TWI Industrial Member Report Summary 1015/2012
By M Consonni
Background
Grade 91 steel (also referred to as P91 in the pipe form, EN designation X10CrMoVNb 9-1) is now widely used in fossil fuel power stations, where it has found application in new build and particularly for headers in existing power plant to replace lower alloy steel counterparts. The higher alloy content has ensured that the microstructure of the parent steel (and the HAZ of any weld) will be martensitic for all likely cooling rates. Moreover, the hardness of such as-formed martensite is typically > 400HV.
The initial stage of this project consisted of a review of the current practice for welding and weld repair of grade 91. This showed that PWHT is generally required for any grade 91 steel fabrication before entering service, to relieve residual stresses, to reduce weld metal and HAZ hardness, and to improve toughness. For the same reasons, after any repair welding there is an expectation, and currently a requirement, that a PWHT is carried out. This present project has investigated the possibility of carrying out weld repairs employing the controlled deposition approach, with some variation in electrode diameter being explored. Low heat input levels were used, as this gives not only a reasonably small coarse-grain HAZ grain size but also it restricts the width of the thermally-damaged region at or beyond the edge of the fine-grain HAZ, which is generally the creep-weak region in a welded joint subjected to cross-weld stress. The preliminary trials reported here represent the first stage in exploring whether repair welding of grade 91 steel without a subsequent PWHT is possible.
Objectives
- Carry out a literature review on welding, testing and weld repair of grade 91.
- Investigate the application of a refinement criterion for the weld repair of grade 91 components without PWHT.