EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Low alloyed, creep resistant CrMoV alloys such as 2.25Cr-1Mo-0.25V are often used for high pressure vessels like hydrocracking reactors in the petrochemical industry. Therefore, they are usually joined via submerged-arc welding. For this purpose, the material and especially the weld seams have to withstand high temperatures in combination with high pressures. Fine V and Nb containing carbonitrides which precipitate primarily at dislocations are known to play a key role for obtaining good mechanical properties and creep resistance by stabilising the dislocation substructure.
Nm-sized MX carbonitrides in as-welded 2.25Cr-1Mo-0.25V multi-layer submerged arc weld metal were systematically investigated by means of atom probe tomography and transmission electron microscopy. Particular attention was payed to the influence of multiple-reheating caused by multi-layer welding on the precipitation behaviour of the carbonitrides. Furthermore, comparative thermodynamic calculations were performed. Multi-layer welding leads to continuous precipitation and partly dissolution of fine MX precipitates during reheating. These carbonitrides consist of Nb, V, C and N. Due to multiple reheating, more and more carbide formers like Cr, Mo, V and C are incorporated. Nonetheless, no precipitation of MX carbonitrides was detected by means of atom probe tomography in the upper, non-reheated layer of the weld metal. Thanks to the combination of high resolution investigations of the fine carbides and additionally performed thermodynamic calculations, we were able to build an improved understanding of the precipitation behaviour of MX carbonitrides during submerged-arc multi-layer welding of 2.25Cr-1Mo-0.25V.