics; Media Management; Milling (machining); Networks; Original Article; Residual stress; Stress propagation; Tool steels; Wire brushing; Fatigue; Polishing; Wire brushing; Crack networks; Residual stress; EDM
• DOI: 10.1007/s00170-010-2751-y
• ISSN: 0268-3768

Machining the EN X160CrMoV12 tool steel by electro-discharge machining (EDM) process generates significant modifications of microgeometrical, microstructural and mechanical properties of the upper layers of the machined components. In this paper, the role of these modifications in controlling the stability, under cyclic loading, of the propagation of the crack networks generated by EDM is discussed. High cycle fatigue tests (2 × 10 6 cycles) show that the presence of these cracks in brittle layers, i.e. white layer, quenched the martensitic layer, and a field of tensile residual stresses (+750 MPa) results in a loss of 34% of endurance limit comparatively with the endurance evaluated for the milled state that generates crack-free surfaces. It is shown, in this work, that the detrimental effect of these crack networks can be controlled by putting in compression the upper layers of the EDM surfaces. Indeed the application of wire brushing to EDM surfaces generates compressive residual stresses (≈−100 MPa) that stabilise the crack networks propagation and therefore restores to the EDM surfaces their endurance limit value corresponding to the milled state. Moreover, removing the crack networks by polishing generates a stabilised residual stress value of ≈−130 MPa. This results in an improvement rate of about 70% of the endurance limit comparatively with the EDM state and of 26% in comparison to the milled state. These rates could be further increased by the application of the wire brushing process to the polished surfaces that reached 75% and 30% comparatively to the EDM and milling states respectively. In this case, a stabilised surface residual stress of about ≈−150 MPa was measured on the specimen surfaces.