Evaluation of Volume Fraction of Retained Austenite in Quenched Carbon Steel

Abstract

Quenching process is a type of heat treatments carried out to improve the mechanical properties of a given steel by making it undergoes phase microstructural and crystallographic changes. The thermal properties and the electrical conductivity may be also altered. The heat treated steel could contain a significant amount of retained austenite that does not transform to martensite upon quenching. The amount of retained austenite presents in the quenched steel depends on the composition, austenitizing temperature, quenching rate , … etc.

Several techniques were developed to calculate the amount of austenite in heat treated steels such as metalographical analysis by optical microscopy and scanning electrical microscopy,X-ray diffraction, magnetization ,... etc.

In the present work, two types of low carbon steels denoted later as S₁ and S₂ are heat treated. The heat treatment cycle consists of heating the steels to a temperature of 950℃ for 30 minutes soaking time to have a fully austenitic structure followed by rapid cooling (quenching) in three different media mainly water, used engine oil and corn oil. The volume fraction of retained austenite accompained with such heat treatments was basically evaluated by X-ray diffraction and was further investigated by measuring the electrical resistivity and applying Mathiessen's rule. The results obtained by X-ray diffraction and by electrical resistivity measurements were compared in order to check the compatibility between them.

It is concluded that, quenching process causes a dramatic increase in the electrical resistivity of the as supplied steels. The results also indicate that the volume fraction of retained austenite increases with increasing the alloying elements in the steel as well as with the severity of quench (cooling rate). However, although the two techniques employed in this work to calculate the amount of retained austenite show the pre mentioned trend, however, the compatibility and the conformation between them are poor. The results obtained by the two techniques can not be well-related