Creep life assessment of an overheated 9Cr-1Mo steel tube

Crude oil heater 9Cr–1Mo steel tubes from a refinery plant were studied, after 24 years of service at nominally 650 °C and 27 MPa, to predict their remanent lives. The investigation included dimensional, hardness and tensile measurements in addition to accelerated stress rupture tests between 650 °C and 700 °C and microstructural examination. Tube specimens were taken from two sections, the overheated side and the side which only saw the nominal operating temperature. The method employed involved the prediction of the increase in temperature with increasing sediment deposition during the operating life times using an FEM model. In addition the predicted temperatures are used to derive appropriate creep properties at relevant temperatures in a 3D pipe FEM creep analysis to predict the pipe deformation rate. All compare well with the actual service exposed pipe measurements and layer deposition. The overheated side revealed a small loss of creep strength in a stress rupture test. A layer of sediment (appr. 10 mm thickness) consisting basically of sintered carbon (coke) spread over the inside of the tube was acting as a thermal barrier causing the temperature to rise above 650 °C. Analysis for the overheated side predicted an upper bound temperature of ≈800 °C and a life of about 50 h suggesting that failure by creep rupture could occur rapidly in the sediment region.

Research highlights
► Remanent life of 9Cr–1Mo steel tubes after 24 year of service was predicted.
► Accelerated creep testing and metallographic investigation was performed.
► prediction of the increase in temperature with increasing sediment deposition was made using FEM.
► 3D pipe FEM creep analysis was used to predict the pipe bending deformation.