255 11.D7 We are to determine, for a cylindrical piece of 8640 steel, the minimum allowable diameter possible in order yield a surface hardness of 49 HRC, when the quenching is carried out in moderately agitated oil. From Figure 11.6, the equivalent distance from the quenched end of an 8640 steel to give a hardness of 49 HRC is about 12 mm (15/32 in.). Thus, the quenching rate at the surface of the specimen should correspond to this equivalent distance. Using Figure 11.8(b), the surface specimen curve takes on a value of 12 mm equivalent distance at a diameter of about 70 mm (2.75 in.). 11.D8 This problem is concerned with the precipitation-hardening of copper-rich Cu-Be alloys. It is necessary for us to use the Cu-Be phase diagram (Figure 11.17). (a) The range of compositions over which these alloys may be precipitation hardened is between approximately 0.2 wt% Be (the maximum solubility of Be in Cu at about 300∞C) and 2.7 wt% Be (the maximum solubility of Be in Cu at 866∞C). (b)
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