Metallurgical Analysis of Crack Initiation of Wire-Cut Electrical Discharge-Machined Spline Actuators Made of 17-4 PH Stainless Steel

摘要

Spline actuators made of investment cast 17-4 PH (precipitation hardening) stainless steel were found to contain micro-cracks. The cracked actuators were subjected to optical and scanning electron microscopy and hardness testing, which revealed that the failure occurred due to fatigue crack initiation and growth after electrical discharge machining (EDM). The rehardened layer produced by the EDM remained after machining, and the cracks and surface irregularities associated with this layer provided sites for crack initiation and growth, which ultimately caused rejection of parts. Close dimensional tolerances on actuators require post-heat treatment EDM. Thickness of the recast layer was measured to be about 38–55 μm, and precipitation in vicinity of the machined surface is a potential source for corrosion. Post-machining polishing by means of fluidized bed granules was employed to remove recast layer and associated precipitates. Test results proved that removal of surface layers improved the microstructure and the resistance to crack formation. The post-EDM polishing and subsequent annual inspections proved that problem was solved.