Local melting of nodular cast iron by plasma arc

Abstract

The microstructural changes in the fusion boundary area of nodular cast iron (NCI) were investigated by plasma arc local-melting with and without a filler metal. The NCI cylinder specimen was locally melted by the static plasma torch for a very short time, adopting the non-keyhole method. The microstructures produced in the fusion region, fusion boundary region and heat-affected zone (HAZ) were examined metallographically. In the absence of filler metal, the fusion boundary area is composed of ledeburite in the fusion region, and martensitic, troostitic—ferritic layers for the short arc-time and sorbitic, sorbitic—ferritic layers for long arc times in HAZ. Although the formation of white iron in the fusion region may be caused by a relatively high cooling rate, the major cause is due to supercooling effects produced by the decrease of magnesium content in the molten iron directly under the plasma arc. Using nickel and Ni—Fe filler metals, Nimartensite appears in the deposited fusion boundary, and the HAZ region is composed of ledeburitic and martensitic or sorbitic layers. The ledeburitic layer thickness varies with the melting point of the filler metal. The diffusion of nickel from the deposit metal to the HAZ occurs at least until the fused base metal in the HAZ.