Abstract: The pipe nipple on the side of K101B valve of one hydrogenation splitting equip exit was cracked in use. The cracking reason was analyzed by means of macroscopic and microscopic examination, chemical composition analysis, hardness testing, energy spectrum analysis and so on. The results showed that there was geometrical discontinuity in pipe nipple welded construction, the change of construction section was large, led to severe lateral stress concentration. Widmanstatten structure was the weakest in heat affected zone, where the crack initiated and extended under the alternating load, and led to break at last. Then the measures were put forward.