The Effect of Cooling Rate and Austenite Grain Size on the Austenite to Ferrite Transformation Temprature and Different Ferrite Morphologies in Microalloyed Steels

The effect of different austenite grain size and different cooling rates on the austenite to ferrite 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 tvvo 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 ofaustenite to ferrite, pearlite and sometimes other ferrite morphologies transformationtemperatures. the results obtained in this tvay 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 fur 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 ofcooling rate, the austenite to ferrite transformation temperature decreases and volume fraction ofintragranular ferrite increases.