Feasibility of particle reinforcement in the casting of a ductile iron gate valve

Abstract

The mining industry relies on gate valves to shut off flow in pipe systems. The demand for, specifically, ductile iron valve housings is steadily increasing in mining applications where cost, wear, and corrosion is a concern. The South African iron and steel industry faces several challenges as the number of foundries in the country have experienced a decline of 22%, where 24 foundries have closed down between 2007 and 2014. To increase global competitiveness of small foundries the cost and thus cast iron volumes used in the casting of gate valves must be reduced. Reduced volumes of cast iron, used in the casting of valve housings, will inevitably result in thinner wall thicknesses and the more likelihood of failure, due to reduced strength. In this study strengthening mechanisms of ductile iron in thin walled castings have been reviewed; and the feasibility of an appropriate casting method, based on a strengthening mechanism, have been tested. In similar studies, strengthening of metallic alloys have been achieved through dispersion of particles in a matrix, through the method of compo-casting; however, little research has been done on further improvement of cast iron. Fly ash, a by-product of coal fired power stations, is an abundant resource, and have been used, in recent studies, to, successfully, improve mechanical properties, such as the tensile strength, impact strength, wear resistance, and hardness of aluminium alloys. The aim of this study was to improve the strength of SG42 ductile iron, through the incorporation of fly ash particles, using conventional casting methods. Compo-casting was used as casting method wherein 0.8 wt% of fly ash particles were added to the melt, the melt was stirred, and cast, in the semi-solid phase, to improve the wetting of the particles by the melt and obtain a homogenous casting. Through the examining of fracture surfaces, by scanning electron microscopy, it was found that wetting of the fly ash particles were not sufficiently improved to successfully achieve the necessary strengthening mechanism. As a result, the tensile strength of the fly ash reinforced ductile iron experienced a decrease. The impact strength experienced a decrease through the addition of fly ash particles to the matrix. Further it was found that the porosity of castings increased as a result of semi-solid casting.