Corinne Chovet

Microstructure and Creep Rupture of P92-grade weld metal

The search for new more creep-resistant materials for supercritical components of modern power plants stimulates also welding research to provide joints adequate to withstand highly demanding operation conditions. Seeking for improvement of existing welding consumables, experimental Ti-doped P92 grade weld metals were developed of exceptionally long life in creep rupture tests. In response to this request SEM and TEM investigations were carried out to identify the features of microstructure, which could be responsible for so different behaviour in the creep rupture tests. In this research the microstructures of the Ti-doped FCAW P92 weld metals were also compared with these of a standard P92 pipe material as well as with the submerged arc weld metal deposit W0 manufactured by flux-cored consumable containing no addition of Ti. Transformation of microstructures of this “reference” P92 pipe material and of the Ti-free P92 weld metal was accomplished in the accelerated creep tests on Gleeble physical simulator. An attempt has been made to correlate the fracture appearance of the creep-ruptured samples with the fine structure in their head/grip and gauge portions. In the microstructure of non-affected by the creep strain head portions of the creep-ruptured samples appeared numerous delta-ferrite grains with no carbide precipitates. Evidences were found that creep voids and cracks formed fast in the initially precipitate-free delta-ferrite grains of the sample W1, while in sample W2 during the creep test a peculiar strain-induced precipitation hardening of the delta-ferrite appeared, however no Ti was identified there in the ultra-fine precipitates.

Effect of Various Factors on Toughness in P92 Saw Weld Metal

In order to increase efficiency in thermal power plants, new grades of 9 %Cr steels have been developed and are now being used. Even though these steels are used at high temperature, where toughness is not a matter of concern, it is important that the welded joints show a good toughness at room temperature, for fabrication and construction steps and for start up / shut down considerations. As a consequence the best toughness / creep compromise has to be obtained to guarantee all the requirements. The present work aims at evaluating the effect of various chemical elements on weld metal toughness in P92 steels. All-weld metal characterizations using submerged arc process were done. Chemical elements which were varied are carbon, chromium, nitrogen and tungsten. Variations of W, C and Cr within the base material range did not significantly affect toughness of the weld metal. However nitrogen content has a great influence on toughness level, decreasing N content resulting in a toughness improvement. The detrimental effect of B and Ti on toughness of weld metal for P92 steels has also been confirmed. An optimised chemical composition has been defined on the basis of this work. This solution features a promising toughness / creep compromise, as very good toughness at room temperature and satisfactory creep behaviour have been obtained.

Additional Recommendations for Welding Cr-Mo-V Steels for Petrochemical Applications

The scope of this paper is the welding of high temperature creep resistant 2 1/4Cr-1Mo-1/4V steels for heavy wall pressure vessels for hydrogen service (petrochemical applications), plates such as ASME SA 542-D Cl4a, SA832 Gr 22V and forgings such as ASME SA 336 F22V, SA 182 F22V. Welding of these steel grades is widely covered by standards and codes, especially by API Recommended Practice P 934-A. The aim of this paper is to provide additional recommendations, applicable to submerged arc welding, in order to ensure sound welds and satisfactory service properties of welded joints. Parameters such as, selection of flux/wire combination, re-baking of flux, welding heat input, ISR and PWHT setting and monitoring, are of primary importance. In the present paper, recommendations concerning these parameters will be given, on the basis of our experience.