Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation
|Chloride stress corrosion cracking (CSCC) requires chloride and stress. To eliminate one is to eliminate the phenomena. In short, you can't have CSCC if one element is missing.
A 7 year old furnace made out of 309L stainless steel developed cracks in a particular region of the vessel. These cracks traveled for nearly 120 degrees circumferentially around the sidewall. The cracks were chased back to their origination. They all started from the juncture point at every butt joint of a stiffening ring. This butt joint, pictured at the left, was not welded solid. This stiffening ring had approximately 25 segments, all of which were loosely butted together and then skip welded.
The extent of the cracking caused the shell to partially buckle because there wasn't anything left holding the vessel up in this region. This situation was hidden from sight because the vessel was insulated.
To determine the cause of the cracking and to assess the present condition of the vessel, some 10 samples were taken all the way around the vessel in the vicinity of the cracks. These samples had cracks in them, and other samples did not. All were analyzed, and the samples without cracks were destructively tested.
|The cracks would start out as starburst type breaks at the tip of the first weld of each butt joint, as shown in the picture to the right. You can see the backside of the shell at the crack by clicking on it. Several of the leading edges of the cracks were recovered (click here) and were analyzed.|
|The leading edge of one of the cracks can be seen below to the left. This is intergranular corrosion. The crack follows the path of least resistance from a metallurgical viewpoint. The reason the crack follows the path along the grain boundaries is because the chromium is severely depleted in these regions and makes it prone to chemical attack. A closer look at the cracks between the grains is shown to the bottom right.
The picture at the bottom right shows the grain boundary condition. It was etched with a stronger concentration to illustrate the sensitivity of the region to corrosive elements. Under normal conditions the metal at this grain boundary would not have disappeared so easily. This phenomena is known as sensitization. This condition was prevalent throughout all of the samples, with and without cracks.
|It was clear that the entire vessel had been sensitized, yet the cracks did not have a random pattern. They didn't just start anywhere. They started only at the region where there was a greater stress. The stress was generated by the fact that since the stiffening ring was not solid it would bend at the butt joint. This created an additional bending stress. If that was the case then the cracking should have been transgranular, but it wasn't.
Something else was causing the cracks to start. Additional testing revealed the presence of chlorine. However, one couldn't just say that it was the chlorine acting by itself that caused the problem because then there should have been cracks everywhere else regardless of the location of the stiffening ring butt joints, and this was not the case.
Therefore, the only way that these cracks could have started was if the additional stress and the chlorine, working in conjunction with each other, initiated the cracks and caused them to spread.
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