STUDIES IN 9 Cr WHITE IRON ALLOYED WITH MANGANESE AND COPPER

Abstract

Effeot of heat-treatment on the hardness, microetruoture and resistance to corrosion in a 5 Y. NaC1 solution has been studied for a 9Cr white iron alloyed with 6 '/,Mn and 1 ''l. Cu for assessing the electrochemical behaviour of marteneite + carbide and related microstruoturee namely marteneite + auetenite + carbide, auetenite + dispersed carbide + massive and auetenite + massive carbide* Heat treatments. involved air cooling and oil quenching from 900, 950,, '1000, 1050°C after holding at each of these temperatures for 4, 6., 18 and 10 bre reepeotively, Corrosion studies were confined to oil quenched specimene which were invee-. tigated with or without a stress relieving heat treatment consist- ing of 60000 (.5 h followed by either air cooling or oil quenching. It emerges that mioroetructuree based on austenitic matrix and containing both dispersed mod. massive carbide+ have superior corrosion resistance than the as-cant micraetruoture oomprieing of merteneite + auetenite + carbide. Presence of oarbidee is not detrimental to corrosion, effect of dispersed carbides ie governed by their size and dietributtion. Presence of a new phase' (resembiing the sigma phase ),formed t an heat.treating from 1050°C, advereel' of eote'the corrosion resistance,,, Stress-. relieving has been found to be detrimental from the point of view of corrosion resistance which decreases with an increase in the test duration. Heat-treatments for producing the most useful microstructures have been identified. key parameter in con- trolling the corrosion resistance appears to be the amount and stability of auetenite. The results which provide useful clues to the design of future compositions, suggest that there is a good possibilityt of designing corrosion resistant cast irons by, employing the white iron route, 0