Abstract
This work examines the microstructural and mechanical response of API 5L X70 pipeline steel subjected to low-temperature isothermal annealing at 200 °C for 4 hours. Unlike conventional high-temperature treatments that often cause grain coarsening, this study focuses on the effects of a mild thermal cycle on the material’s internal energy using a multi-scale analytical approach. Williamson-Hall X-ray diffraction (XRD), scanning electron microscopy (SEM), and computational image analysis were employed to quantify recovery mechanisms.
Results indicate that annealing at 200 °C significantly reduces lattice microstrain (from ε = 6.53 × 10^–4 to 0.19 × 10^–4), primarily through dislocation rearrangement and partial annihilation rather than full recrystallization. The stored energy decreases substantially from 0.313 to 0.014 MJ m^–3. SEM-based quantitative analysis shows a more homogeneous microstructure, with the number of detected sub-grain features rising from 2.533 to 10,485 and the mean projected area decreasing by roughly 75 %. Despite these structural modifications, the steel maintains its mechanical integrity, with Vickers hardness decreasing moderately to 208.22 HV.
Overall, annealing at 200 °C efficiently relieves residual stresses while preserving structural stability and load-bearing capacity. The integrated experimental and computational framework presented here provides a reliable methodology for evaluating recovery mechanisms and their influence on the performance of X70 pipeline steel.

Key words
API 5L X70 steel, low-temperature annealing, microstrain relaxation, static recovery, stored energy, microstructural homogenization

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