Weld repair of grade 91 steel without post-weld heat treatment

Abstract

A weld repair technique to be used without post-weld heat treatment has been developed for use on grade 91 steel. The approach makes use of standard (non- modified) 9CrMo weld metal. The work has extended an approach used successfully on the low alloy steel ½CrMoV, which utilised a low strength 2CrMoL weld metal, to the more advanced steel grade 91, using an equivalently lower strength weld metal, standard (non-modified) 9CrMo. This has considerably lower creep strength than matching modified 9CrMo weld metal. Two variants of standard 9CrMo weld metal were chosen for investigation: a specially commissioned low carbon 9CrMoL version, with carbon below the normal minimum for this grade of weld metal (0·05 wt-%), and a conventional batch of 9CrMo weld metal, but selected to have carbon in the bottom half of the normal range. Comparison between the 9CrMoL weld metal and the standard 9CrMo weld metal, on the basis of residual stress level and creep and fracture toughness properties, has shown the latter to be the better option. The most likely repair scenario envisaged was to a retrofit grade 91 header on a UK coal fired power station. The goal was to achieve a lifetime for such a repair greater than the 4 year period between major overhauls for a typical power station of this type, corresponding to >20 kh operating hours.