Jinsung Jang

Methodologies for Long-Term Creep Life Extrapolation of GR. 91 Steel

A modified 9Cr-1Mo steel (ASME Grade 91, Gr.91) is regarded as one of the structural materials of Gen-IV reactor systems, such as a sodium-cooled fast reactor (SFR) and very high temperature reactor (VHTR). Since the reactor systems are designed for a lifetime of 40–60 years at the elevated temperatures, the long-term creep life extrapolation is very important for a design application. In this paper, a number of creep rupture data were collected for the base metal of Gr. 91 steel. Long-term creep life extrapolation was carried out using the Larson-Miller Parameter (LMP), Orr-Sherby-Dorn (OSD), Manson-Haferd Parameter (MHP), and Wilshire’s approach. In determining the master curves in OSD and MHP methods, a hyperbolic sine (sinh) function was firstly proposed for improving the creep life extrapolation of Gr. 91 steel. Parametric constants for each method were reasonably optimized. It is found to be C=33–36 (optimum: 35) in LMP, Q= 578.4 kJ/mol in OSD, Ta = 526.34 and log ta = 17.074 in MHP, and Q c * = 325 kJ /mol in Wilshire model. Using the optimized constants, the long-term creep life for each method was successfully extrapolated for up to 106 h. An accuracy of each method was compared using a standard deviation of error, and the best method was reasonably drawn.

Geometric variations and surface residual stresses in U-bending processes of an UNS N06690 row-1 heat exchanger tubes

Surface residual stresses as well as wall thickness and ovality changes after U-bending process on UNS N06690 row-1 heat exchanger tubes, were estimated. Surface residual stresses were measured by Hole Drilling Method(HDM), calculating the stresses from relieved strains of 3 rosette strain gages. After bending of the tubes, dimensional tolerances for wall thickness and ovality were satisfied with ASTM requirements. Residual stresses at the extrados were introduced with compressive stress(-) by bending operations, and its maximum value reached-319 MPa in axial direction at in position. Tensile residual stresses(+) of MPa, MPa were introduced in the intrados surface at position of Maximum tensile residual stress of 170 MPa was detected on the flank side at position of ,i.e., at apex region. It appeared that higher stress gradients were generated at the irregular transition regions. In the trend of residual stress changes with U-bend position, the extrados is related with the changes of ovality and the intrados is related with the changes of wall thickness.