The Effect of Austenite Grain Size on the Growth of Different Ferrite Morphologies in a Nb-Microalloyed Steel

Abstract

The effect of austenite grain size on the austenite to ferrite transformation temperature and different ferrite morphologies and growth behaviour in one Nb-microalloyed (HSLA) steel has been investigated. Three different austenite grain sizes were selected and cooled for obtaining austenite to ferrite and growth behaviour of ferrite. Moreover, samples with specific austenite grain size have been quenched, partially, for investigation of the microstructural evolution. The optical microscopy observation suggested that the nucleation site of ferrite is on edge and inside of austenite grains in Nb- microalloyed steels. Micrographs of different ferrite morphologies show that at high temperatures, where diffusion rates are higher, grain boundary ferrite nucleates both at the edge and corner of austenite grains and grows into both austenite grains. As the temperature is lowered and the driving force for ferrite formation increases, intragranular sites inside the austenite grains become operative as nucleation sites and suppress the grain boundary ferrite growth. With more undercooling,intragranular ferrites are seen to nucleate and grow more extensively , indicating the beginning of displacive transformation. Furthermore, growth rate of intragranular ferrite shows that by increasing of austenite grain size, the growth rate of intragranular ferrite increases extensively and growth rate of grain boundary ferrite decreases. The growth kinetics of grain boundary ferrite shows that this transformation is controlled by the diffusion of carbon in the austenite ahead of the interface.

Role of Ti and N in line pipe steel welds

“…19,[21][22][23] TiN precipitates hold a B-N orientation relationship with ferrite matrix. 22 Fine intragranular ferrite that grows from the precipitates can section the prior austenite grains into many colonies.…”

The Influence of Austenite Grain Size on the Mechanical Properties of Low-Alloy Steel with Boron

“…Some studies concerning the influence of the prior austenite grain size on the austenite to ferrite transformation temperature and different ferrite morphologies in Nb-micro-alloyed (HSLA) steel have been performed [6]. Similar studies have been performed to investigate the impact of the prior austenite grain size on the morphology and mechanical properties of martensite in medium carbon steel [3] and to predict the austenite grain growth in low-alloy steels [7], but there is still a lack of knowledge about the influence of the prior austenite grain size on the basic mechanical properties in low-alloy boron steels with high resistance to abrasive wear.…”

Welding fume reduction by nano-alumina coating on electrodes – towards green welding process

“…This occurrence could be associated with the increase in the amount of oxide inclusions in the weld pool with the addition of nano alumina. According to the reports by Esmailian (2010), oxide inclusions in the weld pool acted as favorable nucleation sites for the fine ferrite formation. The micrograph obtained for run 10 exhibited more acicular ferrites, while that for run 11 had more fine grained widmanstatten ferrites with little pearlite, which could be favorable in providing better tensile and yield strength to the resultant weld (Bodnar and Hansen, 1994).…”