The prediction of changes in mechanical characteristics as a function of heat treatments and of steel composition has always been the aim of metallurgical research. If, however, it is specified that the heat treatments to be taken into consideration are as varied as those undergone by a large forged piece--a bar, a sheet, a weld--and that the composition must cover all carbon steels and weakly alloyed steels, the problem would seem to be insoluble as a general question. The use of diffusion laws can greatly facilitate the solution of this difficult problem. The mathematical consequences are developed of Fick's equations, or the relation of Dushman and Langmuir, and finally, of the well-known Arrhenius law.