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Cause analysis of stainless steel tank leakage
Mar 11, 2020

(1) appearance inspection after water holding test leakage inspection, it was found that there was a leakage point at the connection between the pipe and the shell. After careful observation, there were obvious small and fine cracks in the welding seam.

 

(2) macroscopic inspection of fracture observed shell cracking site, found that the crack in the shell is located in the welding seam area close to the connection welding, showing the characteristics of sawtooth morphology.

 

(3) chemical composition inspection conduct chemical composition analysis on the samples of the shell and the nozzles. The chemical composition of the shell conforms to GB/ t4237-2007 standard, and the nozzles conform to GB/ t14976-2012 standard, and the data conform to the standard requirements.

 

(4) metallographic detection: the metallographic samples are prepared by intercepting test blocks at the cracked part of the shell. The samples are first observed by microscope without corrosion. The grade of nonmetallic inclusions in the shell test blocks is C1.5 and D1.5 according to GB/ t10561-2005 standards, and the grade of inclusions is relatively low. Further observation shows that the crack extends from the shell surface to the interior in an intercrystalline manner, and the overall appearance of the crack is a network. Combined with the crack appearing in the position near the welding seam, it should be in the welding heat affected zone, so the crack is the crystal crack in the welding heat crack.

 

The structure of the shell is austenite and a small amount of carbide. Further observation revealed that the crack began in the heat-affected zone of the weld, and the black part was where the crystal fell off. Black carbides can be seen near the grain shedding and at the grain boundary.

 

(5) the hardness test block was intercepted at the crack of the shell, and 3 of the 4 points were greater than the standard value of 210HV.

 

(6) cause analysis based on the analysis of the above, shell and take over the welding process, adjacent to the shell heat affected zone, the temperature is 500 ~ 850 ℃, the formation of welding sensitization zone, shell plate hardness value exceeds bid at the same time, the solid solution treatment effect is poor, not all solid solution and the carbon in the austenite increases the tendency of forming welding sensitization, a combination of both make shell to produce intercrystalline corrosion cracking, thus causing cracking leakage.

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