Heat treating is often used to carbonize metal parts, such as gears. The thickness of the carbonized layer is a critical output variable from this process, and it is usually measured by performing a carbon analysis on the gear pitch

(the top of the gear tooth). Six factors were studied in a 26−2 IV design: A = furnace temperature, B = cycle time, C =

carbon concentration, D = duration of the carbonizing cycle, E = carbon concentration of the diffuse cycle, and F =

duration of the diffuse cycle. The experiment is shown in Table P8.16.

(a) Estimate the factor effects and plot them on a normal probability plot. Select a tentative model.

(b) Perform appropriate statistical tests on the model.

(c) Analyze the residuals and comment on model adequacy.

(d) Interpret the results of this experiment. Assume that a layer thickness of between 140 and 160 is desirable.