STUDIES IN Mn-Cr-Cu WEAR RESISTANT WHITE CAST IRONS

Abstract

A comprehensive review of the literature revealed that abrasion resistant white irons can be classified as (i) plain carbon and (ii) alloyed. The plain carbon white irons comprise of a microstructure consisting of pearlite + carbide (Pc) whose attainment in a desired section size is controlled by the C/Si balance and/or by chilling. They have applications only under service conditions involving light to moderate wear because of (i) a. restrictior!' on the maximum attainable hardness and (ii) the practical limita-tion(s) involved in manufacturing them (in relation to chilling). Cr-white irons represent an improvement over the plain carbon variety with regard to (i) the ease of formation of the (P+C) microstructure and (ii) the possibi-lity of attaining a higher level of hardness (and therefore of resistance to wear) through a combination of alloying and heat treatment. The development of alloyed white cast irons followed two distinct paths. In the U. S.A. q the International Nickel Co. initially developed the low alloy versions Ni-hazd. 1 and 2 which exhibited better wear resistance and shock resistance compared with plain carbon/Cr-C varieties. They were followed by the higher alloy version Ni-hard 4. The main object of developing it was to bring about some increase in toughness through achieving a better eutectic carbide distribution. The other group of alloys more popular in Western Europe V consists of 27.Cr type, 15 Cr-3 Mo type (developed by Climax-Molybdenum Co.) and the more recent 13 Cr variety alloyed with Mn, Mo and/or Ni. Features common to the development of Ni-hard and Cr based white irons are (i) increasing the hardness of the matrix phase by increasing hardenability through alloying with a view to attaining metastable phase(s) in place of pearlite, (ii) increasing the overall hardness further by converting cementite into an alloy carbide and (ii) enhancing the shock resistance/ toughness through improved carbide distribution (applicable mostly to Ni-hard 4 iron). A critical reappraisal of the physical metallurgy of white cast irons as also their wear characteristics revealed that there was a definite need to reassess (i) the most suitable microstructure from the point of view of abrasive wear and (ii) possible alternative paths of attaining such a microstructure; other than those listed above. The present investigation was undertaken in response to (ii) above and 'essentially comprised of conceiving/ designing a series of low cost Fe-Cr-Fin-Cu alloys based on fundamental considerations., assessing their heat treatment response and characterising them for wear. The alloys which were air induction melted and sand cast (25 mm around cylinders), were investigated by employing hardness and microhardness measurements, optical and scanning electron vi microscopy, X-ray diffractometry and EPMA techniques and their wear characteristics determined in terms of weight loss as a function of time in a limited number of instances. The experimental work comprised of subjecting eight alloys, containing approximately 3,4,5 and 67. Mn and 17. Cu at two different Cr contents ( 6-6.5%. and 9-9.57. respect-ively), in the form of 25 mm round X 18 mm long specimens, to heat treatments comprising of air cooling from 750, 800, 850 and 900°C after holding them at each of these temperatures for periods ranging from 2 to 10 hours. Microstructural examination was carried out predominantly by optical metallo-graphy to assess how alloy content and heat-treating schedule influenced the final microstructure. Hardness and micro-hardness measurements provided a quick yet reliable indication of the nature of the microstructure/microconstituents formed. Detailed structural examination by X- ray diffractometry proved helpful in deciding upon the nature of the matrix phase, the type of alloy carbide formed and whether or not austenite was retained. It was possible to establish a correlation between the data thus obtained and the data generated with the help of EPMA studies, involving the effect of heat treatment schedule on the distribution of alloying elements into different phases/microconstituents. The EP data is of fundamental interest and also very useful in the optimal design of alloys. Finally, the alloys were characterized for wear by utilizing two different methods/rigs- vii (i) in which a single specimen abrades under a known0 weight against a rotating abrasive wheel and (ii) in which 9 specimens, mounted on a ring coupled to the shaft of a vertically driven motor, abrade against a sand water slurry. An attempt has been made to establish.a correlation. between the wear characteristics and the microstructure. Crack propagation studies were carried out to a limited extent to assess the nature of the crack propagation path. Results obtained revealed that the crack propagation was not preferential. Based on these findings$ inferences have been drawn with regard to (i) the optimum Mn/Cr combination and the optimum heat-treating schedule for obtaining the stipulated end properties (hardness/wear) 9 (ii) mechanism of hardening, (iii) interrelation between microstructure, hardness and wear and (iv) the most suitable microstructure from the point of view of wear.