| This research examines and evaluates the influence of the strain, grain size, and strain mode (uniaxial and triaxial) on the sensitization process in type 304SS and 316SS. In addition, comparisons of the sensitization kinetics in these materials were performed. The 304SS samples strained in tension (uniaxial) to 20% and 40% were shown to more dramatically affect the degree of sensitization (DOS) as well as the kinetics at 670°C than 316SS. On the other hand, samples strained by cold rolling (triaxial) not only alter the onset of sensitization development, but also accelerates the desensitization (healing) in both materials. It was shown that as the grain size increases in 304SS samples strained by tension, the onset of sensitization is delayed. Only samples (304SS and 316SS) strained to 40% (rolling) showed the presence of the sensitization-desensitization process. On the other hand, by decreasing the grain size to 11 mum in 304SS, the sensitization-desensitization process occurs instantaneously (0.1 hr.) whether the samples were strained by tension or cold rolling. However, in 316SS with 10mum grain size, this process occurs over a longer period of time (2 hr.). There is a limitation of the EPR technique when it is used in small grain size materials (11mum and 10mum). It is due to the narrow chromium depleted zone surrounding the carbide. Since the sensitization-desensitization process occurs rapidly, the narrow Cr-depleted zone is not affected by increases in temperature and aging time. The estimation of the diffusivity of chromium was achieved by using a modified algorithm (FORTAN 77 software). It was found that the Cr-diffusivity at 670°C in 304SS samples strained by tensile and cold rolling at 20% and 40% strain are higher by about two orders of magnitude than those in 316SS samples at the same conditions. Arrhenius plots were used to evaluate and to analyze the mechanism of diffusion by which sensitization occurs as a function of grain size (11mum and 175mum) in 304SS unstrained and 20% tensile strained samples. The activation energy (Qa) decreases from 55.1 kcal./mol to 32.2 kcal/mol as the strain increases from 0% to 20%, in samples with a 175mum grain size. While Qa at 11mum is maintained nearly constant with strain; 1.9 kcal/mol and 2.0 kcal/mol at 0% and 20%, respectively. The kinetics of chromium diffusion in medium grain sizes is influenced by dislocation pipe diffusion while grain boundary diffusion predominates at small grain sizes. Electrochemical Potentiokinetic Reactivation (EPR) test, optical microscopy, and transmission electron microscope (TEM) were the techniques used in this work. |
