Failure Analysis of Duplex Stainless Steel Tube Cracking after Hydraulic Expanding

The duplex stainless steel tube of a heat exchanger cracked after hydraulic expanding. The failure reasons of the duplex stainless steel tube were studied by macroscopic observation, metallographic analysis, scanning electron microscopy, energy dispersive spectroscopy and electron backscatter diffraction. The results show the crack source is located at the single austenite phase zone on the stainless steel tube surface. Abnormally large austenite grains and composition segregation were found near the cracking region, and micro-crack had already existed before hydraulic expanding. Stress concentration at the tip of the crack caused the crack to propagate to the inner wall of the tube and the tube was eventually penetrated.