The effect of nodularity and nodule count on the mechanical properties of A.D.I.

Abstract

In Victorian times almost, anything was likely to be made of cast iron street lamps, domestic fire places, railings, window frames and ornamental water fountains. One important reason for the widespread use of cast iron was the fact that in those days not all pig iron produced was suitable for conversion into commerciable steel. In particular those pig irons high in phosphorus and sulphur were suitable only for ornamental castings which required little strength. Today cast iron is used widely for engineering purposes and its technology, like that of other alloys, continues to be developed at highly sophisticated levels. Grey cast irons differ from other engineering materials in some important respects. These are highly sensitive to the effects of cooling rate in the mould, that is section thickness and its non-elastic behaviour under tensile stress and differing stress/strain properties in tension and compression. The prime reason for the differing properties in tension and compression is that under tension the matrix is not continuous in a grey cast iron but is broken up by graphite flakes which have little strength in tension and give rise to stress concentrations in the matrix. The effect of this is that the stress/strain curve for tensile stress does not follow a straight line and on removing the initial stress a permanent deformation remains. The effect however is small for tensile stresses normally applied to grey cast irons and can usually be ignored. The low average value of the modulus of elasticity of grey cast irons in comparison with steels is also explained by the presence of graphite flakes which act as discontinuities in the matrix. Typical mechanical properties for grey cast iron are tensile strengths in the range 150-400 N/mm2 with 0% elongation. The introduction of spheroidal-graphite iron in 1948 was without doubt the most important event during this century in the iron trade since in S.G. iron the engineer finds a worthy competitor to cast steel but often at much lower cost. Nodular irons differ from grey cast irons in that the free graphite is present as compact isolated nodules rather than as flakes dispersed throughout the material. The formation of the spheroidal graphite is effected by adding small amounts of cerium or magnesium to the molten iron just before casting. Since both of these elements have strong carbide-forming tendencies, the silicon content of the iron must be high enough (at least 2.5%) in order to prevent the formation by chilling, of white iron in thin sections. Tensile strength of 370 N/mm2 with an elongation of 17% are usually achieved following' annealing while tensile strengths of 800 N/mm2 with an elongation of 2% are usually achieved following normalising, air-quenching and tempering. It was not long after it was established that heat treatments used for steel are also applicable for S.G. irons that the first A.D.I. was produced. A combination of mechanical properties superior to those obtained by normalising, hardening and tempering treatments can be obtained by austempering. Austempering in the lower temperature range yielded properties with tensile strength of 1600 N/mm2 and 1-2% elongation whilst austempering in the upper temperature range yielded properties with tensile strength of 1100 N/mm2 up to 16% elongation. It soon became realised that many variables influenced the properties obtained. The heat-treatment variables are austempering temperature and time and austenitizing temperature and time. Work carried out by Grech (25) showed that for the alloy investigated here, (1.6% Cu/1.6% Ni), optimum mechanical properties can be obtained following austenitizing at 9QOoC for 2 hours and austempering at 35QoC for 3 hours. Other variables such as composition effect have been reasonably well researched though there are still some unanswered questions. Other variables that affect properties are the quality of the as-cast material namely, the nodule count and nodularity. These have to date never been investigated. It is therefore the aim of this project to establish the effect of these variables on the mechanical properties of A.D.I. Objective: The aims of this thesis were: 1) to study the effect of nodularity on the mechanical properties of an alloyed Cu/Ni A.D.I. To obtain different percentage nodularity, the magnesium contents were altered between 0.02% and 0.06% 2) to study the effect of nodule count on the mechanical properties of A.D.I. Test bars of different section thicknesses (namely 25,50,75 and 100mm) were cast. The different section thicknesses produce different cooling rates which in turn effect the nodule count.