On the Formation of a Non-Traditional Pearlite Morphology

Abstract

Continuous cooling and isothermal dilatometry was performed for a binary Fe-0.3C alloy and a ternary Fe-0.3C-1.0Mn alloy at slow (< 0.1 oC/s) cooling rates and isothermally at temperatures below the equilibrium eutectoid reaction temperature but above the bainite start temperature (625 to 715 oC). Some of the test conditions produced an unusual morphology in which fine scale ‘sub-grains’ are decorated with carbide, with additional discrete carbide particles inside the ‘sub-grains’. A detailed investigation into the network carbide formation indicates formation during austenite decomposition, as opposed to a post lamellar transformation coarsening or spheroidization reaction, but only for select temperatures, and apparently only during isothermal conditions.

Microstructure of Pearlite Independently Nucleating on Grain Boundary Proeutectoid Phase in 100Mn13 Steel

“…When pearlite forms in steels, austenite grain boundary is its preferential nucleation position. − The pearlite can nucleate directly from the original austenite grain boundary and become a lamellar colony by synergistic growth of two constituents (ferrite and cementite) into austenite. − In addition, the pearlite also can develop from proeutectoid phase, which means that either proeutectoid ferrite or cementite precedes pearlite and becomes pearlite. − The ferrite always acts as proeutectoid phase in hypoeutectoid steel, ,,, whereas proeutectoid cementite is dominant in eutectoid and hypereutectoid steel. ,, As an example, Mangan has observed lamellar pearlite developing from proeutectoid cementite in Fe-0.77C-0.58Mn steel. This kind of pearlite also exists in aged Fe-12Mn-0.8C high carbon high manganese steel. , Furthermore, the distinguishing feature of pearlite forming in this way is that pearlitic carbide is the same as proeutectoid carbide, no matter the carbide type or crystallographic orientation. ,, According to this feature, Mangan proposed that pearlitic carbide develops from proeutectoid carbide directly.…”