Metallurgical reaction and deposit fusion boundary microstructure under a stationary plasma arc

Alloy element loss, decarburization in melted region and microstructural change of a deposit fusion boundary under a stationary plasma arc have been investigated by a first-order kinetic and interfacial microstructure. The cylindrical specimens of iron-based alloys, Fe-C, Ni-C, and Co-C alloys were locally melted by a plasma arc using argon plasma gas and Ar+H₂ shielding gas, and the rates of alloy element loss and decarburization in the melted region were measured. Moreover, the fusion boundaries experienced when nickel, iron, Ni-Fe filler metals were deposited on iron, Fe-C, nickel and cobalt base metals, were evaluated metallographically. The initial rate of alloy element loss decreases as follows: Fe-Al>Fe-Mn>Fe-Cr>SGI-Mg>Fe-Ni. The loss reaction mechanism is metallic evaporation and the rate seems to be limited by transport in the gas boundary layer. The magnitude of decarburization is as follows: Ni-C>Fe-C> Co-C. The decarburization rate in Fe-C alloy is assumed to be governed by a process involving mass transfer in the gas phase and the molten metal. However, at low carbon concentrations, the rate appears to be limited by transfer in liquid metal. Fusion boundary deposited with nickel filler metal on iron is regular, but with carbon added to iron, an infiltration of deposit metal into adjacent base metal occurs. The fusion boundary with iron deposited on nickel is irregular where thin Ni-Fe solid solution is formed. In a deposited fusion boundary of cobalt with iron, nickel, and Ni-Fe filler metals, FeCo compound formation occurs, with cobalt dissolving into the nickel deposit metal resulting in a tongue-like structure produced by nickel penetration and a fine columnar substructure formation produced by Ni-Fe diffusion.