tween 375C (707F) and 410C (770F), the elastic modulus drops to zero. As can be seen in Figure 12, the steady-state elastic modulus values between 100C (212F) and 200C (392F) are much higher than those for finite heating rates. Also, the complete loss of stiffness occurs at a lower temperature when the specimen is heated “infinitely” slowly.


The steady-state elastic moduli measured in the present set of experiments are in close agreement with those measured in Reference [4]. This can be expected because the speci- mens in the experiments of Reference [4] were indeed held at elevated temperatures for some time before the elastic modulus measurement was performed. The agreement in- stills considerable confidence in the present measurements.


Retained Room Temperature Elastic Modulus


After the elastic modulus specimens were held at an elevated temperature for a long period of time, the power to the oven was shut off and the cooling system was activated. The oven was allowed to cool until the specimens were back at room tempera- ture. Then, several measurements of the retained room temper- ature elastic modulus were performed for each specimen. The results of these measurements are plotted as a function of the elevated hold temperature prior to cooling in Figure 15.


Figure 15 shows that for hold temperatures up to about 275C (527F) the specimens fully recover their initial room temperature stiffness upon cooling. There appears to be a


(a)


(b)


(c)


(d)


Figure 13. Elastic modulus variation during heating and holding at an elevated temperature until a constant steady- state value is attained. The holding temperatures are (a) 50°C, (b) 200°C, (c) 300°C, and (d) 370°C. The figures also show the measured retained elastic modulus after cooling to room temperature.


16 International Journal of Metalcasting/Fall 10


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