THE STUDY OF HARDENING CHARACTERISTICS OF HIGH CARBON STEELS AND DUCTILE CAST IRON

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Department of Engineering

ABSTRACT
The hardening characteristics of four grades of high-carbon steel materials and ductile iron material were studied. Three of the steel materials were plain high carbon steels with compositions of 0.95%C, 1.05XC and 1.20XC. the fourth steel material was a low-alloy, with 1.205%C and 1.20%W. Standardized specimens were cut from these materials and were first given a normalizing heat treatment before they were hardened. Hardening was done by heating to an appropriate temperature followed by quenching in water or oil. Hardness measurements were made on these specimens in the as quenched condition or after further conditioning treatments. These included: (a) Tempering between 150 and 500oC immediately after quenching, (b) Ageing, (c) Cold-treatment and tempering. Other tests carried out on these materials include Jominy end-quench tests, to determine their hardenabilities, and notched impact tests to determine their toughness both in the as-quenched condition and after tempering at temperatures between 150 and 500oC. All the steel materials responded to hardening by water quenching, but the ductile iron material did not. Only the low-alloy high carbon steel responded to hardening by oil-quenching. It was also found that optimum hardening temperature for these steels was between 800 and 850oC. Tempering characteristics of the steel materials showed that they all soften slightly on heating to 250oC presumably due to stress relief. Beyond this temperature softening was rapid for the plain carbon steels but was arrested in the case of the low-alloy steel for up to 500oC covered In this work. Cold treatment after quenching caused remarkable increase in the hardness of the high carbon steel materials, both in the tempered and untempered condition. This was probably due to the transformation of the retained austenite during the cold treatment. Ageing treatment which occurred tor over two months was also found to cause the same effects. Tempered notched impact tests produced toughness peaks at 350oC In the steel specimens. This gave an optimum tempering temperature for maximum toughness over a narrow range around this temperature, for the high carbon steels which were water quenched and followed immediately by tempering. The critical diameters of the plain high-carbon steels were found to be 16mm, 14mm and 12mm for the 0.95XC, 1.05XC and 1.20XC carbon steels respectively. These confirmed the low hardenability character of plain high carbon steels. However it was 65mm for the W-based low-alloy high carbon steels.