1


Background One of the key points


to achieve as-cast ausfer- ritic microstructure is to define the minimum


cooling rate. Continuous Cooling Trans-


formation (CCT) diagrams were developed for three different alloys with chemistry in the range of 3-5% Ni, 0-0.2% Mo and 0.1- 1% Cu by weight. The change of the minimum cooling rate to prevent the formation of pearlite (pearlite would keep the ausfer- ritic microstructure from forming) was linked to the content of the main alloying elements (nckel,


2


molybdenum and copper). Shakeout and isothermal


transformation temperatures have a major infl uence on the fi nal micro- structure. Isothermal transformation refers to the transformation of the iron’s microstructure at constant temperature. Diff erent thicknesses in the same casting involve diff er- ent processing temperatures. To be successful, engineered cooling must provide a fully ausferritic micro- structure in all the sections of a casting or at least in the sections de- fi ned by the designing engineer. For this reason, the thickness window where completely as-cast ausferritic microstructures are obtainable must


be clearly defi ned. T e goal of the research was to


develop an experimental model able to defi ne the thickness window where the as-cast ausferritic micro- structures can be guaranteed. Ad- ditionally, the model was validated in a semi-industrial process for the chemical composition range. When the thermal moduli of a casting are in the range of the process- ing thickness window, the model defi nes the optimum processing parameters with the aim of obtain- ing mechanical properties (such as ultimate tensile strength and hard- ness) that meet the requirements of the ADI materials.


Procedure To obtain different


cooling rates, cast- ings with different thermal moduli and


several geometries were poured. The studied thermal moduli range was between 0.16 in. and 0.6 in. (0.4 cm and 1.5 cm). The samples produced 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 castings were removed from the molds early and then air-cooled. T e cooling curve of each casting was recorded with a thermocouple inserted in the thermal center. With this information, the cool- ing rate for the diff erent thermal


3


moduli was experimentally calcu- lated for the temperature range of the eutectoid transformation. T e specimens were visually inspected with an optical microscope. T e goal of the metallographic analysis was to fi nd 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 tem-


perature to obtain as-cast ausfer- ritic microstructures was defined and related to the different thermal moduli of the castings. Once poured and solidi-


fied, 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 air- cooled in the temperature range of ausferrite formation. At this time, the samples were intro-


duced into an insulating medium with a low thermal conductivity. The aim of this step is to maintain a constant temperature to enable the ausferritic reaction to occur. The isothermal transformation was defined as 90 minutes for all the samples. Finally, after the isothermal


holding, the samples were air cooled to room temperature and the cooling curves calculated (Fig. 1). The experimental data were used to obtain the relationship between the shakeout temperature and the thermal modulus. Tensile and hardness specimens


were machined from the samples. The ultimate tensile strength (UTS), yield strength (YS) and elongation were measured as per the standard EN 1563:2011. In addition, Brinell hardness mea- surements were carried out per the standard ISO 6506-1:2005.


Results and Conclusions


Based on the results of


the experiment, an Excel spreadsheet model was


developed to establish if a specifi c casting, with specifi c thickness dif- ferences, can be produced through engineered cooling with fully ausfer- ritic microstructures on all sections.


T e inputs of the model are the


minimum and maximum thermal modulus of the casting where an ausferritic microstructure must be guaranteed and the mechanical


Sept/Oct 2015 | METAL CASTING DESIGN & PURCHASING | 41


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60