The effect of boron nitrides (BN) and aluminum nitrides (AlN) on long-term creep life and rupture ductility has been investigated for martensitic 9 to 12Cr steels at 550 to 700 °C. The BN particles form in 9 to 12Cr steels during normalizing heat treatment at 1050 to 1150 °C, suggesting no change in the amount of BN particles during creep. On the other hand, the AlN particles gradually form during creep at 550 to 700 °C, decreasing the concentration of nitrogen free from the AlN particles. The degradation in creep life takes place more significantly with test duration in the steels containing high Al but not in those containing the BN particles. The rupture ductility is evaluated by using a semi-logarithmic diagram of the RA and total elongation, showing the necking dominant and void swelling dominant regions. The BN and AlN particles are responsible for the degradation in RA at low stresses and long times by accelerating the formation of creep voids at interfaces between the BN and AlN particles and alloy matrix.

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.

This paper investigates the cause of premature failures in certain Grade 91 steel components used in UK power plants. The failures were linked to both low material hardness and specific chemical compositions that fell within ASTM specifications but had a low nitrogen-to-aluminum ratio (N:Al). The investigators examined eight material batches, including those involved in failures, new stock, and in-service components with similar properties. Testing confirmed these materials had lower creep resistance compared to standard Grade 91 steel. Microscopic analysis revealed the presence of large aluminum nitride precipitates, which limited the formation of beneficial vanadium nitride precipitates, leading to reduced creep strength. These findings suggest that even within the ASTM specification limits, a low N:Al ratio can negatively impact the performance of Grade 91 steel.