Study on the Cause and Nondestructive Test of Backside Cracks in Stainless Steel Linings

Abstract

In this paper, the cause and nondestructive test technology of backside cracks were studied. On the one hand, failure analysis revealed that, the backside cracks, which were found universally in the stainless steel inner linings of urea reactors in recent years, were caused by the interaction of wet hydrogen sulfide (H2S) and chloride ions (Cl−) stress corrosion cracking (SCC), and the source of the H2S and Cl− was the leak detection steam. On the other hand, the use of Lamb wave inspection was studied because of the inapplicability of other inspection methods. The dispersion curves of phase and group velocities of 316LMod stainless steel linings were derived out according to the Rayleigh-Lamb equation. The main Lamb wave mode of the backside defects was drawn out by 2D FFT to echo signals of the test plate including artificial defects. Then, the Lamb wave testing parameters of 8mm 316LMod stainless steel were defined. The defect equivalent evaluation diagram was given through the defect recognition of the test plate including artificial defects. And the quantitative evaluation of Lamb wave inspection to the backside defects was realized. After all the analysis done, the measures to avoid backside cracks were put forward, and the inspection technology was verified by applications on urea reactors.