The effect of austempering processing variables on the transformation kinetics, microstructure and mechanical properties of the Fe-C-AI family of spheroidal graphite cast irons, in which the silicon is below 0.2wt% has been investigated. These alloys are described as SO Al cast irons. In the preliminary experiments the spheroidisation and inoculation treatments for SO Al were developed and the microstructure and tensile properties in the as cast and normalised SO Al cast iron at 2.2 and 3.2% of Al examined. The results show that the sequence of spheroidisation of SO Al iron differs significantly from those observed in SG Si iron. Whilst the treatment to introduce some Mg into the melt is required in both irons to alter the growth habit of graphite from flake to spheroidal, the behaviour of the irons towards the subsequent inoculation with various inoculating alloys is quite different. A wide range of mechanical properties of SG Al irons can be obtained, similar to SG Si irons, in the as cast or heat treated conditions. The effects of austenitizing temperatures of 850, 900 and 950°C, austempering temperatures of 300 to 450°C and austempering times of 5 to 300 minutes on impact and tensile properties have been investigated. X-ray diffraction has been used to determine the volume fraction, lattice parameter and carbon content of retained austenite and bainitic ferrite produced under different austempering conditions. Optical and scanning electron microscopy were used to analyse the microstructure. It has been shown that the basic mechanisms for the isothermal transformation of austenite to bainite are essentially similar to those of austempered SO silicon cast irons for both lower and upper bainite. This can be attributed to the strong graphitising effect of aluminium which delays the formation of transformation carbides. However, the higher carbon contents of retained austenite in SG Al iron show the stronger carbide inhibiting characteristics of aluminium compared with those of silicon. The first stage starts with the nucleation of bainitic ferrite from the grain boundaries and adjacent to the graphite nodules. The second stage in upper bainite fonnation is the growth of ferrite, during which carbon diffusion occurs ahead of the ferrite-austenite interface thus enriching the surrounding austenite. This is followed in the third stage by carbide precipitation in the high carbon retained austenite. In lower bainite fonnation the nucleating ferrite is supersaturated with carbon, and carbide precipitation occurs inside ferrite plates. The results show that the rate of fIrst stage transfonnation is higher in SO Al than in the corresponding SO Si iron, and the high carbon retained austenite has a longer life time with a high carbon content. Increasing the austempering temperature was shown to increase structural coarseness and retained austenite content, leading to reduced strength and hardness and improved impact properties and ductility. Retained austenite produced in austempered SO Al iron showed more stability at a high austempering temperature of 450°C, compared to SO Si iron. This was attributed to the graphitising potential of aluminium which suppresses the formation of carbide at that temperature. Austenitizing temperature was found to control the carbon content of both the matrix and of the retained austenite. Increasing the austenitizing temperature was shown to increase the matrix carbon content and to slow the bainitic reaction. Microstructures obtained at higher austenitizing temperature revealed two types of retained austenite: one with a film morphology and the other in a blocky form surrounded by ferrite growing in different directions. The length of bainitic ferrite appears to increase with austenitizing temperature. Austenitizing temperature had a greater effect on hardness in the lower bainitic region and on impact values in the upper bainitic region. The results obtained show that the controlling austempering variables for an SO Al iron of a given starting composition and microstructure are closely similar to those of SO Si iron. The transformation mechanisms are also similar, but the transformation kinetics differ and, to a small extent, microstructural differences give rise to differences in properties between the two types of irons.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:520795 |
| Date | January 1991 |
| Creators | Boutorabi, S. M. A. |
| Publisher | University of Birmingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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