With macrographic examination

SEM observation and theoretical calculation

fracture fractography of a spray tube with both mechanical stress and thermal stress in a boiler desuperheater in a power plant was interpreted

and rupture mode and causes were researched.The results showed that rupture of the spray tube was due to fatigue

and further

thermal-mechanical fatigue.Alternating bending stress and thermal stress were main factors of rupture failure.Shallow surface thermal fatigue cracks inside tube were induced by thermal stress.When thermal-mechanical fatigue crack grew up steadily to the shallow surface cracks region

fast growth occurred and local fast rupture region generated which had different fractography with final rupture region of conventional fatigue fracture.