Creep performance of a grade 91 header

Extensive Type IV cracking was found associated with branch and attachment welds on a Grade 91 header after about 58,000 h operation. The inspection of the header was undertaken because it had been established that a number of low nitrogen to aluminum (N:Al) ratio components had been used in construction. This ratio had been identified as a 'risk factor' common to plant failures in this grade of steel. Detailed metallographic investigations subsequently showed the presence of coarse aluminum nitride (AlN) precipitates, a depleted VN-type MX precipitate population, and relatively poor parent creep strength in Grade 91 material with low N:Al ratio. The cracking found in the present case was overwhelmingly on the header barrel sides of the welds. The material, while meeting the chemical compositional requirements of the then ASTM specification for this grade of steel, was confirmed to have low N:Al ratio. This paper summarizes the background and inspection history of the header, summarizes the damage found in 2004, at a later outage in 2006, and details the results of post service examinations. This information, and data from post service cross weld creep tests, are discussed in terms of Type IV creep life in Grade 91 steels. The wider implications for other Grade 91 components in service are considered. (C) 2013 Elsevier Ltd. All rights reserved.