15MnNbR Spherical tanks in a lighter hydrocarbon reservoir were tested by internal MT. Almost whole circle crack located at fusion zone of the weld connecting equator plate and the lower pole plate was found. The cracks are parallel to the weld and the deepest depth is 10 mm. Through hardness testing, metallographic testing and residual stress testing, etc, the causes of cracks were analyzed. The analysis shows that strong assembly, poor welding process controlling and poor effect of post weld heat treatment are the main causes for formation of the cracks.

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