With modern welding methods, satisfactory microstructures in 9%CrMoV (P91) steel can be obtained with a modest variation in hardness and prior austenite grain size. However, there is always a risk that significant deviations in the properties can be obtained, if the welding parameters are not optimized. In the present paper the role of extra coarse grains in the heat affected zone (HAZ) has been studied. Creep tests were carried out at 600°C for parent metal, weld metal, cross weld, simulated extra coarse grained HAZ, and simulated intercritical HAZ of a 9%CrMoV (P91) steel. The parent metal, the cross welds, the weld metal, and the simulated intercritical HAZ had about the same rupture strength except at long rupture times, where the values for the cross welds were considerably lower. In the cross welds, rupture took place in the intercritical HAZ at longer times (Type IV cracking). The simulated extra coarse grains gave considerably longer rupture times, lower strain rate and lower creep ductility than the parent metal and the weld metal. The creep strain behavior was successfully analyzed using the Omega model where the log creep strain rate is linear in the creep strain.

Abstract:Thick section copper canisters are planned to be used as a corrosion protection of nuclear waste disposal containers for long term underground deposal in Sweden. The copper canisters will have the top and possibly the bottom lid welded to the canister walls using electron beam or friction stir welding. Due to the high external hydrostatic pressure and the relatively high temperature of the waste during the first one hundred years the copper will creep. The creep process will close the manufacturing gap between the cast iron container and the copper canister. The creep ductility must be sufficient to avoid cracking of the weld.Specimens cut from the friction stir welds and the electron beam welds have been creep tested at temperatures ranging from 75 to 175 °C. Cross-weld specimens were used for both friction stir and electron beam welds. Weld metal, heat affected zone and base metal were also studied for friction stir welds. The results for the electron beam welds show that the main creep deformation is concentrated to the weld metal where the failure takes place. Weld metal and most cross-weld tests of friction stir weld material show similar creep lives and ductility as base metal tests. Ductility at rupture was found to exceed 30% for friction stir weld specimens, and the Norton power law exponent was determined to be between 30 and 50.

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This paper describes changes to specimen grips and specimen design in order to creep test steels at temperatures a little below the spalling temperature and at low stresses (long times) where a large amount of oxidation occurs. High ductility and oxidation of a creep specimen often mean that the thermocouples lose contact with the specimen surface and the specimen itself cannot be removed from the grips after testing. The improvements involve the use of corrosion resistant alloys with reasonable high temperature strength for the loading bars and grips, a change in grip design and measuring the temperature inside the specimen.

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