This study deals with the solidification behaviour of cast iron with nodular graphite. By measurement of the “apparent shrinkage” the real feeding requirement can be determined under operating conditions. By means of cooling curves and measurements of dilatation the mode of solidification is established. The results are in accordance with today's knowledge of the morphology of crystallisation and offer an operating hypothesis for feeding technique in practice.

It is well known that ductile iron is more prone to the formation of internal defects and to the swelling-out of the mold than grey iron. This is likely related to differences in the way the volume changes due to solidification and graphitization occur for these two cast irons. Direct study of these phenomena is difficult because interdendritic feeding and graphite expansion are intimately related.

A simple physical model is proposed to simulate the effects of a few variables on the tendency for a S.G. cast iron to lead to a casting with internal defects: its chemical composition, inoculation and the rate of heat extraction.

The aim of the model is to calculate the pressure of the residual liquid at any point of the casting during its solidification. A special attention is payed to the pressure drop due to the movement of the residual liquid in the mushy zone. Therefore the evolution of the fractions of liquid, austenite and graphite are taken into account for calculating the local volume changes which are assumed to be the main driving forces for the liquid flow. In the same time, the evolution of the permeability of the mushy zone is estimated for calculating the liquid flow pattern and the pressure drop pattern.

The numerical values for the quantities which describe the solidification of the cast iron are issued from quantitative image analysis of samples quenched during directional solidification (Q.D.S.).

The factors resulting in porosity in various types of cast iron were fairly well understood by 1960. These factors are now reviewed. The importance of the details of the solidification sequence is stressed and the special features of the eutectic reaction in nodular cast iron are emphasized.

By way of unidirectional solidification, the critical parameters of the transition from a metastable to a stable eutectic in technical cast-iron alloys were investigated. A critical growth rate which was determined for near-eutectic alloys can be attributed to the stable-to-metastable transition. The structural parameters for the metastable eutectic were defined as a function of the growth rates.

Gray iron technology has advanced greatly in the past half century. Much has been learned about the mechanisms of nucleation and solidification and about solid state transformations. This information has alloweu gray iron castings to remain competitive for structural components. Low cost and excellent castability make them versatile for a wide variety of industrial components including engine blocks, cylinder heads, housings, manifolds, hydraulic valve bodies and many other castings of similar complexity. Gray cast iron provides the technical advantages of high strength, soundness, good machinability, dimensional stability and uniformity of properties usually required by such cast components. Achieving these characteristics is dependent, however, on achieving proper and uniform structures tnrougnout all sections of the casting. This objective is reaoily attainable in judiciously alloyed gray cast iron.

The effect of small additions of Si, P, Cr, Mn and Ti on transition velocities from grey to white and white to grey of pure Fe-C eutectics has been measured by varying growth rates in directional solidification experiments. Growth undercoolings were measured as a function of growth rate, while white and grey eutectic temperatures were obtained from DTA experiments. As a result of these experiments, alloying elements can be classified into three categories : graphitizing (Si, P), carburizing (Cr) and “opposite effect” (Mn, Ti). Each category is characterized by a given influence on eutectic temperature, growth undercooling, and nucleation of cementite. A qualitative model which permits determination of the influence of alloying elements on these latter three parameters is proposed.

By combining heat extraction laws with kinetic growth laws, the solidification process of nodular, grey and white cast iron have been calculated. The risk for formation of white structure during solidification of grey and nodular cast iron was calculated. The calculations was done for different thickness of the casting as well as, as a function of the number of growing units. The fraction of white structure was also calculated.

Minor additions (0.05-0.2 wt.%) of sodium chloride, hexachloroethane and elemental sulphur to commercial white iron melts have been found to enhance the kinetics of first stage graphitisation during subsequent annealing of white iron samples. The optimum dose of sodium chloride and hexachloroethane addition is around 0.1%. Yield strength and ductility of annealed test bars treated with NaCl or C2Cl6 compare favourably with those of untreated test bars. Sulphur treatment causes slight deterioration in mechanical properties. Fully grown nodules in both treated and untreated samples appear porous under SEM. Possible mechanisms of acceleration of graphitisation in the treated samples have been suggested.

The present article aims to show that high intensity compressive prestressing applied to white iron to be malleabilized, alters the final structure of the iron after malleabilization. The graphite nodules are finer and more numerous and ferritisation is greater. The phenomenon increases, with several factors as follows: intensity, repetitionmultiaxiality of prestress and hot prestressing.

Initial results of the study of correlations between finer graphite structure and certain mechanical properties show that there is little change to static properties, but some dynamic properties are improved (1).

It is well known that the gaseous elements have detrimental effects on the annealability of white cast iron for malleablizing. The present knowledge, however, is based on the white cast iron containing about 1.0 % Si. Therefore the author made a series of experiments by using the white east irons with silicon content ranging from 1.0 % to 1.8 %, and carr ied out multiple regression analyses of the measurements. The statistically meaningsful results can be summarized as follows:

1) In the silicon range of 1. 01 ∼ 1. 60%, the first stage graphitization is retarded by the soluble nitrogen. However, when silicon exceeds 1. 6 %, the opposite is true.

2) In the silicon range of 1. 01 ∼ 1. 40 %, the second stage graphitization is retarded by the soluble nitrogen. The degree of' the retardation increases as the specimens contain less silicon. When more than 1. 40 % Si is present, the second stage graphitization proceeds independently of soluble nitrogen content.

3) Oxygen does not at. all influence the first stage graphitization, while the second stage graphitization is slightly retarded by oxygen. This is due to nodule count, which decreases as oxygen content increases.

The kinetics of first stage graphitization was studied in white cast irons of different compositions. These irons were presubjected to various isothermal anneals at 300–500°C, after being step-quenched from the intercritical (austenite and carbide) region. As a consequence of such pretreatment, the kinetics of post first stage graphitization were substantially accelerated. Both light and scanning electron metallography hive indicated that such acceleration arises due to the creation of vast areas of austenite/carbide interfaces upon cooling to and during subcritical transformations. These interfaces act as favourable nucleation sites for graphite and additionally they provide shorter diffusion paths for carbon. The latter also allows faster growth of graphite nodules. Effects of varying Si and Hn contents of the cast iron were determined. The practical significance of such treatment in providing] faster malleabilization and uniform distribution of graphite nodules is thus evident.

The individual and combined addition of nickel and aluminium to S.g. iron influences several solid state transformations. Nickel depresses the temperature range for reversion of ferrite and pearlite to austenite on heating, while aluminium raises it. Combined addition of nickel and aluminium restores the temperature range closer to that for unalloyed iron. But nickel enhances the kinetics of ferritisation during tempering and weakly retards recrystallisation of ferrite grains. Aluminium strongly retards both the ferritisation and recrystallisation processes. Ferritisation kinetics is enhanced and recrystallisation of ferrite grains is also hastened in alloys containing nickel and aluminium. Alloys containing nickel and aluminium also respond to agehardening treatment in the temperature range 450–550°C after a solution treatment-cum-ferritising anneal at 700°C. On the basis of X-ray diffraction and electron microscopic studies, a probable mechanism of ageing has been proposed.

As heat treatments of metallic alloys generally require high temperatures, energy saving cycles are increasingly looked for. Castings offer the advantage over wrought products of possible hot shake out as they are just made.

The object of this study was to observe the structure of a SG cast iron resulting from hot shakeout right after the end of solidification, followed or not by a 1 000°C holding and subsequent heat treatment.

The austenite then presents the peculiarity of not having been re-generated by heating up from an initial structure as it is the case in conventional cycles.

This study has clearly showed the influence of composition heterogeneity within the solidification cell on the response to heat treatment. Along the solidification cell boundary, an increase in carbon and manganese contents was found while silicon decreased. Along the SG nodules, the opposite is true. Between these two zones, gradients vary. The presence of these three areas results in having three different CCT curves and, therefore, a varying response to heat treatment. This is even magnified by the section size of the sample because the cell size increases, but this is minimized by the decrease in quenching rate.

Structural differences observed in hot shake out samples and re-heated samples, both held or not at 1 000°C, and then cooled down, are directly related to these composition heterogeneities:

- hot shakeout cast iron shows a high hardenability which varies with the holding time at 1 000°C;

- after a ferritizing treatment, an homogeneous matrix is difficult to obtain from a hot shakeout cast iron;

- after austempering the structural heterogeneity of the cell boundary is greater for the hot shakeout cast iron;

- after the pearlitic transformation, the structure of the hot shakeout cast iron is similar to that resulting from conventional treatment.

For an economical purpose, an austenitic spheroidal graphite cast iron grade, containinq nickel-manganese, has been developed to replace Ni-Resist cast iron, type D2M, for cryogenic applications.

Several tests were carried out to produce semi-industrial. heats of 200 kg so as to cast samples with a thickness ranging between 25 and 75 mm, and a variable nickel and manganese content. The resulting grades were compared each other and with Ni-Resist cast irons, before and after heat treatment, for their metallographic structure, their hardness, their mechanical tensile properties at room temperature and their impact values at temperatures down to - 150° C.

The results show that it is possible to produce cast iron grades with 7 to 10 % manganese and 9 to 11 % nickel contents, featuring, at quenched state, better mechanical tensile properties at room temperature than Ni- Resist D2M with a 22–25 % Ni content, for roughly equivalent impact values at low temperatures. For as-cast material, the properties obtained vary with material thickness, and ductility is steeply decreasing for thicknesses under 30 mm.

Weldability of Ni/Mn cast iron can be compared to Ni-Resist D2M weldability; machinability however seems more difficult, especially when boring quenched material.

Generally speaking, the overall properties of these new grades and their lower cost-price if compared to Ni-Resist cast iron, should lead to promising outlets for low temperature uses and/or applications requiring high-level mechanical properties.

In gray cast iron carbon diffuses irreversibly and the result is a growth remaining castoff voids and graphite redistribution. The origin of primary growth is in the mechanism of carbon diffusion which is controlled by a ratchet at graphite where carbon is carried by vacancy flow. Growth of flake cast iron in oxygen reduced air at low oxygen content is accounted for by self carburizing due to a reaction of penetrated oxygen and graphite flakes. For the irreversible carbon migration by gaseous carrier another ratchet is suggested. Cracking theory for primary growth is replaced by castoff void theory, and oxidation theory is modified.

The thermal expansion of grey cast iron was studied in six different alloys. The expansion was determined at two different heating and cooling rates. It was found that the expansion was influenced by the heating and cooling rates. The thermal expansion was larger than the thermal expansion in steel and the thermal expansion also increased with increasing temperature. The thermal expansion was theoretically analysed. The temperature dependence was accounted for, by considering the fact that the carbon content in austenite and ferrite increases with increasing temperature.

The influence of microstructure on the properties of austempered ductile cast irons have been studied along with an analysis of the necessary process control to insure consistent properties. Both austempered ductile iron transformed at high temperatures with a matrix structure of carbide-free ferrite and substantial amounts of austenite, and austempered/bainitic ductile iron transformed at low temperatures with a more characteristic bainitic structure also containing austenite have been studied. The good toughness and ductility of these high strength ductile irons is due to the microplasticity associated with both the ferrite and austenite phases; however carbide precipitation during the later stages of austempering reduces room temperature toughness considerably and results in a change to a quasi-cleavage fracture mode. The influence of casting quality control and heat treatment control on the microstructures and properties obtained are discussed.

Experiments were carried out on unalloyed ductile cast iron to evaluate the % retained austenite (%RY) and its lattice parameter as a function of austenitizing time and temperature for austempering temperatures ranging from 270 to 420°C. Results are related to expected carbon levels in the gamma iron matrix at the austenitizing temperature. It is shown that the rate of austenitization can be described as a two step process and experiments demonstrate that 900°C austenitizatlon is complete after 8m. An 1100°C homogenizatdon has been shown to have a small effect upon %RY and rate of austenitization.

Interest in austempered bainitic ductile irons has increased dramatically in the past few years as more and more successful applications are being reported. The remarkable combination of properties attainable has caused this material to emerge as a new class of ductile iron. Austempered ductile irons achieve strength levels as high as twice those of standard ductile iron grades at the same level of toughness and ductility, see Figure 1. In addition they respond to work-hardening treatments at the surface and thus have exceptionally high bending fatigue strength and wear resistance. These good properties are directly related to the unique bainite-austenite microstructure produced by austempering.

Using a dilatometer the isothermal transformation of austenite to bainite has been studied in ductile cast iron with 0.05 % Mn and a silicon content varying from 2.4 to 3.8 %. The alloys were austenitized to a carbon content in the matrix of 0.65 %. It appears that silicon retards the formation of carbides in the upper bainite region (400 °C), resulting in an amount of retained austenite up to 40 % present in the final structure at room temperature. Silicon improves the strength; in the lower bainite region the yield strength in particular. An elongation up to 10 % or more is obtained after austempering at 400 0C independent of the silicon content.