As-cast ausferritic ductile iron materials obtained by a controlled cooling process

Resumen

Within the framework of the spheroidal graphite cast irons, the ADI materials are the ones that present the best mechanical properties. Due to their strength/ductility relationship, they can replace forged steel in some applications and due to their strength/weight ratio, even aluminium alloys. These materials base their optimal mechanical properties on their ausferritic microstructure. This ausferritic microstructure is conventionally obtained applying a heat treatment process called austempering to the as-cast foundry castings. This is an aspect that hinders the market expansion of the ADI materials, because it implies a cost increase, an energy consumption growth, and a lead time extension, together with a complexity increase due to the co-work between the foundry and the heat treater. Trying to overcome this handicap, on the present research work, the obtention of spheroidal graphite iron castings with an as-cast ausferritic microstructure is approached, that is, without the need of a subsequent heat treatment. This way, the negative aspects that the austempering heat treatment can involve could be overcome. To that aim, the technology necessary to face an engineered cooling process was studied. The practical basis of the actions taken during the cooling process so that an as-cast ausferritic microstructure could be achieved, was to perform them as simple as possible, trying to facilitate the future industrial application of the methodology. For this reason, air cooling was defined as the way to avoid the pearlitic nose and an insulating medium was considered to maintain a constant temperature during the isothermal transformation, step that allows the formation of a fully ausferritic microstructure. Together with the necessary technology, the main process parameters were studied, and their optimal working ranges were defined. These parameters include: Alloying elements: The minimum necessary alloy content in order to evict pearlite formation was specified. Nickel, molybdenum, and copper were used to this purpose, being these three the main alloying elements for the ADI materials and three of the elements that most strongly affect the austemperability of the alloy. Shakeout temperature: The influence on the final microstructure and on the mechanical properties of the shakeout temperature is studied. This is the moment at which the accelerated cooling rate due to the air-cooling starts, once the castings are extracted from the sand mould. Austempering temperature and time: These are the parameters that have a major influence on the ausferrite formation and consequently on the mechanical properties. A deep study of the kinetics of the ausferrite formation is performed, defining the temperature and holding time effect on the ausferritic microstructure (upper or lower ausferrite) and on the mechanical properties. The processing window in terms of time is defined. Having defined the technology and the optimal working ranges of the main process parameters for an engineered cooling process, an advanced characterization of the material is performed. Dynamical properties, low temperature performance and corrosion behaviour are analysed and seen if they agree with the conventionally obtained ADI materials results. The former analyses were carried out considering standard cast samples. The next step was to produce a technology demonstrator prototype. A steering knuckle was chosen for that purpose. The highest and the lowest section sizes were studied, and their final microstructure defined. This was related to the mechanical properties. Finally, trying to approach the industrial application of the as-cast ausferritic technology, a mathematical model was developed. This model establishes if a specific casting, with certain thickness differences, can be produced through engineered cooling and if it is possible to obtain fully ausferritic microstructures on all its sections. If the technology application is feasible for that casting, then the minimum alloy content and the optimal shakeout and austempering temperatures are defined for the different sections. The austempering temperature is related to the ultimate tensile strength and hardness, so that its optimal value is defined based on the desired mechanical properties. Resulted from all the developed experiments, the as-cast method to obtain an ausferritic matrix in ductile iron castings was proved to be a valid methodology to produce cast parts as an alternative to conventionally austempered ADI materials. This technology is now ready to be industrially applied.