The aims of the research programme are: - To obtain thin Si-Al casting with spheroidal graphite and a possibly non-existent primary chill depth. - To obtain bainitic structures in the casting, as cast alloying and Austenite stabilizers in appropriate percentage, in addition to Si and Al, such as Ni and/or Mo. The innovative aspects of this research are in the current industrial production of spheroidal graphitic thin castings with bainitic structure compared with those with normal pearlitic structure. The economic benefits of this research are the production of castings for mechanical uses which have the required qualities without the necessity of heat treatment, which can cause deformations. Castings, with good mechanical strength and a reduction of weight will have potential applications in the motor car industry. The cooperation between a Technical Corporate Research Centre such as C.T.I.F. with its specialised equipment, and an SME such as Elletrometallurgica Fusani with its practical experience, will be of great benefit to the success of the project. The technical innovation consists of using aluminium as an alloying element and simultaneously inducing graphitisation to produce a new bainitic grey iron with spheroidal graphite nodules. These new alloys could be used economically for thin-walled castings in automotive components such as exhaust collectors, oil pumps, gear boxes, small gears, etc.
Preliminary experiments showed that very high amounts of stable austenite can be easily obtained in a silicon aluminium cast iron after austempering in the upper range. The addition of copper or nickel is adequate to adjust bainitic hardness and control the transformation mechanism. If necessay, the addition of small amounts of molybdenum may improve hardness properties.
A suitable chemical composition is 3.2 to 3.7% carbon; 2.3 to 2.7% silicon; 1.4 to 1.8% aluminium; 0.2 to 1.0% manganese; 1.0 to 1.5% copper; 0.03 to 0.05% magnesium and 0 to 0.3% molybdenum. The composition shows good behaviour during austempering. It also has a strong potential for graphitization during solidification, enabling the production of thin castings without easily melted carbides.
A microprobe analysis of microegments showed an irregularity in the distribution of silicon in the matrix. In SG cast iron containing aluminium, the amount of silicon increases gradually from the area located near the graphite particles to the area at the end of solidification. This irregularity probably promotes the formation of homogeneous and stable structures but impedes the starting of the second stage of the transformation.
The creation of unwanted carbides introduce porosities which both weaken and reduce the melting point of the end product. To counteract this effect, a technique was developed using both inert gas and filtration to protect the melt. Additional improvements were made to the process by using bottom pouring.
It was also shown that the form of graphite present in east iron is an important determirant of its strength. This can be controlled by being very attentive to the temperature used, and to the precise duration of each phase in the austempering process. Experiments showed that bainitic transformation occurs optimally at temperatures between 350 C and 375 C.
The new process is industrially viable, and technical asssitance is available to companies interested in making use of the techniques developed.
THE INNOVATIVE ASPECTS OF RESEARCH ARE IN THE CURRENT INDUSTRIAL PRODUCTION OF SPHEROIDAL GRAPHITIC THIN CASTINGS WITH BAINITIC STRUCTURE, IN COMPARISON WITH THOSE WITH NORMAL PEARLITIC STRUCTURE.
REGARDING THE ECONOMIC INTEREST THE SEARCH AIMS AT THE PRODUCTION OF CASTINGS FOR MECHANICAL USES WHICH HAVE REQUIRED QUALITIES WITHOUT THE BURDEN OF THE HEAT TREATMENT, WHICH CAN CAUSE DEFORMATIONS.
THEIR CASTINGS, WITH GOOD MECHANICAL STENGTH AND A REDUCTION OF WEIGHT WILL HAVE POTENTIAL APPLICATIONS IN THE MOTOR CAR INDUSTRY. THE ASSOCIATION OF A TECHNICAL CORPORATE RESEARCH CENTER SUCH AS CTIF WITH ITS SPECIALISED EQUIPMENT, AND AN SMI SUCH AS ELLETROMETALLURGICA FUSANI WITH ITS PRACTICAL EXPERIENCE, SHOULD BE OF GREAT BENEFIT TO THE SUCCESS OF THE PROJECT. THE TECHNICAL INNOVATION CONSISTS TO USE ALUMINIUM AS AN ALLOYING ELEMENT AND TO INDUCE AT THE SAME TIME PHENOMENA OF GRAPHITISATION IN ORDER TO PRODUCE A NEW BAINITIC GREY IRON WITH NODULES OF SPHEROIDAL GRAPHITE.
THESE NEW ALLOYS COULD BE USED ECONOMICALLY FOR THIN WALLED CASTINGS IN AUTOMOTIVE COMPONENTS SUCH AS EXHAUST COLLECTORS, CARTERS, GEAR BOXES, SMALL GEARS, ETC...