Abstract
The thermal treatment involved in galvanising affects the mechanical properties of cold drawn pearlitic steels. There is a trough in the ductility at short heating times, which recovers after further heating, but only at the expense of strength. The objective of this work was to gain an understanding of the underlying processes that lead to these changes in properties. A 0.85 wt-%C, vanadium microalloyed, fully pearlitic steel was studied. Samples were heat treated for up to 90 s in a salt bath at 500°C to simulate the galvanising process. The effects of the postdrawing heat treatment on the microstructure and carbon atom distribution were investigated using differential scanning calorimetry, SEM, TEM, and atom probe field ion microscopy. Carbide fragmentation occurs during the heat treatment, representing the onset of the spheroidisation process. The carbon concentration in the ferrite matrix remains low in all heat treatment conditions. However, localised regions of ferrite containing approximately 2.5 at.-%C are observed, indicating the presence of Cottrell atmospheres around dislocation lines. Planar regions enriched in carbon are also seen, which demonstrate segregation to ferrite subboundaries. A simple model of the carbon redistribution process is proposed.
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