Nucleation Sites of Austenite from Lath Martensite in Fe-0.2%C-X Alloys

Abstract

The influence of alloying elements on the preferential nucleation sites of austenite formed from lath martensite in low carbon low alloy steels by heating above A_C1 temperature was studied. The main results obtained are as follows.
1) In plane carbon steel and low alloy steels containing 1%Mo, 0.2%Nb, or 1%Cr (carbide forming elements) as the third element, prior austenite grain boundaries act as preferential nucleation sites of austenite during the early stage of austenite formation. As the precipitation proceeds, austenite tends to nucleate also inside prior austenite grain.
2) In an Fe-C-1%Ni alloy, not only prior austenite grain boundaries but also packet boundaries of lath martensite act as the preferential nucleation sites of austenite during the initial stage of austenite formation. As the precipitation proceeds, block boundaries and lath boundaries also become the nucleation sites of austenite. The similar precipitation behavior was observed in the alloys containing 2%Mn, 2%Cu, 1%Si, or 1%Co (non-carbide forming elements) as the third elements.
3) The effect of the amount of third elements on the preferential nucleation sites of austenite was also studied in the alloys containing Si or Ni. In both cases, nucleation of austenite inside prior austenite grains is more dominant in the higher alloys.
4) The effects of the formation temperature of austenite and prior austenite grain size on the preferential nucleation sites of austenite were found to be small.
5) In the alloys containing Ni or Si, nucleation of austenite inside prior austenite grains was clearly observed. Such a phenomenon was approximately explained by the decrease of boundary tension due to micro-segregation of alloying elements on the grain boundary.