Abstract
In 2004, extensive Type IV cracking was discovered in the branch and attachment welds of a modified 9Cr (Grade 91) header after 58,000 hours of service. The header, installed as a retrofit in a 500MW unit in 1992, was inspected early due to concerns over the incorporation of low nitrogen-to-aluminum (N:Al) ratio components, a factor previously linked to premature failures of this steel grade in the UK. Investigations confirmed the presence of coarse aluminum nitride (AlN) precipitates, a depleted VN-type MX precipitate population, and reduced parent and Type IV creep strength in low N:Al ratio material. Cracking predominantly occurred on the header barrel sides of the welds in material that, despite meeting ASTM compositional requirements, exhibited this unfavorable N:Al ratio. This paper summarizes the inspection history, detailing crack distribution observed in 2004 and a subsequent outage in 2006. The findings are analyzed in the context of Grade 91’s Type IV creep life shortfall and its dependence on chemical composition, with broader implications for other Grade 91 components in service.
