Microstructure evolution and optimization of the tempering heat treatment of a medium-carbon low-alloy steel

Abstract

In this research, the effects of processing parameters on different phenomena occurring during tempering of a medium-carbon low-alloy steel were investigated. Conducting highresolution dilatometry machine, samples were cooled with three cooling rates of 4.8, 18 and 180 °C/min, to produce martensite, martensite mixed with bainite and bainite, respectively. These microstructures correspond to different locations of a large-size forged ingot after quenching. Subsequently, non-isothermal tempering up to 600 °C with different heating rates was carried out to study various tempering stages and phase transformations occurring during tempering especially for bainite. Results indicated that tempering behaviour differed remarkably with the primary microstructure, and among all the tempering stages, only retained austenite decomposition and carbide precipitation have occurred during tempering of a bainitic microstructure. Furthermore, isothermal tempering for three temperatures of 350, 550 and 600 °C with different holding times up to 16 h was performed to study retained austenite decomposition and carbides evolution during tempering of a bainitic specimen containing retained austenite. Results showed that full decomposition of retained austenite occurred only at 600 °C. Also, after 16 h of tempering at 600 °C, carbides tend to be rich in chromium and mostly Cr7C3 and Cr23C6 carbides were identified. Hardness increase was observed during carbides precipitation, while it decreased when carbide coarsening started.

Date	20 Aug 2018
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Mohammad Jahazi (Supervisor)