Manufacture of Transformer Steel

To make a transformer steel with the proper hysteresis characteristics, it has to be loaded with silicon up to a certain prescribed content. This is accomplished by exposing a steel sheet that is low in silicon content and of 2 mm thickness to an atmosphere of SiQ4 that dissociates to Si(g) and Cl2(g). The silicon gas dissolves in the steel up to 3 wt% at equilibrium. The treatment is to be carried out at 1255 K. Silicon diffusivity in steel at this temperature is 8.2 X 10 m /s. 

We start by noting that we can set the initial concentration C = 0 in view of the low silicon content of the steel to be treated. The surface concentration 

some 16 days are required for the target concentration to be attained. This is entirely due to the low diffusivity of silicon in the steel, which is some three orders of magnitude lower than typical diffusivities in liquids. To reduce the inordinate length of treatment, the operation will likely have to be carried out at a higher temperature. Since the diffusivity in solids varies exponentially with temperature (see Equation 3.17) a modest increase in the latter will quickly lead to substantially higher diffusivities and thus shorten the duration of the treatment considerably. 

---