12CrMoV is generally rated for service up to 560°C, placing 550°C at the upper edge of its useful range. At this temperature, the steel’s vanadium additions precipitate fine carbides that pin dislocations and provide creep resistance, allowing it to outperform simpler Cr-Mo grades.
However, maintaining strength at 550°C is not automatic. Research on 12CrMoV steels creep-tested at 550°C shows that excessive nickel content (above roughly 0.6 wt%) accelerates the loss of creep rupture strength, producing a sigmoidal drop in long-term performance. Bolting service adds another concern: stress relaxation. Fasteners must maintain clamping force, yet 12CrMoV bolts undergo relaxation at high temperature, necessitating periodic retightening, and ageing can further degrade creep and impact properties.
For boiler fasteners at 550°C, 12CrMoV can perform adequately only if nickel is tightly controlled and the application tolerates the relaxation behaviour inherent to this grade. Where long-term joint integrity without frequent retightening is critical, higher-alloy bolt steels may be the safer choice.
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