6.2 Steel is normalized by heating above the critical temperature, soaking, and then air cooling. This process increases the strength of the steel, refines the grain, and homogenizes the structure. An experiment is performed to determine the effect of temperature and heat treatment time on the strength of normalized steel. Two temperatures and three times are selected. The experiment is performed by heating the oven to a randomly selected temperature and inserting three specimens. After 10 minutes one specimen is removed, after 20 minutes the second is removed, and after 30 minutes the final specimen is removed. Then the temperature is changed to the other level and the process is repeated. Four shifts are required to collect the data, as are shown below.

(a) Analyze assuming an exchangeable structure.

(b) By the actual conduct of the experiment an AR(1) structure is reasonable. Reanalyze accordingly.

(c) Discuss these analyses. Shift 1 2 3 4 Time (min) 10 20 30 10 20 30 10 20 30 10 20 30 Temperature (°F) 1500 63 54 61 50 52 59 48 74 71 54 48 59 1600 89 91 62 80 72 69 73 81 69 88 92 64