| Stainless steel has been used in many fields due to its excellent resistance to general corrosion, such as, automotive industry, water industry, building industry, household electrical appliances industry, environmental industry, industrial provision, and so on. However, it is vulnerable to IGC or even leads to stress cracking corrosion when exposed to an environment with temperature between 450 and 900℃. So it is very important to develop some methods that can be used to investigate the IGC of stainless steel as early as possible. So far, there are a number of standard methods, such as the Strauss test, Huey test, or Streicher test can be used to estimate the DOS of austenitic stainless steel. Not quantifying the DOS, in addition, these tests are also destructive and slow-a situation that is not welcome at plant site. Therefore, it is very urgent to develop non-destructive, quantitative and rapid methods to investigate the IGC of stainless steel.The main reason is that Cr-rich carbides and intermetallic phases precipitate along grain boundaries. Adjacent to the carbides, there is a chromium-depleted zone that can be preferentially attacked in corrosive environment. In order to correlate the IGC with grain boundary characteristics, many studies have been devoted to evaluate the depleted zones quantitatively by empirical or analytical models. For example, Stawstrom and Hillert, Was and Kruger, and Bruemmer have applied thermodynamics method to study the chromium concentration at grain boundaries and in the matrix, and reasonably good agreement with experimental chromium concentration profiles has been obtained. However, experimental observations show that there is a delay in reaching the minimum chromium concentration, i.e. the minimum chromium concentration will only be reached after a finite time rather than at the beginning of precipitate growth. Then Sahlaoui divided the sensitization into two stages-the nucleation stage and the grain growth stage. Recently, Yin and his teammates proposed a new model which contained a three dimensional distribution of chromium concentration. They investigated the influence of sensitization time, temperature and grain size on the distribution of chromium concentration. Generally speaking, these models are successful to evaluate the stages of sensitization and desensitization to the IGC, but they can only produce the chromium concentration profiles rather than giving an overall view of the chromium distribution.The main purpose of this dissertation is to develop some non-destructive, quantitative and rapid methods to investigate the IGC of stainless steel and study the influence of all kinds of experiment conditions on it. Moreover, the precipitation, the distribution of chromium concentration and the influence of experiment conditions on IGC can be simulated by CA with reasonable models. In this dissertation, the factors that affected the IGC of stainless steel (304,316) were investigated by electrochemical potentiodynamic reactivation test. Some further investigations on IGC of the materials were carried out by field emission scanning electron microscope (SEM) and electron back-scattering diffraction (EBSD). Furthermore, the precipitation evolution of Cr-rich carbides and the distribution of chromium concentration were simulated by CA, and which vividly showed the effects of solution and sensitization treatments on IGC. The influence of re-crystallization on Cr-rich carbides' precipitation and the three dimensional distribution of chromium concentration were also simulated. The results clearly showed the microstructure evolution of the re-crystallization under different conditions and exhibited the change of precipitation along with the deformation. At last, the IGC of austenitic stainless steel (304) was investigated by potential step test, and the effect of experiment conditions were studied in detail. Furthermore, the effects of solution treatment time and sensitization time on the DOS were studied too. The main contents and research results are as following:1. Investigation of the IGC of stainless steel (304) by electrochemical methodThe samples of stainless steel (304) were first solution treated at 900,1000 and 1100℃for 0,0.25,0.5,1,2 and 6 h respectively, and then sensitized at 650℃for 12 h. In order to investigate the influence of sensitization treatments, the saples were first solution treated at 1100℃for 1 h, and then sensitized at 600,650 and 700℃for 0-72 h. From the DOS obtained from EPR tests, it could be seen that the DOS decreased as solution treatment temperature and time increased. Without solution treatment, the DOS was as high as 85.1%, but the DOS with solution treatment decreased sharply. For example, when the samples were treated at 1100℃for 0-6 h and then sensitized at 650℃for 12 h, DOS decreased from 85.1% to 10.4%. When solution treatment temperature was 900℃and 1000℃, the changing trends of DOS were the same as that at 1100℃. On the other hand, the DOS increased as sensitization temperature went up. When the samples were treated at 1100℃for 1 h and then sensitized at 600-700℃for 24 h, DOS increased from 21.9% to 33.3%, which meant the IGC became severe along with the increase of sensitization temperature. The SEM results, showing the influence of solution treatment temperature and time, and sensitization temperature on IGC, were in accordance with the electrochemical results. To study the factors that affected the IGC, the relationship between grain size and chromium concentration at grain boundaries was investigated. The results agreed with the theoretical calculation, which indicated it was the grain size that caused the change of IGC. Generally speaking, it was the diffusion of chromium atoms that led the influence of sensitization temperature on IGC. The higher the temperature was, the more Cr-rich carbides precipitated. So the IGC increased as sensitization temperature went up. Furthermore, the precipitation of Cr-rich carbides and the distribution of chromium concentration were simulated by CA, which vividly showed the effect of solution and sensitization treatments on IGC.2. Investigation of the IGC of stainless steel (316) by electrochemical methodThe samples of stainless steel (316) were first solution treated at 1000,1050 and 1100℃for 0,0.25,0.5,1, and 2 h respectively, and then sensitized at 700℃for 48 h. In order to investigate the impact of sensitization treatments, the samples were first solution treated at 1100℃for 1 h, and then sensitized at 600,650 and 700℃for 0-96 h. From the DOS obtained from EPR tests, it could be seen that the DOS decreased as solution treatment temperature and time increased. Without solution treatment, the DOS was as high as 90%, but the DOS of the samples with solution treatment decreased sharply. For example, when the samples were treated at 1100℃for 0.25-2 h and then sensitized at 700℃for 48 h, DOS decreased from 35.2% to nearly 0%. When solution treatment temperature was 1000℃and 1050℃, the changing trends of DOS were the same as that at 1100℃. On the other hand, the DOS increased as sensitization temperature went up, which meant the IGC became severe along with the increase of sensitization temperature. The SEM results, which showed the influence of solution treatment temperature and time, and sensitization temperature on the IGC. were in accordance with the electrochemical results. According to the EBSD results, the fraction of low∑-CSL grain boundaries didn't increase as solution treatment temperature increased, which implied that it was not the low∑-CSL grain boundaries that led to the change of IGC. Besides, the fraction of low∑-CSL grain boundaries for the samples with sensitization did not decrease as sensitization temperature increased. So it was not the change of low∑-CSL grain boundaries that affected the IGC. However, from the SEM results, it could be seen that the grain size changed regularly along with solution and sensitization treatments, which could explain the influence of different experiment conditions on IGC. Therefore, the grain growth, the precipitation of Cr-rich carbides and the distribution of chromium concentration were simulated by CA, which vividly showed the effects of solution and sensitization treatments on IGC.3. Investigation of the influence of cold work on the IGC of stainless steel by CAThe effect of cold work on the IGC of stainless steel (304) was studied by CA, and the re-crystallization, the influence of sensitization temperature and the deformation on the IGC of stainless steel were simulated, which vividly showed the effects of solution and sensitization treatments on the precipitation of Cr-rich carbides and the distribution of chromium concentration. The simulation results were in good agreement with the theoretical calculation, indicating the correctness of the simulation. From the simulation, it could be seen clearly that the re-crystallization grain size decreased as the deformation increased due to the increase of nucleation sites, which were caused by the increase of deformation storage energy. As it is known, the IGC becomes severer as the grain size gets larger. As to the influence of the sensitization temperature, it was caused by the rate of chromium diffusion, which was affected by sensitization temperature.4. Investigation of the IGC of stainless steel (304) by potential step testCombing the virtues of potentiostatic pulse technique and the electrochemical reactivation test, the IGC of stainless steel (304) was studied by potential step test. The influence of experiment conditions on IGC was discussed in detail and the best experiment conditions were determined through experiments. From the results, it could be seen that the passive potential had little influence on activation current. However, the active potential had very obvious effect on it. At lower active potentials, the current could reach a maximum value and kept constant in a very short time. But at higher active potentials, the current needed a much longer time to reach the maximum value. Therefore, all the experiments were carried out at a lower level. The SEM results showed that IGC occurred to all the samples, which meant the potential step test could be used to investigate the IGC of stainless steel. Compared with the traditional method, this method was much faster but less destructive to stainless steel.
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