Structure and Strength of Isothermally Heat-Treated Medium Carbon Ti-V Microalloyed Steel

Stefan Dikić,Gvozden Jovanović,Nenad Radović,Abdunnaser Fadel,Dragomir Glišić

doi:10.3390/met11071011

Abstract

Isothermal transformation characteristics of a medium carbon Ti-V microalloyed steel were investigated using light microscopy, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), and by uniaxial compressive testing. Samples austenitized on 1100 °C were isothermally treated in the range from 350 to 600 °C and subsequently water quenched. The final microstructure of the samples held at 350 °C consisted of bainitic sheaves and had compressive yield strength, approximately from 1000 MPa, which is attributed to high dislocation density of low bainite. At 400 and 450 °C, acicular ferrite became prevalent in the microstructure. It was also formed by a displacive mechanism, but the dislocation density was lower, leading to a decrease of compressive yield strength to approximately 700 MPa. The microstructure after the heat treatment at 500 °C consisted of coarse non-polygonal ferrite grains separated by pearlite colonies, principally dislocation free grains, so that the compressive YS reached a minimum value of about 700 MPa. The microstructure of the samples heat-treated at 550 and 600 °C consisted of pearlite and both grain boundary and intragranular ferrite, alongside with some martensite. After 600 s, austenite became stable and transformed to martensite after water quenching. Therefore, the presence of martensite increased the compressive YS to approx. 800 MPa.

Highlights

Demands for materials that provide good mechanical properties, along with weight reduction and cheap manufacturing process, are constantly increasing
That temperature austenite transforms by bainitic mechanism, forming either bainite or acicular ferrite (indicated in the diagram as bainite start temperature (BS)(AF))
The microstructure of the samples isothermally treated at 350 ◦ C, Figure 2a, consists mainly of bainitic sheaves (BS)—parallel bainitic ferrite plates emanating from the prior austenite grain boundaries but smaller sheaves of intragranularly nucleated ferrite plates can be observed

Summary

Introduction

Demands for materials that provide good mechanical properties, along with weight reduction and cheap manufacturing process, are constantly increasing. Medium carbon microalloyed steels are intended to attain similar strength/toughness levels to that of quenched and tempered steel by air cooling from the temperatures of hot working. In this manner, the process of quenching and tempering would be avoided, which would shorten the production time and considerably lower the expenses [1]. The main role of microalloying elements—titanium (Ti), vanadium (V), and niobium (Nb)—is to control grain size at high temperatures and to increase yield strength through mechanisms of grain refinement by austenite grain boundary pinning and retardation of recrystallization, as well as through the mechanism of dispersion strengthening of ferrite. The influence of carbides and nitrides of microalloying elements is rather circumstantial; by providing the suitable sites for heterogeneous nucleation of the products of austenite decomposition [4]

Methods

Results

Conclusion