A sheet of steel with an initial nitrogen concentration of 0.066% is exposed to an atmosphere containing 0.2% nitrogen. After 6 hours, the concentration of nitrogen 5 mm into the surface of the steel sheet is 0.090%. At what temperature is this process taking place? Assume that D0 = 4.7 x 10-7 m2/s, QD = 76.6 kJ/mol.