Effect of the Condition of Short Time Continuous Quenching and Tempering Heat-Treatment on Improving the Toughness of the Web and Base of Rail

Abstract

To shorten treatment time, heat treatment conditions were investigated, taking into account the possible high toughness of the web and base portions of rails by means of continuous quenching and tempering.
The investigation on the relations between fracture toughness and heat treatment conditions led to the following optimum conditions ; namely, finishing temperatures in quenching from 100 to 150°C, rapid heating at heating rates of 10°C/sec or less for tempering, and tempering temperatures from 650 to 700°C.
In continuous quenching and tempering where heating rate is fast, when martensite volume fraction after quenching exceeds 40%, fracture toughness is not affected and remains nearly constant. This may be because the microstructure formed under the above-mentioned conditions consist of tempered fine bainite-martensite mixture having a microstructure similar to that of tempered martensite, and thus, shows nearly the same fracture toughness as that of tempered martensite.