Analysis of different acicular ferrite microstructures generated in a medium-carbon molybdenum steel

Different acicular ferrite morphologies have been produced in a medium-carbon steel by isothermal treatments carried out at 400, 450, 500 and 550 deg. C. These treatments were interrupted at different times to analyse the evolution of the microstructure at each temperature. It has been found that at low transformation temperatures sheaves of parallel ferrite plates, similar to bainite but intragranularly nucleated, are formed, at intermediate temperatures a more interlocked microstructure is produced and at high temperatures, individual plates are generated following an incomplete reaction phenomenon. Taking as a reference a previous work, in the present one the role of molybdenum on the transition between these acicular ferrite morphologies has been investigated. The results show that molybdenum raises the temperature for which the transition between an acicular sheaf morphology and a more interlocked microstructure is produced, probably as a consequence of the reduction in the carbon mobility in ferrite and in austenite