Vol.26, No.2, 2026, pp. 244–248
https://doi.org/10.69644/ivk-2026-02-0244

This paper presents a comprehensive methodology for the remaining life assessment of carbon steel St.35.8/II boiler screen tubes after an extreme operating lifetime of 306,581 hours. The assessment is carried out using non-destructive in-situ plastic replica metallography and Brinell hardness tests. The investigated boiler screen tubes located at an elevation height of 10 m are directly exposed to the highest heat flux inside the furnace where actual tube wall temperatures exceed the nominal fluid parameters of 300 °C and enter the creep regime and exhibit advanced microstructural degradation. This is characterised by progressive-to-complete spheroidisation of pearlite colonies and continuous accumulation of secondary carbide networks along ferrite grain boundaries. Measured in-situ hardness on these degraded regions reaches up to 174 HB, which is attributed to a combination of secondary phase carbide dispersion and surface work-hardening artifacts introduced during in-situ mechanical grinding preparation. Conversely, optical microscopy examination of surface replicas up to 500× magnification reveals no oriented creep cavitation pores, formally classifying the tubes as Class 0 (undamaged condition) according to the conventional Wedel-Neubauer creep classification scheme. The study highlights a critical safety limitation where traditional models based solely on cavitation underestimate the risk of premature brittle failure in low-carbon boiler steels. Consequently, a modified engineering assessment model incorporating carbide morphology parameters is proposed to prevent catastrophic intergranular failure.

Keywords: • St.35.8/II boiler steel • remaining life assessment • Wedel-Neubauer classification • in-situ replica metallography • hardness anomaly