The flux formulation of stainless steel flux-cored wires usually contains small amounts of bismuth in the form of Bi2O3 to improve slag detachability. The resulting bismuth content in the weld metal is about 180-200 ppm. At service temperatures below roughly 500 ºC there are no detrimental effects from bismuth on quality and mechanical properties of the weld, but at higher temperatures segregation of bismuth to the grain boundaries occurs. The following issues have been experienced:
- Reduced ductility at temperatures above 650ºC
- Inter-granular cracks at temperatures above 700ºC with fracture surface showing presence of bismuth
The vast majority of stainless steel weld deposits are put into service below about 250°C, but within power generation and process industries, extended service can exceed temperatures of 500°C. It is in these weldments that bismuth creates problems, which may also occur when carrying out a post weld heat treatment (PWHT) when weld overlaying carbon steel or after repair of castings.
Böhler Welding offers two series of bismuth-free stainless steel cored wires. These have the same weldability and slag detachability as bismuth alloyed stainless cored wires. Low-temperature CVN impact toughness and lateral expansion are also at the same level. The two series are:
- All-positional rutile cored wires feature excellent weldability in all positions. They are used for joining high-temperature resistant stainless steel. The fast freezing slag system allows relatively high amperes in positional welding.
Flat-horizontal rutile cored wires are optimized to maximize the parameters, and consequently are used for high deposition rate, in flat-horizontal positions. They are ideal for cladding low-alloyed, creep resistant steels but can be
used also for joining high-temperature resistant stainless steel when welding in flat position.
Both type of wires are easy to operate – both in manual and mechanized welding– and show excellent welding characteristics with conventional non-pulse power sources.
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