3325.7 - Solution This problem is solved by using Equation...

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5.7 A sheet of steel 1.5 mm thick has nitrogen atmospheres on both sides at 1200 ° C and is permitted to achieve a steady-state diffusion condition. The diffusion coefficient for nitrogen in steel at this temperature is 6 10 -11 m 2 /s, and the diffusion flux is found to be 1.2 10 -7 kg/m 2 -s. Also, it is known that the concentration of nitrogen in the steel at the high-pressure surface is 4 kg/m 3 . How far into the sheet from this high-pressure side will the concentration be 2.0 kg/m 3 ? Assume a linear concentration profile.
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Unformatted text preview: Solution This problem is solved by using Equation 5.3 in the form J = ! D C A ! C B x A ! x B If we take C A to be the point at which the concentration of nitrogen is 4 kg/m3, then it becomes necessary to solve for x B, as x B = x A + D C A ! C B J 2200 # 5 % & 220d Assume x A is zero at the surface, in which case x B = 0 + ( 6 ! 10-11 m2/s ) 4 kg /m3 2200 2 kg /m3 1.2 ! 10 2200 7 kg /m2 - s # 5 % & 220d ( = 1 ⋅ 10-3 m = 1 mm...
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