STUDIES IN SOME Mn-Cr-Cu MARTENSITIC AND AUSTENITIC WHITE CAST IRONS

Abstract

A comprehensive review of the literature revealed that corrosion resistant alloy cast irons can be broadly classified into three categories namely (a) ferritic (high silicon cast irons), (b) austenitic (Ni-resist cast irons) and (c) martensitic (high Cr cast irons with or without Mo). The first variety has useful applica-tions where resistance to corrosion under oxidising conditions is the essential requirement. The poor mechanical strength and shock resistance associated with high silicon irons preclude their gene-ral engineering applications. Of the remaining two, the Ni-resist cast irons, although extensively used in a variety of conditions, have a low strength and are unsuitable at operating temperatures 0 900°C or more. The martensitic variety of cast irons exhibit a relatively higher strength and can be employed upto higher service temperatures. Their shock resistance can be improved by lowering the carbon content. A critical analysis of the data on austenitic and martensitic cast irons revealed that little information is available on the structure-property relations in general. Furthermore, systematic information is lacking on the electro-chemical and the deformation behaviour of micro-structures commonly encountered in alloy white irons namely "martensite + carbide", (M + A), Austenite + Carbide (A + MC) and their allied counterparts. Detailed information on these aspects is likely to prove useful in ascertaining whether microstructures exhibiting good resistance to aqueous corrosion and useful mechanical properties could be attained through the 'white iron' route. It would be equally pertinent to investigate whether these micro-structures could be generated by utilizing low cost alloying elements (Mn, Cu etc.) in preference to the conventionally employed alloying elements such as Ni, Mo etc. The present investigation was undertaken in response to the above queries. It essentially comprised of conceiving/designing some new low cost Fe-Mn-Cr-Cu alloys based on fundamental conside-rations, assessing their heat-treatment response and characterizing them on the basis of corrosion behaviour and mechanical properties. The alloys, which were air induction melted and sand cast (25 mm round cylinders and 120x22x8 mm rectangular strips), were investi-gated by employing hardness measurements, optical and scanning electron microscopy, quantitative metallography, x-ray diffractome-try, EPM analysis, compression testing and their electrochemical behaviour determined by the weight loss and potentio-static methods. Computational techniques were extensively employed for data analysis. The experimental work comprised of subjecting disc and recta-ngular specimens of the four alloys, containing 06% and 0 8% Mn* and 0 4% Cr nominal alloy compositions at two different Cu* levels namely 0 1.5 and 0 3%, to heat-treatments comprising of holding for 2,4,6,8, and 10 hrs at 800,850,900,950,1000 and 1050°C followed by oil' quenching. This treatment was preferred over air cooling because a more uniform micro-structure was obtained. Optical metal-lography was extensively used to assess how the alloy content and heat-treating schedule influenced the micro-structure which comprised of : (i) P/B + M + MC with and without RA in the as-cast state, (ii) M + MC + DC with and without retained austenite (RA) on heat-treating from upto 900°C, (iii) A + MC + DC or A + MC with and without M (in traces) on heat-treating from upto 1000°C, and (iv) A + MC + New phase (bridge type carbide) on heat-treating from 1050°C. The volume fraction of massive carbides (MC) decreased with temperature or with soaking period at a given heat-treating tempe-rature. The decrease was marked at temperatures > 1000°C. Simulta-neously, massive carbides were rendered discontinuous from the early stages of heat-treatment. The 'rounding-off' tendency set in at 1000°C. Dispersed carbides (DC) formed corresponding to the 800°C, 10 hr heat-treatment directly from austenite. They underwent coarse-ning with an increase in temperature and or soaking period. The extent of coarsening which was marked at 900 and 950°C, has been represented by the 'coarsening index' (CI). The dispersed carbides dissolved on heat-treating from 1000°C. X-ray diffractometry was helpful in establishing that four types of carbides formed in the experimental alloys namely M3C, M23C6, M5C2 and M7C3. At heat-treating temperature > 1000°C only the M5C2 and M7C3 carbides existed. The new phase [NP] formed on heat-treating from 1050°C was formed by N & G type transformation. Its volume fraction initially increased with soaking period upto 4 hrs and decreased thereafter. Hardness measurements provided a quick yet reliable indication of the mathematical modelling it was possible to establish that the hardness is related with the heat-treating temperature and time by an equation of the form : H = Cl e c2/T + ( C3 + C4T)t where, H = VHN30 • temperature in °K • time in seconds the four constants were different for the four alloys. From the point of view of mechanical properties the marten-site bearing micro-structures were brittle and were characterized by low compressive strength (CS) and % strain. The austenite based micro-structures gave high values of compressive strength and % strain. The key parameter in influencing the deformation behaviour was the amount and stability of austenite. The effect of massive carbide (MC) on the deformation behaviour was a function of the combatibility, volume fraction and morphology. The behaviour was governed by their size, shape and distribution. The new phase (bridge carbide), formed on heat-treating from 1050°C, adversely affected the deformation behaviour. Parameters controlling the deformation behaviour are also the key parameters in controlling the overall corrosion behaviour. Stress relieving in general proved to be detrimental due to enhan- ced galvanic action. The presence of new phase was also found to be detrimental from the corrosion resistance point of view. It was further concluded that the austenite-carbide couple proved extreme-ly satisfactory from the point of view of corrosion resistance because the experimental alloys did not undergo any localized attack.......................