Iranian Journal of Materials Science and Engineering

Abstract:

transformation temperature and different ferrite morphologies in one Nb-microalloyed (HSLA) steel has been

investigated. Three different austenite grain sizes were selected and cooled at two different cooling rates for obtaining

austenite to ferrite transformation temperature. Moreover, samples with specific austenite grain size have been

quenched, partially, for investigation on the microstructural evolution.

In order to assess the influence of austenite grain size on the ferrite transformation temperature, a temperature

differences method (TDM) is established and found to be a good way for detection of austenite to ferrite, pearlite and

sometimes other ferrite morphologies transformation temperatures.

The results obtained in this way show that increasing of austenite grain size and cooling rate has a significant influence

on decreasing of the ferrite transformation temperature.

Micrographs of different ferrite morphologies show that at high temperatures, where diffusion rates are higher, grain

boundary ferrite nucleates. 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. The results indicate that increasing the austenite grain size increases the rate and volume fraction of

intragranular ferrite in two different cooling rates. Moreover, by increasing of cooling rate, the austenite to ferrite

transformation temperature decreases and volume fraction of intragranular ferrite increases.