Aluminium Diffusion Coatings on Medium Carbon Steel

In the present study, medium carbon steel has been hot dip aluminized (HDA) typically at 973 K/1–2 m, as well as calorized (CAL) by a cementation process, typically 1173–1223 K/2–4 h in a controlled atmosphere. It is found that in HDA coatings, the outermost pure Al layer is followed by predominantly hard intermetallic phases such as FeAl3 and Fe2Al5, the latter forming a serrated interface with the base. In CAL coatings, depending upon the process parameters (such as pack composition, temperature and time of calorizing), phases such as FeAl and Fe3Al form predominantly and the other Al-rich phases are presented to a lesser extent.Since the HDA presents a quasi-equilibrium situation with regard to phases formed between the base metal and the Al-coating (in view of the very short time of treatment), an attempt has been made to study the phase changes accompanying subsequent isothermal holding (773–933 K/10 m). This leads to the formation of stable equilibrium phases, possibly Fe3Al and solid solution next to the base, resulting in improved endurance to thermal shock (1123 K/WQ) and bending. In the CAL process, parameters have been optimised to produce a mixture of favourable phases, as defined by Pilling-Bedworth parameter.