Abstract
Since the 1970s, bismuth is widely used as an auxiliary ingredient in stainless steel flux-cored wires to improve slag detachability. But, for components subject to post-weld heat treatment (PWHT) and/or applications at high temperature, bismuth has been confirmed to have a negative effect on weld metal ductility. It has been suggested that this is due to grain boundary bismuth segregation, and it has been debated whether it is as bismuth or as bismuth oxide Bi2O3. There are also reports on cracks found in weldments after service at elevated temperatures. This has affected the specifications, and API RP 582 has included a maximum bismuth content of 20 ppm in the weld deposit when welding with austenitic stainless steel flux-cored wires for applications above 538 °C, including PWHT. This demand required development of a range of flux-cored wires intended for overlay welding (cladding) of creep-resistant steels and joining stainless steels for high-temperature applications. Standard E347, E309L and E308H wires have here been compared with bismuth-free versions in as-welded condition and after PWHT. All-weld metal has been subject to mechanical, hot ductility and Varestraint testing. Results show that bismuth-free wires have higher ductility, and this was confirmed also when welding in single V-butt weld joints. Electron microprobe analysis (EPMA) modified for high precision mapping is used to illustrate that bismuth has a particle-like distribution without any clear relation to oxygen.
Highlights
Flux-cored arc welding (FCAW) results in good surface finishes, which makes post-fabrication cleaning easier when welding stainless steel
Some areas such as the inside of nozzles and fittings cannot be covered by clad plates and/or strip cladding and need separate overlay welding. This can be efficiently done using the FCAW process with an E309L buffer layer between the creep-resistant steel and the E347 layer. This type of equipment is typically operated at temperatures below 500 °C, but depending on the alloy grade and requirements on mechanical properties, a final post-weld heat treatment (PWHT) is performed at 660–710 °C, in addition to other intermediate PWHT
Weld metals deposited from bismuth-free FCAW wires have been shown to have high-temperature creep properties on par with those made with other welding processes and consumables [8]
Summary
Flux-cored arc welding (FCAW) results in good surface finishes, which makes post-fabrication cleaning easier when welding stainless steel. This can be efficiently done using the FCAW process with an E309L buffer layer between the creep-resistant steel and the E347 layer This type of equipment is typically operated at temperatures below 500 °C, but depending on the alloy grade and requirements on mechanical properties, a final PWHT is performed at 660–710 °C, in addition to other intermediate PWHT. Weld metals deposited from bismuth-free FCAW wires have been shown to have high-temperature creep properties on par with those made with other welding processes and consumables [8]. The hot tensile tests performed with the bismuth-free E 308 H PW-FD wire still showed high elongation at both 700 °C and 800 °C, which are typical service temperatures in, e.g. the FCC regeneration process.
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