| Summary: | Dissimilar metal welds (DMWs) are widely used in steam vessels in thermal power stations to join low-temperature alloys, such as steels, to high temperature alloys, such as nickel-based alloys. This provides a cost-effective manufacturing solution. However, there is a history of DMWs failing due to creep in service environments. Many investigations have been performed on weld systems and failures in the traditional 2.25Cr-1Mo (P22) steels, but fewer have been performed on newer 9Cr-1Mo steels, such as P91 and P92. Failures involving these newer steels continue to occur, for reasons which are not thoroughly understood. The factors involved are believed to include system stresses, differences in thermal expansion and microstructural evolution due to interdiffusion across the weld interface. The overall aim of this research was to investigate the chemical and microstructural stability of a range of DMWs involving P91 and P92 steels. Much of the work is centred on a 3-bead manual metal arc weld of nickel alloy 625 onto a P92 substrate. This was subjected to tempering (760 °C for 2 hours) and furnace ageing (625 °C for 32 and 125 days). The as-welded and aged states were characterised by a range of techniques, including SEM, TEM, EDX and microhardness testing. Site-specific TEM samples were extracted from the weld interfaces using focussed ion beam (FIB) methods. EDX measurements show iron-enrichment in the weld metal (WM) up to 30 wt%, and a partially mixed zone (PMZ) up to 50 microns from the weld. TEM analysis of the as-welded state reveals the presence of a 1 – 2 micron wide band separating the two alloys, and of different crystallographic orientation to both. Following ageing, diffusion of carbon takes place from the P92 to the WM, leading to a carbon denuded zone (CDZ) in the former and an enriched zone (CEZ) in the latter. Precipitates in the CDZ (M23C6 and MX) dissolve to supply this diffusion, while Nb-rich MX phases have formed on grain boundaries in the CEZ. Additionally, carbides are found to form along the interface between the band and the P92. EDX measurements confirm that the aged WM is enriched in carbon near the interface. Microhardness measurements reveal slight softening of the P92 in response to ageing, and pronounced hardening of the WM. The thermodynamics of the alloys were modelled using the software Thermo-Calc, while diffusion across the interface was modelled using DICTRA. The findings support the trends of the experimental results in terms of diffusion behaviour and phase changes. An industrial case study, ex-service P91 – alloy 625 pressure vessel welds exposed to c. 565 °C for c. 40,000 hours, has also been undertaken. Creep failure occurred during service near the weld interface. SEM showed that the creep crack tip was advancing through the CDZ. Microstructural changes were similar to those in the P92 – alloy 625 system, only more pronounced; the CDZ was found to be almost entirely devoid of standard M23C6 and MX precipitates, having been replaced by a band of carbonitrides (either M23X6 or M6X) of unusual chemistry, parallel to the weld interface. A second industrial case study involved a weld between P91 and P92 steels using the P87 filler metal, recently developed by EPRI, which is designed to minimise interdiffusion. This system, in contrast to those involving alloy 625, shows evidence for only minimal interdiffusion after ageing at 649 °C for 131 days, with no CDZ being observed. These observations are supported by Thermo-Calc and DICTRA calculations. Therefore, systems of this type may be resistant to creep failure in long-term service.
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