This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
[{'content': 'Age hardening is a process in which heating is done: \nSelect and explain. \nA. Up to 200 - 450°C for 15 - 30 min and quenching in water, B. At 700°C for 10 -15 min and quenching in water, C. At 700°C for 1 hour and quenching in water, D. Above 700°C for 30 min and quenching in water', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Age hardening—Process of hardening certain alloys by controlled heating and cooling, which usually is associated with a phase change.\nHeat Treatment of Gold-Copper Alloys\nTo soften the alloy, the casting is placed in a furnace for 10 minutes at 700°C and then quenched in water. All intermediate phases in the alloy are changed to a disordered solid solution\xa0 at\xa0 700°C,\xa0 and\xa0 the\xa0 rapid\xa0 quenching\xa0 prevents\xa0 ordering\xa0from occurring during cooling. The tensile strength, proportional limit, and hardness are reduced by such a treatment, and the ductility is increased. To harden the alloy, the temperature of the furnace is set between 200°C and 450°C and the casting is heated for 15 to 30 minutes before it is quenched in water. Ideally, before the alloy is age-hardened, it should be subjected to a softening heat treatment to relieve all residual strain hardening before the alloy is hardened again by heat treatment to produce a disordered solid solution.\xa0 Otherwise\xa0 the\xa0 amount\xa0 of\xa0 solid-state\xa0 transformation\xa0 will not be properly controlled. In metallurgical terminology, the softening heat treatment is referred to as a solution heat treatment\xa0 and\xa0 the\xa0 hardening\xa0 heat\xa0 treatment\xa0 is\xa0 termed\xa0 age hardening.\n\xa0Age hardening reduces the ductility of gold alloys.\nPhillips 12 ed page 367, 374', 'role': 'assistant'}]