Inconel 690 is commonly used in heat transfer tubes of once-through steam generators (OTSGs), but its material cost is high. This paper establishes a thermo-structural coupling finite element analysis framework to evaluate layered tube wall schemes that reduce the amount of Inconel used. First, using all Inconel 690 as a baseline, an attempt was made to replace it entirely with a low-cost composite material. The results showed that thermal stress and deformation increased significantly, making it difficult to meet structural reliability requirements. To improve the thermo-structural response and reduce costs, a three-layer structure of Inconel 690 / Ni–SiC metal-matrix composite / Inconel 690 is proposed, and the results are compared between a fixed 20 vol% SiC interlayer and a 0–20–0 vol% graded interlayer; with a total wall thickness of 1.5 mm, the interlayer thicknesses are 10%, 20%, and 30%. The advantages and disadvantages of the scheme were evaluated by the maximum displacement, maximum equivalent stress, and peak Tresca stress. The results show that the Ni–SiC interlayer can effectively reduce deformation and alleviate interfacial stress concentration, but the interlayer thickness will bring a trade-off between heat transfer performance and structural performance; compared with the fixed interlayer, the gradient interlayer is more effective in reducing peak stress.

Keywords: i-SMR, Once-through steam generator, Heat-transfer tube, Thermo-mechanical coupling, Ni–SiC metal-matrix composite, Sandwich structure, Interlayer design