Early type IV cracking on retrofit grade 91 steel headers

Type IV cracking has been found on two retrofit grade 91 headers in the UK in less than 80kHrs service, ie approximately half the design life. The headers were installed on 500MW units in 1991 and 1992. This paper summarises the inspection history of the two headers, and the distributions of cracking found. Evidence for the contributions of operating stress, operating temperature, and chemical composition to the cracking is discussed. The cracking incidents are particularly significant because these components can be regarded as representative of the wider population of retrofit grade 91 headers in the UK, and the wider implications for other grade 91 components in service are considered. Early inspection of both headers was undertaken primarily because of the presence of low nitrogen to aluminium (N:Al) ratio casts, previously identified as a factor in earlier premature plant failures in this grade of steel elsewhere in the UK. In addition, small scale samples taken from one of the headers were shown to have creep strength comparable to that of a known weak reference material, tested both in the form of small scale and conventional creep specimens. This material was also shown to have poor Type IV strength, implying similar poor Type IV strength in the header material. As a result early Type IV cracking was predicted and inspection recommended.