Interfacial microstructure observation and nanoindentation measurements in mild steel/HT780 clad plate

The microstructures at the cladded interface between two low alloy steels, a mild steel and HT780, having different transformation temperatures, were characterised using optical microscopy, SEM and nanoindentation. To study the effects of heat on the near interface structures, the HT780 clad steel plate was welded and heat treated. It was found that cladding produced an interfacial layer with greater hardness than was found in either steel type tested and that carbon was enriched in this layer. During welding, the interfacial layer broadened and its hardness increased in the heat affected zone, which was heated to approximately the transformation temperature, but then softened again at temperatures above Ac3. Hardening of the interfacial layer occurred under various cooling rates. These microstructural changes are potentially due to the difference between the transformation temperatures of mild steel and HT780.