13.15) A long cylindrical bar of steel which contains 3 ppm of hydrogen is dehydrogenated by a two-step vacuum process. The first step is treatment at 150C for time period t1, followed by the second step at 300C for time period t2. If t1=2t2, calculate: a) the total time (t=t1+t2) to reduce the average composition to 1.5ppm of hydrogen, and b) the center composition after the two-step treatment. Data: DH=1.0exp(4000/T) with DH in cm2s1 and T in K. The diameter of the bar is 2cm. Finite system - use charts