<![CDATA[SA210 Grade A1 steel is widely used for economizer and waterwall tubes in subcritical boilers. In these components, local overheating can degrade the ferrite–pearlite microstructure before severe wall thinning becomes apparent. This study evaluates an integrated metallurgical approach that combines degradation-stage identification and relative hardness loss with dimensional and operating evidence for failure diagnosis of SA210 Grade A1 economizer tubes. Controlled accelerated exposure was performed at 600–720 °C for 1–100 h, followed by optical metallography and Vickers hardness testing. The resulting microstructures were classified from Stage A to Stage F. Progressive thermal exposure produced lamellar fragmentation, carbide spheroidization, carbide coarsening, and a corresponding decrease in hardness. The most severe laboratory condition produced Stage F degradation with a 27.4% relative hardness loss. Two failed economizer tubes exhibited comparable Stage F-type microstructures and substantial relative hardness reductions of 27.2% and 30.6%, while their remaining wall thicknesses were above the calculated minimum requirement. The latter value was derived from an instrument-converted portable hardness measurement and was regarded as supporting field evidence, not as a direct substitute for laboratory Vickers hardness. Taken together, the evidence was most consistent with overheating-induced metallurgical degradation, whereas the available data did not support generalized wall thinning or a recorded pressure excursion as the primary driver. The approach can identify severe thermal degradation that may not be evident from thickness measurement alone, but it is not a standalone remaining-life model.]]>
