Metallurgical Characterization of Bainitic Steel by Eddy Current Method
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TECHNICAL ARTICLE—PEER-REVIEWED
Metallurgical Characterization of Bainitic Steel by Eddy Current Method S. H. Khan . A. Nusair Khan
Submitted: 24 June 2016 / in revised form: 26 September 2016 / Published online: 25 October 2016 Ó ASM International 2016
Abstract Bainite is a decomposition product of austenite consisting of an aggregate of ferrite and carbides. In general, it forms at temperatures lower than those where very fine pearlite forms and higher than those where martensite begins to form on cooling. In the present work, a bainitic structure steel comprised of plate-like ferrite grains, inside which carbide particles can be seen, have been prepared. The microstructural evaluation is performed at different temperatures. These steels are characterized by optical microscopy, hardness testing, and eddy current testing system. It is found that there is a relationship between hardness and eddy current. Thus, eddy current technique found effective tool to identify the condition of microstructure at different temperatures. Keywords Bainitic steel Eddy current testing Low alloy steels
Introduction Chromium hot-worked steels have good resistance to heat softening because of their medium chromium contents and the addition of carbide forming elements such as Mo and V. The low carbon and the low total alloy contents promote toughness at the normal working hardness of 392–596 Hv (40–55 HRC). Higher W and Mo contents increase hot strength but slightly reduce toughness. Vanadium is a major microalloying element added to a broad range of constructional and engineering steels. The precipitation of S. H. Khan (&) A. N. Khan Army Public College of Management and Sciences, Rawalpindi 46000, Pakistan e-mail: [email protected]
vanadium carbides and nitrides results in grain refinement and precipitation strengthening upon tempering [1–3]. Vanadium is also added to increase the erosive wear resistance at high temperature [4]. The outstanding characteristic of these steels is their toughness, which distinguishes them from most other tool steels. They are more shock-resistant than other hotworked die steels and have greater hot hardness. Their extraordinary shock resistance makes them preferable on most hot work tasks and especially when it is necessary to cool the dies in service with water or other flushing mediums. Typical applications of these steels include diecasting dies, forging dies, punches, piercers, and mandrels for hot work, hot extrusion tooling, shear blades for hot work, and all types of dies for hot work that involves shocks. Because of their high toughness, these steels are also employed for cold work where shock resistance is essential. Typical cold work applications include machine knives, heavy-duty shear blades, coining dies, notching dies, master hubs, and pneumatic tools. These steels are also used for ultra-high-strength structural parts [5]. Further, vanadium micro-alloyed medium carbon steels are currently used in automobile industry due to the improved tensile and impact properties that are a
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