Numerical reliability of hot working processing maps

The extensive use of hot working processing maps witnessed in the literature calls for a critical evaluation of the relevant data analysis procedures. Data elaboration procedures are shown to have a critical bearing on the structure of the maps. The single most critical point lies in the numerical estimate of second derivatives. Typical procedures based on cubic splines are not acceptable for numerical analysis. Errors of estimated processing maps obtained by alternative numerically sound procedures give rise to statistically non-significant results if experimental data of typical accuracy are employed. Reliability problems can be circumvented by direct estimate of derivatives form the data, but then a large number of experiments with several strain rates are required.