Effect of machined surface condition on fatigue strength of Ni based superalloy Alloy 718

Abstract

The effect of processed surface condition on low cycle fatigue life of Ni based super alloy, Alloy 718, was investigated. Observation of turning surface indicates that the roughness is fairly small, i.e. Ra < 1 μm, however, many scratch are found. Compressive residual stress is measured on the surface. Specimen subsurface condition consists three characteristics including fine grained and plastically deformed layers and bulk material. Fine grained layer is divided into three layers. Specimens were prepared by electropolishing in order to remove the fine grained and plastically deformed layers. Low cycle fatigue tests were performed on these specimens. Under strain range Δε = 0.8 %, fatigue lives of the specimens with turned surface are longer than those of the electropolished specimens. On the contrary, the surface conditions have no effect on fatigue lives under Δε = 1.4 %. Fatigue cracks initiate from surface scratches and inclusions on the turned surface, while they are initiated from slip band for electropolished specimens. Under the test conditions in this study, residual stress has the strongest correlation with the low cycle fatigue lives affected by the surface conditions. To consider scratch, residual stress and microstructures are important to decide low cycle fatigue design and machining method.

Keywords

• low cycle fatigue
• crack propagation
• crack initiation
• surface condition
• ni based super ally
• residual stress
• machining