Repair of 9%Cr Creep Resistant Steel without PWHT

Objectives

Evaluate different strategies for the repair of 9%Cr-Mo steels without PWHT

Project Outline

Repair welds will be made in 9%Cr-Mo steel using a number of procedures.

Temper bead techniques

Temper bead welding has been successfully used for lower grade power plant steels, and has the potential to be employed for 9%Cr steels.

Autogenous re-melting techniques

For high strength steel, re-melting of weld metal via an autogenous process has been used to refine and temper underlying metal and eliminate brittle coarse-grain HAZ. A patent currently exists to this effect, which describes following each MAG bead with an autogenous TIG re-melt run. By applying a re-melt technique to the welding of martensitic power plant steels, it is hoped that enough tempering can be achieved to remove the need for PWHT.

The use of dissimilar weld metals

For creep-aged P91 material, it may be possible to use lower grade consumables, eg grade 22 consumables, as the remaining life of the component has already been compromised through exposure to stress at elevated temperatures. Alternative alloys may offer improved oxidation resistance, better ductility, and reduced risk of cracking without PWHT.

Ni-base consumables are often used when PWHT is not possible. However, inspection of such joints is not easy, and therefore these repairs are generally deemed temporary. If the weld deposit could be made easier to inspect by matching the inter-boundary spacing to that of the parent, the repair could be considered permanent; this may be more suitable for the coarser-grain parent materials.

Low transformation temperature weld metals containing approximately 10%Cr and 10%Ni have been known to reduce the level of residual stress during weld cooling, as a consequence of the transformation to martensite occurring at low temperatures (~200C), thereby offsetting the contraction which occurs during cooling. The reduction of residual stresses may be enough to remove the need for PWHT.

The above strategies may be combined to some degree, in order to achieve the most satisfactory solution. Highly restrained welds will be deposited, and the decay of residual stresses monitored during exposure to elevated temperature. Detailed microstructural studies will be carried out to aid understanding the creep behaviour. Also, evaluation will include the determination of cross-weld hardness and creep rupture life. The measurement of hardness at intervals during the creep life will be explored for both stressed and unstressed material, in order to investigate how the measurement of hardness can be used as a predictor of remnant life.

Relevant Industry Sectors

Power generation (thermal and nuclear)

Technical and Economic Benefits

Shorter repair times

Reduced costs, particularly during unplanned outages, if successful repairs can be carried out without a subsequent PWHT

Case Study