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Steel balls 12 mm diameter are annealed by heating to 800°C and then slowly cooling to 127°C in air at 50°C. The heat transfer coefficient for air is 20 W/m2K. Calculate the time required for cooling process. The properties of steel are taken as k=45 W/mK, r = 7830 kg/m3 and  
cp = 600 J/kg K.                

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