Type IV cracking in ferritic power plant steels

There have been concerted world wide efforts to develop steels suitable for use in efficient fossil fired power plants. Ferritic alloys containing between 9 and 12 wt-% chromium are seen as the most promising materials in this respect, especially for thick walled components such as headers and the main steam pipe in boilers. However, the performance of the improved steels has often not been realised in service, because premature failures occur in the heat affected zone of welded joints in a phenomenon referred to as type IV cracking. This review assesses the relationship between the composition and microstructure of 9-12 Cr steels, the welding and fabrication procedures and how these factors translate into a propensity for type IV failures.