For a steel alloy it has been determined that a carburizing heat treatment of 15 h duration will raise the carbon concentration to 0.35 wt% at a point 2.0 mm from the surface. a.) Estimate the time necessary to achieve the same concentration at a 6.0-mm position for an identical steel and at the same carburizing temperature. b.) What surface concentration is required to achieve the 0.35% concentration at a depth of 2.0 mm from the surface after 24 h if initial concentration is 0.05%? C in alpha-iron, Do = 6.2 x 10^-7 m^2/s and Qd = 80 kJ/mol.