Characteristics of the rough-cut surface of quenched and tempered martensitic stainless steel using wire electrical disc
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3/12/04
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Characteristics of the Rough-Cut Surface of Quenched and Tempered Martensitic Stainless Steel Using Wire Electrical Discharge Machining C.A. HUANG, G.C. TU, H.T. YAO, and H.H. KUO This article studies the surface characteristics of quench- and temper-treated AISI 440A martensitic stainless steels, which were rough cut using wire electrical discharge machining (WEDM). The microstructure of the recast layer on the cut surface was investigated using scanning and transmission electron microscopes, and the phase compositions were analyzed with an energy-dispersive X-ray (EDX) spectrometer. Experimental results showed that the thickness of the recast layer varied with the heat-treatment condition of the workpiece, the largest thickness was obtained with a quenched specimen, and the thickness decreased with increasing tempering temperature. Intergranular surface cracks were observed only from the as-quenched specimen, whereas surface cracks were not found in the rough-cut specimens after tempering above 200 °C. It is reckoned that reliefing of the thermal residual stress in the quenched workpiece induced the surface intergranular cracks. Microstructures of the recast layer on the rough-cut surfaces of the 600 °C tempered specimen were examined using cross-sectional transmission electron microscopy (TEM) specimens. An amorphous layer exists at some parts of the outermost cut surface. A high density of wire electrode droplets of spherical shape, approximately 10 to 60 nm in size, was found throughout the porous recast layer. Besides, many highchromium containing sigma spheres with sizes of approximately 120 to 200 nm were precipitated at the bottom part of the recast layer, and its formation mechanism was proposed. Adjacent to the recast layer was a heat-affected zone (HAZ) with a thickness of about 4 m, in which temper-induced carbides were fully dissolved. The HAZ comprised basically two distinct regions: the first region adjacent to the recast layer was composed of a lath martensite structure, while the other region was an annealed ferrite structure.
I. INTRODUCTION
MARTENSITIC stainless steels are widely used in plastic injection molds, surgical tools, and mechanical parts, owing to their superior corrosion resistance, high strength, and good wear resistance. In practical usage, these steels are often quenched from 1100 °C to 1200 °C to obtain the martensitic structure, which has the highest strength and hardness because of the saturated solution of carbon and high dislocation density in the matrix. Due to the low toughness of the martensitic structure, a temper treatment between 200 °C and 650 °C after quenching is needed to obtain suitable mechanical properties. With increasing temper temperatures, carbides such as M23C6, M7C3, and M3C precipitate[1] in the matrix, inducing a tempered martensitic structure with multiple phases[2] and enhancing the toughness of material. In general, martensitic stainless steels are quenched and then tempered before shape cutting using wire electrical dis
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