The repair weldability of two types of heat-resistant austenitic stainless castings, HP-Nb modified alloys and 20-32Nb alloys, is studied in the present work.
This investigation has focused on the microstructure evolution and hot ductility behavior of the two types of alloys. The microstructure evolution starting from the as-cast condition, to the condition after service exposure, as well as the condition after simulated repair welding (HAZ simulation) is discussed in the present work.
After service exposure, Ni-Nb silicide and Cr-rich M23C 6 were identified in both types of alloys. However, the HP-Nb alloys have a much higher total volume fraction of microconstituents than the 20-32Nb alloys. The M23C6 phase is more prevalent than Ni-Nb silicide in the HP alloys; while in the 20-32Nb alloys the Ni-Nb silicide is dominant. The 20-32Nb alloys have a much coarser dendritic structure than the HP alloys. The nature of the service-exposed microstructure evolution directly affected the microstructure and hot ductility behavior during simulated HAZ testing conducted with a Gleeble 3800™ thermo-mechanical simulator. For the HP-Nb alloys the results revealed a good metallurgical stability and on-cooling ductility, which can be related to good repair weldability of these alloys. In contrast, service-exposed 20-32Nb alloys showed a severe susceptibility to liquation cracking and significant loss in on-cooling ductility. The liquation cracking is the combination effect of high-silicon concentration at the dendrite boundaries resulting from the dissolution of the Ni-Nb silicide and a NbC constitutional liquation mechanism. The loss in on-cooling ductility that resulted from these two liquation mechanisms persisted from peak HAZ temperature to below 1000°C. The liquation cracking mechanism is discussed in detail relative to the microstructure evolution during HAZ simulation. The high temperature embrittlement (HTE) and strengthening mechanisms are also proposed.
Based on these studies, recommendations on repair welding are proposed from the perspective of controlling microstructure. Control of phase balance appears to be very important in avoiding cracking during repair welding and involves manipulating the relative amounts of species among the element Nb, C and Si. Controlling dendrite size is also important from the standpoint of controlling silicon concentration in the interdendritic regions during repair welding.
|School:||The Ohio State University|
|School Location:||United States -- Ohio|
|Source:||DAI-B 79/09(E), Dissertation Abstracts International|
|Keywords:||Heat-resistant stainless steel casting, High temperature embrittlement, Hot ductility behavior, Microstructure evolution, Repair weldability|
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