Effect of microstructure on the corrosion and deformation behavior of a newly developed 6Mn-5Cr-1.5Cu corrosion-resistan
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A comprehensive review of the literature on corrosionresistant alloy cast irons currently in use, n a m e l y , (a) ferritic (high Si), (b) austenitic (Ni-Resist), and (c) martensitic (high Cr with o r without Mo), revealed that the ferritic irons are most usefully employed under oxidizing conditions. Their poor mechanical strength and shock resistance preclude their general engineering applicationY ,2j Ni-Resist irons, although very useful in a variety o f environments, have a low strength, suffer from graphitic corrosion, and are unsuitable at operating temperatures -->800 °C. 12,31 The high-Cr irons can be employed up to higher service temperatures.12] Their shock resistance is improved by lowering the carbon content.t4~ Ni-Resist and high Si-irons suffer from graphitic corrosion--a phenomenon known to promote failure P.N.V.R.S.S.V. PRASADA RAO, Senior Research Fellow, and A.K. PATWARDHAN, Immediate Past Chairman, are with the Metallurgical Engineering Department, University of Roorkee, Roorkee 247667, India. N.C. JAIN, formerly Postdoctoral Candidate, University of Roorkee, is Research Officer, Indian Lead-Zinc Information Centre, New Delhi 110029, India. Manuscript submitted May 6 , 1991. METALLURGICAL TRANSACTIONS A
without warning. The former also undergo pitting, another undesirable form o f corrosion.t2,3,41 The l o w e r mechanical strength o f Ni-Resist irons is a major disadvantage, t2'3m Therefore, a strong case existed for evolving an alternative strategy for designing and developing alloy cast irons w h i c h could combat graphitic corrosion and overcome pitting and other problems. A possibility that held promise was to adopt the "white iron" approach in preference to the "gray iron" route. A critical analysis revealed that there is a general lack of systematic information on the electrochemical and deformation behavior o f microstructures encountered in white irons. An alloy development program, based on utilizing Mn, Cr, and Cu as alloying elements, was therefore initiated by Patwardhan151 to assess whether new compositions, helpful in resisting aqueous corrosion and eventually replacing proprietary compositions, could be developed. The main attribute o f the selected alloy system is its l o w cost, as it excludes the use o f Ni and Mo. The other anticipated advantages are (1) absence o f graphitic corrosion, (2) improved response to aqueous corrosion since austenite and carbide (in white irons) are closer compared to austenite and graphite (in gray irons) VOLUME 24A, FEBRUARY 1993--445
in the electrochemical series, and (3) a possible improvement in the high-temperature response due t o the presence of carbides in the microstructure. The alloys were investigated in the as-cast and heattreated conditions and their transformation behavior studied in detail using optical metallography, X-ray diffraction, electron probe microanalysis, and hardness and microhardness measurements. Qualitative interrelations were provided with a quantitative base by correlating microstructure with the corrosion a
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