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Metal Casting Technologies : March 2007
16 www.metals.rala.com.au TECHNICAL FEATURE ontinuing developments in liquid metal treatment technologies have made major contributions to the quality, performance and reliability of metal castings. Advances in melt conditioning, in magnesium treatments to produce Ductile and Compacted Graphite Irons, and in inoculation techniques have ensured that Cast Irons remain key and competitive engineering materials. The combined interactions and influences of liquid metal treatments and other factors such as alloy composition, trace elements, section size & design and cooling rate etc. are covered in standard texts [1-3]. The aim of this short review is to outline the development of inoculants and inoculation practices, and to also reflect on recent improvements in our understanding and practical control of graphite nucleation. In liquid iron preparation the usual sequence of treatments involves: • Initial control of composition and inherent nucleation by melt pre-conditioning • Mg treatment (when Ductile or Compacted Graphite Irons are being produced) • Inoculation. The report begins by looking at the main aspects of inoculation and the materials used as inoculants, before commenting on how inoculation can be effectively integrated with the other treatments. INOCULATION: SOME KEY POINTS Inoculation is the term used to describe the process of increasing the numbers of nucleating sites from which eutectic graphite can grow during the solidification of flake, nodular and compacted graphite irons. The main aim of inoculation is to minimize the degree of undercooling of liquid iron during eutectic solidification, and hence to make sure that the resultant cast microstructures are completely free from eutectic carbides. Inoculation also plays a major part in the control of eutectic graphite morphology and distribution, and hence in control of the levels of pearlite and ferrite in matrix structures. In flake graphite cast irons inoculation refines the eutectic cell size of the austenite-flake graphite eutectic reducing the continuity of the weak graphite phase and thereby increasing tensile strength. Inoculation is not normally required for higher carbon equivalent flake irons which are used to produce lower strength grades but it is a must for the lower carbon equivalents used for flake iron grades requiring minimum tensile strengths in the range of 250 - 400N/mm2. In flake graphite (grey) irons inoculation is used to: • Prevent eutectic carbides, especially in thin sections and at corners • Ensure a uniform distribution of fine type A graphite flakes • Avoid the presence of undercooled graphite and the associated soft free ferrite in the matrix. The formation of eutectic carbide is called "chill". It increases the tendency for fracture of the casting during handling or service and gives hard spots that seriously reduce machinability. Figure 1 shows schematic views, using wedge test type samples, of the effects of inoculation in reducing chill and refining cell size in a flake graphite iron. Inoculation also avoids the formation of large eutectic cells consisting of the very finely branched form of graphite (undercooled graphite) that can grow at high degrees of undercooling when nucleation of the melt is low. Because of its high surface area undercooled graphite promotes the formation of a ferrite rather than a pearlite matrix (Figure 2). Free ferrite lowers strength, hardness and wear resistance. It also reduces machinability since it encourages the formation of a built up edge reducing both tool life and the quality of machined surface finish. In ductile irons eutectic carbides tend to form in the intercellular regions throughout the casting but are more likely to occur in thin sections (due to high cooling rates as in Figure 3) or heavy sections (due to segregation of carbide formers). The intercellular regions, in between the austenite-nodule eutectic cells are the last zones to solidify and will contain elements which segregate into the liquid: notably manganese, chromium and other carbide forming residuals such as niobium, vanadium and titanium. Inoculation must ensure that sufficient numbers of graphite nodules are nucleated to prevent the formation of intercellular carbides, and also to encourage a C By Dr. John Pearce Inoculation of Cast Irons: Practices and Developments Figure 1: Effect of inoculation on eutectic cell size and chill depth in wedge samples (schematic). (a) before inoculation (b) immediately after inoculation (c) fading due to holding time after inoculation before pouring .