There have been a number of failures of high-temperature, low chrome piping in the power generation and petrochemical industries, some with catastrophic consequences. Several of these failures have been attributed to peaking of longitudinal weld seams. Generally, local weld peaking occurs during pipe manufacturing due to angular misalignment of the rolled plate at the weld seam location (the pipe locally deviates from a true circular cross-section). Furthermore, for many fusion-welded piping fabrication standards, no specific tolerance for longitudinal weld seam peaking exists; that is, some of the high-temperature pipes that have failed in-service met the required original fabrication tolerances. Additionally, depending on original heat treatment, creep damage progression is known to be accelerated by the mismatch in creep properties of the base metal, weld deposit, and heat affected zone (HAZ). This mismatch results in stress intensification and triaxial tension that accelerates the rate of cavity growth near the weldment (typically in or adjacent to the HAZ). Local weld seam peaking can induce significant local bending stresses in the pressure boundary. For piping components that operate in the creep regime, the presence of local peaking can lead to an increased propensity for creep crack initiation/propagation and eventual rupture of the pressure boundary. An overview of some of the well-known historical low chrome piping failures is provided in this paper and a literature review on existing creep analysis methodologies that have been applied to high-temperature piping systems is offered.
Detailed finite element analysis (FEA) is employed in this study and coupled with advanced, non-linear creep simulation techniques to investigate the elevated temperature response of piping with peaked longitudinal weld seams. The objective of this study is to use analytical methods to estimate the remaining life of select low chrome piping geometries and to assess the sensitivity in results to variations in key parameters such as operating temperature, magnitude of longitudinal weld seam peaking, and the effect of pipe heat treatment resulting in a creep property mismatch between the base metal, weld deposit, and HAZ. Additionally, commentary on different creep damage failure criteria is rendered. Specifically, the effect of implementing a damage parameter that adjusts the elastic modulus of the material as a function of creep damage accumulation is examined. The creep simulations utilize the Materials Properties Council (MPC) Omega creep methodology and compare the creep damage progression for multiple postulated cross-sections of 30 and 36-inch diameter 1 1/4 Cr - 1/2 Mo pipes with and without local weld seam peaking. Simulation techniques such as the ones discussed herein are not only valuable in estimating remaining life of inservice piping, but detailed analysis can be leveraged to establish recommended local weld seam peaking fabrication tolerances, appropriate inspection practices, and reasonable non-destructive examination (NDE) intervals for in-service high-temperature low chrome piping systems.
