poured. Te studied thermal moduli range was between 0.16 in. and 0.6 in. (0.4 cm and 1.5 cm). Te samples pro- duced included plates (3.9 x 2.4 in. [10 x 6 cm] and from 0.4 to 3.1 in. [10 to 80 mm] in thickness, varying each 0.4 in. [10 mm]), cylinders with the height equal to the diameter, and keel blocks Y2 (as per the standard EN 1563). To develop the CCT diagrams, the
2
with specific thickness differences, can be produced through engineered cool- ing with fully ausferritic microstruc- tures on all sections.
3 Te inputs of the model are the
minimum and maximum thermal modulus of the casting where an aus- ferritic microstructure must be guar- anteed and the mechanical property requirements. Taking into account these 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 ausfer- rite). As several alloy combinations can be considered, different criteria, such as economical or qualita- tive, 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-
castings were removed from the molds early and then air-cooled. Te cooling curve of each casting was recorded with a thermocouple inserted in the ther- mal center. With this information, the cooling rate for the different thermal
Results and Conclusions Based on the results of the
experiment, an Excel spread- sheet model was developed to establish if a specific casting,
Procedure To obtain different
cooling rates, castings with different thermal moduli and several geometries were
moduli was experimentally calculated for the temperature range of the eutectoid transformation. Te specimens were visu- ally inspected with an optical microscope. Te goal of the metallographic analysis was to find the pearlite occurrence and thus the minimum cooling rate to avoid the formation of pearlite as a function of the alloy composition. Second, the processing temperature to obtain as-cast ausferritic microstruc- tures was defined and related to the dif- ferent thermal moduli of the castings. Once poured and solidified, the test
castings followed a controlled cooling process. At the beginning, all samples were shaken free from their molds at the same time and then aircooled in the temperature range of ausferrite formation. At this time, the samples were introduced into an insulating
mines if the process is feasible for the maximum and minimum thermal 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
medium with a low thermal conduc- tivity. Te aim of this step is to main- tain a constant temperature to enable the ausferritic reaction to occur. Te isothermal transformation was defined as 90 minutes for all the samples. Finally, after the isothermal hold-
ing, the samples were air cooled to room temperature and the cooling curves calculated (Fig. 1). Te experi- mental data were used to obtain the relationship between the shakeout temperature and the thermal modulus. Tensile and hardness specimens
were machined from the samples. Te ultimate tensile strength (UTS), yield strength (YS) and elongation were measured as per the standard EN 1563:2011. In addition, Brinell hard- ness measurements were carried out per the standard ISO 6506-1:2005.
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
Fig. 1. This is an example of the cooling curves charted for different thermal moduli. October 2015 MODERN CASTING | 45
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