Cyclic Analysis of Power Plant Headers and Materials

This dissertation evaluates the fatigue response of a steam header designed to mirror the specifications of an ex-service unit, with a focus on optimizing material selection through a detailed analysis involving cost, performance, and durability. Beginning with a study comparing three different alloy choices, 2.25Cr-1Mo, 9Cr-1Mo-V, and IN740H, headers are developed and compared using the procedures outlined in ASME BPVC. The design of the headers follows that used in the original development, and their performance is evaluated in representative loading transients. Each of the designs is evaluated for their fatigue response using the finite element program Abaqus. The results demonstrate that cost savings would likely outweigh any performance benefit to the current system.
The second portion evaluates the material characteristics of 2.25Cr-1Mo following years of exposure to a harsh operating environment. Material specimens were machined from the ex-service unit and subjected to uniaxial testing at various temperatures. The process is used to establish the Chaboche NLKH hardening coefficients. The selection of the NLKH model was guided by its capability to capture the cyclic behavior of the material. The material results are used to compare the projected performance of the 2.25Cr-1Mo header found using readily available material acquired from virgin specimens and those found from the existing unit. The results demonstrate a markedly reduced strength in the service-exposed material, illustrating the effects of the material transformation that occurs over time. This study highlights the importance of operational wear on the projected performance of the header.
The final portion introduces an automated crack growth algorithm in combination with Abaqus to model the progression of a seam crack within a 2.25Cr-1Mo header. Traditional fatigue assessments consider the formation of surface cracks as the end of usability. However, it is well established that the existence of cracks in headers may be allowable, depending on several factors such as size, location, and material. Additional challenges exist in headers along the tube-header intersections, which suffer from non-uniform crack propagation stemming from the complex thermal-mechanical loading near the intersection. To address this issue, the present work develops an algorithm in Abaqus to use the seam crack capability and Paris law to efficiently perform iterative crack growth simulations. This approach captures the uneven growth response of the crack, providing more realistic service life estimations.