Abstract
This study investigates the combined effect of temperature and notch depth on the Charpy impact toughness of API 5L X70 pipeline steel using an experimental approach coupled with statistical modeling. A 2²factorial design of experiments was implemented to evaluate the influence of two factors: temperature (T), coded as X₁, and notch depth (Nd), coded as X₂. Charpy V-notch impact tests were performed according to the experimental matrix defined by the design of experiments, and the absorbed fracture energy (U) was measured for each condition. The results indicate that the notch depth has a more significant influence on the absorbed energy than temperature within the studied range. Furthermore, the fracture energy increases within creasing temperature, confirming the typical ductile-to-brittle transition behavior of HSLA steels. Response surface methodology was applied to model the variation of absorbed energy as a function of the studied parameters. A first-order interaction model was developed to correlate the experimental parameters (T, Nd) with the fracture energy (U) and to predict its values within the investigated domain. The obtained results contribute to a better assessment of the structural integrity of pipelines under different operating conditions.

Key words
pipelines, temperature, notch depth, Charpy impact test, response surfaces

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