Plasma surface hardening of ASSAB 760 steel specimens with Taguchi optimisation of the processing parameters

Surface hardening of steel components is traditionally done either by oxy-fuel, induction or laser hardening. Plasma arc is a new hardening technique which makes use of a small controlled stream of ionised gas to heat the material to its austenitising temperature. The heat is conducted rapidly into the bulk of the specimen causing self-quenching to occur and the formation of martensitic structures. This paper describes the optimisation of the processing parameters for maximum hardened depth of ASSAB 760 (equivalent to AISI 1045) steel specimens of 6 mm thickness by using a Microplasma-50 plasma arc machine with the Taguchi method. A 4-factor 3-level (L9) orthogonal array (OA) was used in the experiment. The optimum settings for the four chosen processing parameters, namely arc current (35 A), torch traversing speed (5 mm/s), plasma gas flow rate (1 l/min) and arc gap (2.3 mm) were established and a confirmation experiment was done. The maximum depth of the hardened zone obtained was 0.36 mm and the maximum hardness observed was 753 Hv. Lath martensitic microstructures were present in the hardened zones.