Subcritical Thermal Degradation of SA210 Grade A1 Ferritic–Pearlitic Steel: Microstructural Evolution, Hardness Loss, and Strength Reduction

Ferritic–pearlitic boiler tube steel undergoes progressive microstructural degradation during subcritical overheating, impairing mechanical integrity well before severe wall thinning occurs. To quantify this degradation trajectory, we established a stage‐resolved framework linking subcritical thermal exposure, microstructural degradation, Vickers hardness loss, and tensile strength reduction in SA210 Grade A1 boiler tube steel. Isothermal annealing was conducted at 600, 660, and 720°C. Holding times reached up to 100 h at 600°C and 660°C, and up to 30 h at 720°C, producing degradation states ranging from the intact as‐received condition to an advanced carbide‐degraded morphology, classified under the Toft–Marsden six‐stage scheme. Higher temperature severely compressed the degradation timeline. Reaching Stage D required 100 h at 600°C, 30 h at 660°C, and merely 1 h at 720°C. Image analysis revealed a reduction in the pearlite‐derived dark constituent area fraction from 25.44% ± 2.27% in the as‐received condition to 11.63% ± 1.55% at Stage F, tracking the dissolution and spheroidization of continuous pearlite colonies. Ferrite grain coarsening remained minor, with the ASTM grain size number shifting from G = 9.49 ± 0.04 to G = 9.16 ± 0.03. Mechanically, median hardness fell from 167.9 HV to 121.8 HV, corresponding to a 27.4% loss, alongside a 29.55% drop in mean UTS (508.02 ± 9.71 MPa to 357.92 ± 14.61 MPa). A temperature–logarithmic time regression modeled the hardness loss with R ²  = 0.965, indicating that a 60°C thermal increment produced a softening effect equivalent to a tenfold increase in dwelling time. Relative UTS loss scaled proportionally with relative hardness loss, following ΔUTS (%) = 0.989Δ H (%) − 0.194, with R ²  = 0.962. When integrated with metallographic staging and thermal history, hardness testing serves as a robust quantitative proxy for nondestructive strength assessment in degraded boiler tubes.