This study conducts thermomechanical analyses of various steam headers to evaluate the cost-effectiveness of different material choices, demonstrate the influence prior service exposure may have, and model damage within these systems. Several headers designed in accordance with ASME BPVC were evaluated as potential replacements for an existing unit. The existing header, constructed from 2.25Cr-1Mo Grade 22 material located in Charlotte, NC, provided the basis for the operational specifications used in the proposed solutions. To achieve improved durability and cost-effectiveness, the study focused on identifying more robust alloys as potential replacements. In this context, P91 and IN740H were specifically considered due to their superior properties. Each header was subjected to thermo-mechanical loading, reflecting conditions experienced by the in-service header unit. The material's response to these conditions was captured using Abaqus finite element software. Among the various material models available, the analysis for each header was conducted using the elastic perfectly plastic material model. This particular approach was adopted in light of the constrained data pertaining to the IN740H alloy and to facilitate a homogeneous comparison across different alloys. The low cycle fatigue, LCF, and response of each header were approximated by incorporating the Ostergren damage parameter. This methodology searched the region of the header to identify the region within the header exhibiting the highest damage coefficient, subsequently assessing its impact on the overall reduction in fatigue life. Intriguingly, the study's findings revealed a lack of correlation between the predicted damage under realistic loading conditions and the known issues observed in the P22 unit. This observation led to the hypothesis that the strength of the ex-service unit had been altered by the prior service exposure. It is well documented that prolonged exposure to high temperatures can result in carbide coarsening and phase changes in 2.25Cr-1Mo steel. Test specimens were taken from the ex-service unit and subjected to uniaxial testing at various temperatures to substantiate this hypothesis. This data was used to calibrate a new material model for the P22 alloy. An identical header configuration was reviewed using two sets of material properties to elucidate the differences between the materials. The first header was modeled using properties characteristic of virgin material, while the second incorporated properties derived from the ex-service test specimens. In order to accurately capture and compare the behavior of these materials, a Non-Linear Kinematic Hardening, NLKH, model was selected. This choice was motivated by the NLKH model's capability of capturing cyclic behavior in metals and the relative simplicity of obtaining coefficients. The coefficients for the model were acquired by evaluating test and publicly available data. To aid the comparison of the material's response, simplified loading conditions were established to represent a common start-up and shut-down cycle, as well as a common and limit case transient within the tube header junction. The concluding segment of the project focuses on evaluating damage mechanisms in the P22 header. This evaluation consists of iteratively propagating a theoretical crack using an algorithmic approach. The theoretical crack is incorporated into the header using Abaqus' seam crack capability. Then, by combining Paris law and an algorithmic approach, the crack is propagated through the material. The crack propagation phase of modeling incorporates the sub-modeling approach, which allows for high levels of accuracy to be obtained in an efficient manner. The results are then compared to those found using XFEM to propagate the crack. By applying these dual techniques, the study aims to provide a basic understanding of crack growth behavior in a P22 header using the techniques available in Abaqus. This study establishes a comprehensive framework for evaluating headers and similar components subjected to thermomechanical fatigue. Our methodology, which integrates advanced material modeling with fatigue analysis, offers a replicable framework for other researchers.