Abstract
The main goal of the current paper is focused to investigate the effect of isothermal heat treatment temperature and time on microstructure and strength in a medium carbon vanadium titanium free micro alloyed steel. Isothermal heat treatment was carried out in the temperature range 350 to 600 C° at different holding times varying from 2s to 1200s followed by water quenching. Samples were investigated using optical microscope (OM) and scanning electron microscopy (SEM) paired with energy dispersive spectroscopy (EDS) and by compressive testing using a servo-hydraulic testing machine. The results show that, the final microstructure of samples held at high temperatures (550 and 600°C) consists of polygonal intra-granularly nucleated ferrite idiomorphs, combined with grain boundary ferrite and pearlite were produced and followed by retained austenite that transformed to martensite upon quenching (incomplete transformation phenomenon). At intermediate temperatures (450 and 500 °C) an interlocked acicular ferrite (AF) microstructure is produced, hence acicular ferrite becomes prevalent in the microstructure at (450 °C). The microstructure after the heat treatment at 500°C consists coarse nonpolygonal ferrite grains separated by pearlite colonies. However, at low temperatures (400 and 350°C), the final microstructure of the samples held at 350°C consists of bainitic sheaves, where the sheave of parallel acicular ferrite plates, similar to bainitic sheaves but intra-granularly nucleated were observed, which called in some references as sheaf type acicular ferrite for samples isothermally treated at 400°C. Yield stress was determined by compression testing on samples with final Microstructure, the results show that, the observed change in the microstructure is related by a marked decrease of compressive yield strength, approximately from 1000 to 700 MPa.