TECHNICAL DEVELOPMENT REPORT


EFFECT OF COPPER ON THE CORROSION RESISTANCE BEHAVIOUR OF AS-CAST AND DESTABILIZED CR-MN WHITE CAST IRONS


I. Chakrabarty Banaras Hindu University, Varanasi, India Copyright © 2011 American Foundry Society Abstract


The effect of the addition of copper to Cr-Mn white cast irons in the as-cast and destabilized conditions and their performance in the chloride corrosive media were studied. This research was part of the development programme to consider substitute alloys for the existing costly wear resistant alloys like Ni-Cr or High Cr white cast irons for wear resistant applications. In the as-cast condition, a predominantly austenitic matrix was developed by the addition of Mn(~5%) and Cu(~3%) to the base iron with 2.8%C, 1.7%Si, and 6%Cr. These metastable austenitic alloys were subjected to destabilization ausaging treatment. The corresponding transformation behavior was


Introduction


Conventional alloyed white cast irons used for wear resis- tant applications are mostly either high chromium or nickel- chromium white cast irons. However, there has been a con- tinuing interest in the development of substitute alloy irons to provide either a partial or total replacement of costly and scarce alloying elements like Ni, Mo etc. in the aforesaid al- loys. Manganese, a comparatively cheaper alloying element, has been used to replace the more expensive elements in al- loys like Ni-hard type irons.1-4


in chromium cast irons can be successfully suppressed by the addition of manganese.5


The pearlitic transformation Because of its strong austenite


stabilizing characteristics, the manganese percentage is re- stricted in as-cast martensitic irons. However, the austenite in the matrix can be subsequently reduced by a suitable de- stabilization treatment which would transform the retained austenite into martensite embedded with fine secondary carbides. The work hardening characteristics of the high manganese austenitic irons have also been reported to result in the high wear resistance during impact-abrasive wear.6 The alloy cast iron with an austenitic matrix perform well in impact-wear applications.4,7,8


Many cast iron components


employed in mining and farm machinery, slurry pumps etc. are subjected not only to wear but also to corrosion in ag- gressive environments. In this respect, the addition of copper to Cr-Mn white cast irons appears to be beneficial because it partitions mainly to the austenitic phase5 sistance to aqueous corrosion.9-11


and enhances re- The aim of this article is to study the effect of the addition of copper to the as-cast and International Journal of Metalcasting/Winter 11


characterized by optical microscopy, hardness measurement, x-ray diffraction analysis, and electron probe micro-analysis etc. The corrosion rate in 5% NaCl aqueous solution were measured by potentiostatic method and the corroded surfaces were examined under a scanning electron microscope with EDX facility. The observations prove the beneficial effect of copper on increasing the corrosion resistance of Cr-Mn white cast irons.


Keywords: corrosion resistance, ausaging, corrosion- erosion, slurry media


heat treated microstructure and the corresponding electro- chemical corrosion behavior of Cr-Mn white cast irons.


Experimental


The alloys were made in a basic lined 4kg. indirect arc fur- nace from white cast iron foundry returns and ferroalloys. Electrolytic grade copper strips were used for the addition of copper. Cylindrical test bars (2cm. diameter and 15cm. long) were cast in resin bonded sand molds at 1500C (2732F). Table 1 shows the composition of the alloys cast and the percentages of retained austenite content in as-cast condi- tion. The retained austenite contents were measured by x-ray diffraction analysis using Mo Kα radiation.


ture) of the alloy, followed by air cooling to room temperature. The equilibrium austenite at this lowest possible temperature has minimum solid solubility for carbon and other alloying elements. So, during aging treatment at this temperature, the excess alloying elements diffuse out of the alloy saturated aus- tenite matrix in the form of fine secondary alloy carbides. The primary eutectic carbides are formed during eutectic solidifi- cation. Consequently, the martensite start (Ms) temperature is raised above room temperature and with the corresponding crit- ical cooling rate still remaining low, the alloy depleted austenite will transform to martensite during subsequent cooling to room


The alloys were subjected to destabilization or ausaging treat- ments to destabilize the austenite in the as-cast matrix. The ausaging treatment involves aging at the lowest possible tem- perature in the γ+ carbide phase field (just above A3


tempera- 49


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