KINETICS OF FERRITE TO AUSTENITE TRANSFORMATION IN A HIGH-STRENGTH LOW-ALLOY STEEL CONTAINING NB

The kinetics of ferrite to austenite transformation has been studied i n a high strength low alloy steel containing Nb as the carbide forming element. The growth rate and volume fraction measurements were made u sing optical microscopy. Three distinct stages of the transformation h ave been observed. The dissolution of pearlite occurs in the first sta ge. In the second stage, the growth of austenite occurs by the diffusi on of carbon into austenite. However, due to paucity of the carbon dif fusion field the transformation at this stage can not continue under a full diffusion field but occurs under a truncated diffusion field. In the third stage, austenite grows into ferrite by the diffusion of Mn in ferrite. The parabolic growth rate constants observed experimentall y match well with the calculated values.