Microstructure Aspects of a Newly Developed, Low Cost, Corrosion-Resistant White Cast Iron
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THE literature on commonly used corrosion-resistant alloy cast irons namely ferritic (high Si), austenitic (high Ni), and high Chromium with or without Molybdenum revealed that the high Si irons are mostly utilized under oxidizing conditions.[1–4] Their poor mechanical properties restrict their wide spread engineering applications.[4] Although Ni-resist irons show good corrosion resistance to many environments and are hence frequently used, they lack sufficient mechanical strength P.K. SAIN, Research Scholar, C.P. SHARMA, Professor, and A.K. BHARGAVA, Professor, are with the Department of Metallurgical and Materials Engineering, Malaviya National Institute of Technology, Jaipur, India. Contact e-mail: [email protected] Manuscript submitted August 30, 2012. Article published online November 9, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
and suitability at a service temperatures higher than 1073 K (800 °C).[5,6] Single phase microstructures are most suitable in resisting corrosion for obvious reasons. The effectiveness of a two-phase microstructure in resisting corrosion depends on the morphology of the second phase, its volume fraction, the nature of the matrix-second phase interface, and the difference in electrochemical potential of the two constituents. For example, in cast irons, the microstructure comprising austenite and carbide is better than those comprising austenite and graphite in resisting corrosion. For better corrosion resistance, second phase particles should be nearly spherical and their volume fraction should be optimum in order to avoid galvanic cell formation and pitting corrosion. Work on low cost alloyed white cast iron, based on the use of relatively low cost and indigenously available alloying elements such as manganese, chromium, and VOLUME 44A, APRIL 2013—1665
copper, revealed the presence of desirable microstructure (austenite plus carbides) only at and above air cooling at 1173 K (900 °C).[7–11] In order to retain austenite in the as-cast condition, in the present work, the alloy based on the Fe-Mn-Cr-Cu-Ni system was studied. The results are presented in this paper. II.
Table I. C 2.53
Chemical Composition of the Investigated Alloy (wt pct)
Si
Mn
Cr
Cu
Ni
S
P
1.05
9.90
1.61
1.50
2.23
0.010
0.108
EXPERIMENTAL PROCEDURE
The composition was decided with the intention to develop low cost corrosion-resistant white cast iron rather than Ni-resist iron. Indigenously available cheap manganese was selected to replace the costly nickel. The nominal composition of the alloy was Fe-2.5 pct C-10 pct Mn-1.5 pct Cu-2 pct Ni. 20 9 2 9 2 cm3 rectangular strips were casted. Samples of 2 9 2 9 2 cm3 size were cut using a diamond cutter. The chemical composition of the investigated alloy was determined using the spectroanalysis method. The results for chemical composition of the alloy are given Table I. Heat treatment cycles comprised of air cooling from 1073 K, 1123 K, 1173 K, and 1223 K (800 °C, 850 °C, 900 °C, and 950 °C) after holding for periods ranging from 2 to 8 hours with an inte
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