Effect of cooling rate on phase transformation and microstructure evolution in a large size forged ingot of medium carbon low alloy steel

Abstract

In the present study, the influence of cooling rate on the kinetics of phase transformation is investigated in large size forged ingots made of medium carbon low alloy steel. In particular, the volume fraction of martensite after quenching is determined. Dilatometry tests were conducted on Gleeble® 3800 thermomechanical simulator to determine the transformation temperatures and identify the phases after different heat treatment cycles. The heat treatment tests were carried out at 900 °C with a heating rate of 5 °C/s, followed by holding for 1800 s after which the samples were cooled at 3 and 15 °C/s. Optical and electron microscopic analyses, X-Ray Diffraction and micro-hardness measurements were utilized to identify and quantify the volume fraction of martensite and retained austenite for each condition. The calculated volume fraction of martensite using mathematical analysis was compared with those calculated by Koistinen-Marburger equation.