Causes of early fatigue cracking of S32750 duplex stainless steel instrument pipe joints

Cracking occurred in the straight joint of S32750 duplex stainless steel instrument pipe on an offshore platform. The cracking reason of the instrument pipe joint was analyzed by means of macroscopic observation, fracture analysis, chemical composition analysis, hardness test, metallographic examination, scanning electron microscope and energy spectrum analysis. The results show that due to the damage defects on the surface of the joint and the brittleness of the material, under the action of alternating load, cracks initiated and propagated on the surface, resulting in early fatigue cracking of the joint.