Microstructure analysis of the martensitic stainless steel surface fine-cut by the wire electrode discharge machining (WEDM)

In this research, quenched and tempered martensitic stainless steels, AISI 440A, were subjected to multi-cutting passes by wire electrical discharge machining (WEDM). The WEDM is widely applied to final surface shaping of harden steel. The steel was roughly machined at first cutting pass, semi-finished by two cutting passes, and then finished by one cutting passes, all machined by WEDM. The negatively polarized wire electrode (NPWE) was used for all of these four cutting passes. Some finished specimens were further extra-finished by using the positively polarized wire electrode (PPWE). Microstructures of the finished surfaces using NPWE as well as PPWE were studied with scanning and transmission electron microscopes (SEM and TEM). Chemical composition was analyzed by an energy-dispersive X-ray spectrometer (EDX) integrated in SEM or TEM. The study of the finished surfaces with NPWE shows that a heat-affected zone (HAZ) of about 1.5 μm thick was developed. The HAZ is composed of very fine equiaxed martensitic grains of about 200 nm with concentrated dislocations, but the temper-induced carbides were not found. A few spherical deposits of wire electrode material were also registered. Oxides with ca. 10 nm in diameter were detected around the deposits. The spherical deposits were characterized as “bull eye” in TEM according to their appearance. For the surface finished with PPWE, no obvious HAZ was detected, but a very thin (<50 nm) and uniform amorphous layer composed of wire electrode and workpiece material was found. The structural difference between the two finished surfaces was explained based on the theory of electrical discharging and metallurgical physics.