property requirements. Taking into account these
Fig. 1. This is an example of the cooling curves charted for different thermal moduli.
inputs, the model analyzes the required alloying elements in the first step. By means of an iterative method, the model calculates the minimum nickel, molybdenum and copper content to prevent the formation of pearlite (which would prohibit the formation of the desirable ausferrite). As several alloy combinations can be con- sidered, different criteria, such as economical or qualitative, could be the decisive factor in selecting the proper alloy. In the second step, the model deals with the shakeout process. Based on the relation between the shakeout temperature and the thermal modulus, the model deter- mines if the process is feasible for the maximum and minimum ther- mal modulus of the component and, if it is, the optimum shakeout temperature. The third step deals with the
isothermal transformation tem- perature window. For the same maximum and minimum thermal modulus, the model determines if it is feasible to achieve the target microstructure and, if it is, defines their optimal isothermal transfor- mation temperatures, based on the required mechanical properties in terms of ultimate tensile strength and Brinell hardness. The two critical temperatures— shakeout and isothermal transfor- mation temperatures—have to be inside defined ranges that permit the formation of an ausferritic microstructure that meets the requirements of the ADI materi- als. Depending on the different thermal moduli of the casting, these temperatures will change. Based on these changes, the model calculates the thickness window in which this methodology is feasible and by extension, if a given casting could be produced with the engi- neered cooling process. Mechanical properties differ
based on the thermal modulus and processing temperature, which result in different ADI grades
42 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2015
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